Institute for Crustal Studies
1999/2000 Annual Report


Next Page

Previous Page

Table of Contents




Awards Administered

Crustal Materials

 

Frank Spera

12/15/97 - 12/14/01

Department of Energy, DE-FG03-91ER14211

7/15/99 — 6/30/01

National Science Foundation, EAR - 9972922

Magma Rheology, Mixing of Rheological Fluids, Molecular Dynamics Simulation
and Lithospheric Dynamics

Experimental Rheology of Magmatic Emulsions and Foams: Effects of
Composition, Temperature, Shear Rate and Bubble Content

Construction of a high temperature rheometer based on a concentric cylinder design has been completed. This device can investigate the rheology of magma, a complex mixture of solids, melt and vapor bubbles, at temperatures from 700 °C to 1375 °C for shear rates in the range 10-5 to 1 s-1. The instrument has been calibrated and certified using NBS standard glasses and is accurate to within 0.02 log10 units in viscosity throughout the temperature and shear rate range. This 2-sigma uncertainty is better than the NBS quoted value of 0.029 log10 units. A paper was published in Review of Scientific Instruments where details may be found. Experiments are presently underway exploring the relative viscosity, defined as the ratio of the viscosity of the mixture to the viscosity of single phase melt for a rhyolite from the DOE sponsored drilling site at Obsidian Dome, near Long Valley California as well as melt-vapor emulsions of molten albite and molten orthoclase compositions. Results suggest that for shear rates exceeding a critical value, the relative viscosity decreases with increasing bubble content whereas for shear rates below the critical threshold, the relative viscosity increases as the bubble content increases in accord with the classical result derived by G.I. Taylor 60 years ago. The goal of this work is to develop comprehensive non-linear constitutive relations for melt-vapor emulsions in the range of shear rate and temperature relevant to crustal magmas. A short paper has been submitted to Earth and Planetary Science Letters detailing some of these results and presently another paper is being prepared for submission to Journal of Volcanological and Geothermal Research. Other work completed this year includes an extensive set of MD simulations applicable to molten and glassy CaAl2Si2O8 at temperatures and pressures characteristic of continental lithosphere, study of the glass transition in glassy anorthite and MD study of melts in the system NaAlO2-SiO2. This work was published in American Mineralogist. Other MD simulations for compositions in the system Na2O-SiO2-H2O are also underway. A final sub-project concerns porous media thermohaline convection. We have confirmed that in low porosity geologic media, a doubly-advective instability sets in that leads to chaotic behavior for Darcy Rayleigh numbers and buoyancy ratios in the range found in nature. These simulations take account of the temperature and composition dependence of fluid properties (H2O +NaCl) and allow for the importance of dispersion (rather than simple molecular diffusion) of solute. This doubly-advective instability has application to diagenesis, the formation of crustal ore deposits and the evolution of metamorphic terrains. This work is in press at Earth and Planetary Science Letters.

http://magma.geol.ucsb.edu/

 

Frank Spera

Wendy Bohrson

3/15/97 - 2/28/00

National Science Foundation, EAR-9614381

Geochronological, Isotopic and Petrological Constraints on Magma
Dynamics at Mt. Etna

Approximately 30 distinct flows representing most of the historical and many pre-historical eruptions from Mount Etna have been studied by image analysis. The crystal size distributions for plagioclase, clinopyroxene, olivine and Fe-Ti oxides have been analyzed using standard image analysis and stereological methods. Major and trace element analyses of pre-historical Mount Etna lavas and 40/39Ar dates of these same lavas have been completed.

For the image processing, in plots of the rate of change of cumulative number of crystals per length (n) versus size of crystals, distinct trends are found that can be associated with crystal residence times and volume of specific eruptive units. Future work will correlate the geochemistry and petrology of specific eruptions with their textural and crystal-size attributes. Work on the pre-historical lavas has revealed that through time, the source of magmas may have changed. Work is currently being completed that will characterize the nature of this change.

http://magma.geol.ucsb.edu/

 

Crustal Structure and Tectonics

 

Tanya Atwater

9/1/98-8/31/99

The Boyd Foundation

Support of the Creation & Testing of an Interactive Animated Geological
Teaching Module for Use in Santa Barbara Middle Schools

Professor Tanya Atwater and Artist/Animationist John Iwerks have been creating geological animations for use in undergraduate classes at UCSB and in the local K-12 school system. Animation topics include plate tectonic history of the Pacific hemisphere, of western North America, and of California/Baja California (animated map views), Cenozoic geological history of the Transverse Ranges (animated block diagrams), Pleistocene ice ages and their effects on sea level, wave-cut topography, and species isolation (map and cross-sectional views). One goal of this work is to bring alive geological processes, history and landscapes in the minds of the general populace, to create earth awareness. Another goal is to clarify plate tectonic history for western North America and the San Andreas system for the general geological community.

Atwater uses some of these animations in her classes and offers some on her web page http://www.geol.ucsb.edu/~atwater/Animations/Animations-FR.html for general public access and downloading. With Iwerks she is producing an experimental video-tape that interleaves the animations with local landscape photographs and paintings and paleontological images. This tape, along with relevant paper copy, will be used and critiqued by a group of (self-selected) master teachers during the 1999-2000 school year. The testing will be monitored and summarized by Education Graduate Student Allison Takeo to help us work out the best mode of delivery for each of various levels of students, from 6th to 12th grade.

Temporal variation in natural methane seep rate due to tides and other factors, Coal Oil Point area, California.

 

James R. Boles

Jordan F. Clark,

Libe Washburn

Ira Leifer

7/1/99-6/30/01

University of California Energy Institute, UCSB 19990760

University of California Energy Institute, UCSB

Temporal Variation of a Major Natural gas Seep,
Santa Barbara Channel, California

A large natural methane seep, occurring a kilometer offshore in 64m of water, has been monitored on an hourly basis for 10 months. About 16.4 cubic meters (580 MCF) of methane/day is captured from the seep by two large steel tents (each 30m by 30m) open at the bottom to the ocean. The gas is piped to shore where it is metered and processed. We have analyzed the variation in seep rate as a function of anthropogenic and natural factors. The affect of hydrocarbon production from the nearby South Ellwood oil field on this seep is not clear. There is no clear relation between seep rate and change in formation pressure in key wells over a 10 year period, and from a month long platform shutdown. Results, however, show a clear tidal influence on the gas flow rate; these results are the first documentation of the effect of tides on natural gas seeps.

High tide results in a reduced flow, whereas low tide correlates with an increase in flow rate. Time series analyses clearly shows all of the four principal diurnal tidal components at 12.00 hr, 12.42 hr, 23.93 hr, and 25.82 hr. The correlation indicates that each 30 cm (foot) increase of sea height results in a decrease of 3.0 to 3.8 cubic meter/hour or 0.5% of the hourly flow rate. A simple physical model indicates that the observed changes can best be accounted for by a pore activation model, whereby gas is released from small pores at low pressures but release is inhibited at higher pressure. Our study implies that sea level drops, even those as small as tidal variations, can result in significant increase in degassing from sediment. Major sea level changes might result in substantial degassing of the sediment.

 

Cathy Busby

1/1/98 - 8/31/00

American Chemical Society - Petroleum Research Fund, 32624-AC8

High Resolution Sequence Stratigraphic Framework of a
Tectonically Active Forearc Basin

Mesozoic rocks of the Baja California Peninsula form one of the best-exposed and most areally extensive, well preserved convergent margin complexes in the world. Busby proposes that this convergent margin shows an evolutionary trend that is typical of arc systems facing large ocean basins: a progression from highly extensional through mildly extensional to compressional strain regimes. This proposal focuses on the southernmost end of the Peninsular Ranges forearc basin complex of southern California and northern Baja California, Mexico, the Rosario embayment. Busby suggests that the Peninsular Ranges forearc basin complex formed as forearc strike slip basins under a compressional convergent margin strain regime, where strong coupling promoted the development of strike slip faults during oblique subduction. Strike slip basins are known to be the most tectonically active and complex basin type of all, characteristically showing very rapidly alternating subsidence, uplift and tilting events of great magnitude. This makes them ideal for the kind of detailed "tectono-stratigraphic analysis" that is proposed here. Busby and a graduate student will spend 3 to 4 months in the field mapping the entire basin at a scale of 1:50,000 or better using aerial photographs, satellite imagery and topographic bases. Packages of strata will then be dated using foraminifera, nannofossils, macrofossils and magnetostratigraphic data.

 

Cathy Busby

7/1/98-6/30/00

UC MEXUS, UCSB 08981162

Sequence Stratigraphic Studies in Baja California, Mexico

The Rosario Embayment in northwestern Baja California represents the most extensive area (3600 km3) of continuous exposure of Cretaceous-Paleocene strata in California or Mexico. Exposures in the area are excellent and show few postdepositional faults, which makes it ideal for testing sequence stratigraphic models. Busby and her collaborators have completed several detailed studies of the individual depositional systems in this basin. However, geologic mapping of the entire basin stands at a reconnaissance level, barely enough to suggest that tectonics provided the primary control on the evolution of the basin. This resulted in a sequence stratigraphic framework that does not fit standard models based on eustatics. To test this model, detailed datings and paleobathymetries are needed. This study will provide the kind of high-resolution dates and bathymetric data needed to determine eustatic vs. tectonic controls on sea-level fluctuations in the basin.

 

Bradley Hacker

4/1/98-3/31/00

National Science Foundation, EAR-9728643

Xenoliths from the North-Central Tibetan Plateau
INDEPTH III Geologic Team

This proposal is for a multi-year, interdisciplinary study of the exposed crustal section of the Talkeetna island arc to address the rate and mechanism of continental growth at convergent margins. The Talkeetna arc section, particularly in the Nelchina-Tonsina region, is a crustal section through an accreted, Jurassic subduction-related magmatic arc, from volcanic, volcaniclastic and sedimentary rocks at the top to residual mantle peridotites at the base. Detailed studies of exposed, island arc sections can provide a crucial link between geophysical observations and geochemical studies of volcanic rocks in active arcs. In the same way that ophiolite studies have provided an ideal counterpoint to both marine geophysics and analysis of mid-ocean ridge basalts in developing a complete picture of crustal accretion at oceanic spreading ridges, studies of arc sections will be essential to progress in understanding arc magmatism and crustal genesis over the coming decade. http://www.geol.ucsb.edu/~hacker/

 

Bradley Hacker

7/15/98-6/30/01

National Science Foundation, EAR-9809840

The Thermal, Petrological and Seismological Structure of Subducting
Oceanic Lithosphere

We compiled and assessed a database of the relevant thermodynamic properties of phases expected in mafic and ultramafic rocks at subsolidus pressures up to 8 GPa. These data are used to calculate mineral physical properties (e.g., density, Vp, Vs) at elevated P and T. Rock physical properties are derived using a Voigt-Reuss-Hill average of mineral physical properties weighted by mineral proportions.

A phase diagram for 5 ultramafic rock compositions was constructed using Holland and Powell's Thermocalc program and natural mineral compositions, and modified in light of recent experiments on hydrous phases at high P. A phase diagram for mafic rocks was constructed from Thermocalc and from petrological studies that reported mineral modes, mineral compositions, and MORB bulk-rock compositions. These phase diagrams permit direct assessment of phase proportions, reaction stoichiometries, H2O content, and, in combination with the rock physical property calculations described above, density, Vp, and Vs of mafic and ultramafic rocks in subduction zones.

Application of these principles to a thermal model results in a cross section detailing the mineralogy, H2O content, density, Vp, and Vs, of a specific subduction zone. These predictions are then compared to models of slab devolatilization related to arc magma generation, models of devolatilization-caused earthquakes, calculations of slab buoyancy, and observations of slab wave speed.

For example, the Philippine Sea plate beneath SW Japan shows seismic velocities of Vp=6.8 and Vs=4.0 km/s in the upper 60 km (Ohkura, 2000). These observations are well fit by calculated velocities for epidote-blueschist facies mafic rocks, the expected equilibrium mineral assemblage, implying that phase transformations are keeping pace with increasing P and T. In contrast, in at least 5 other colder Pacific subduction zones, high-frequency energy that travels up a slab waveguide from intermediate-depth earthquakes is delayed 5-9% relative to long-wavelength energy propagated through the surrounding mantle. This may reflect lawsonite + amphibole stability or partial transformation in the eclogite stability field.

http://www.geol.ucsb.edu/~hacker/

 

Bradley Hacker

1/15/99-12/31/01

National Science Foundation, EAR-9814889

Exhumation of Ultrahigh-Pressure Rocks in the
Scandinavian Caledonides

Unraveling the high-temperature exhumation history of Norwegian UHP eclogites is severely hampered by the paucity of Caledonian igneous rocks, which could otherwise provide opportunities for measuring P, T, deformation, and the ages of high-temperature minerals. Each of the few known plutons is therefore viewed with considerable interest. The Sognekollen pluton is particularly noteworthy because it is the only pluton involved with the Nordfjord-Sognfjord Detachment Fault--the premier, large-magnitude exhumation structure in the Western Gneiss Region of Norway. The crustal section in the Sognekollen area is a W-plunging synform that exposes eclogite-bearing orthogneiss overlain by metasedimentary rock, metavolcanic rock, and capping Devonian sedimentary rocks. We calculate equilibration conditions of c. 2.0 GPa and 500°C for eclogites reported by Chauvet et al. (1992). Kyanite-staurolite-chloritoid-garnet-(coesite pseudomorphs?) rocks within the metasedimentary section yield univariant equilibria at comparable pressures and temperatures, implying at least 70 km of exhumation. The Sognekollen pluton is a leucocratic garnet-epidote bearing body with associated felsic dikes that intruded along the metasedimentary/metavolcanic rock contact. Pooled U/Pb isotopic ratios from two of the dikes yield a 1.3 Ga upper intercept and a 437+/-19 Ma lower intercept, which we interpret as the crystallization age of the dikes. Zircons from the pluton itself are U rich and show significant Pb loss, but similarities in rock compositions imply that the pluton and dikes are consanguineous. The metasedimentary host rocks to the pluton underwent regional amphibolite-facies metamorphism at 600-1000 MPa. They document strong down-to-the west extension with a range of outcrop- to crystal-scale structures. Discrete mylonite zones, cataclastic faults, and outcrop-scale brittle faulting indicate that the same deformation regime continued during cooling through greenschist-facies temperatures until c. 395 Ma (hornblende and muscovite 40Ar/39Ar ages from Chauvet and Dallmeyer, 1992). http://www.geol.ucsb.edu/~hacker/

 

Bradley Hacker

7/1/98-6/30/00

National Science Foundation, EAR-9725667

Collaborative Research: Petrotectonic Study of Ultrahigh-Pressure Rocks from
the Kokchetav Massif, Northern Kazakhstan, and the Maksyutov Complex,
South Ural Mountains, Russia

SHRIMP analyses of Kokchetav zircons with diamond inclusions have yielded a mean age of 530 7 Ma; two grains without diamonds contained 2.0 Ga cores (Claoue-Long et al. 1991). Our 40Ar/39Ar analyses show cooling through muscovite and biotite closure at 528-529 Ma, within a few million years of the U/Pb ages. Nearby units show cooling as recent as 507-511 Ma.

 

Jeff Lee

7/1/99 — 5/31/01

National Science Foundation, EAR-9902968

Collaborative Research: Dynamics of Intraplate Fault Systems in the
Northern Basin and Province

This collaborative project is investigating the dynamics of a system of intraplate faults in the northern Basin and Range province using paleoseismic techniques on range front faults in central and eastern Nevada. In light of ongoing research on contemporary strain accumulation across this system of faults using GPS, it is timely to obtain a precise chronology of late Quaternary strain release across the province. The two data sets will provide an unprecedented basis for modeling the dynamics of Basin and Range faulting.

To date our research efforts have on range front faults of the Cortez Mountains and Schell Creek Range. For the Cortez Mountains we have (1) completed detailed surficial mapping and fault scarp profiling of the 60-km long range front fault zone, (2) permitted, opened, and initiated logging of three trenches across an alluvial scarp at the mouth of Fourmile Canyon, (3) sampled various units in the alluvial gravels offset by and overlapping the scarp for 14C and 26Cl geochronology, and (4) sampled a 6-m high bedrock scarp for surface exposure age dating. For the Schell Creek Range we have (1) completed reconnaissance surficial mapping of a 70-km long range bounding fault segment and (2) examined a fluvial scarp that transects the main fault zone appropriate for application of alluvial scarp dating.

 

Jeff Lee

Phil Gans

4/1/96 - 3/31/00

National Science Foundation, EAR-9526861

Geometry and Timing of Gneiss Dome Formation, Southern Tibet, China

This project combines geologic mapping, structural studies, metamorphic petrology, and geochronology to characterize and document the structural and tectonic evolution of gneiss domes in southern Tibet. 

A manuscript, entitled Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints", will soon be published by Tectonics. In addition, we presented a talk at the Fall, 1999 AGU meeting entitled "Contraction and extension in southern Tibet: Structural, petrologic, and thermochronologic constraints from the north Himalayan gneiss domes" and an invited talk at University of Idaho entitled "Contraction and extension in southern Tibet: Structural, petrologic, and thermochronologic constraints."

Ar/Ar analyses were completed on 18 mica and hornblende samples from the Mabja Dome; problems with the mass spectrometer require that 15 additional samples be reanalyzed. These should be completed by the end of the year. Apatite fission track analyses on seven samples from the Mabja Dome were completed. These data indicate rapid cooling uniformly across the dome from the middle to late Miocene.

 

Bruce Luyendyk

7/1/99-6/30/00

University of California Energy Institute, UCSB 1990762

Variation in Discharge from Marine Hydrocarbon Seep at Coal Oil Point, CA: Implications for Offshore Oil Production

Oil and natural gas are found escaping from the sea floor at many locations on the world’s continental shelves. These seeps are suspected to be a significant source of atmosphere, ocean, and beach pollution as well as a source of the greenhouse gas methane. One of the largest seeps known is offshore Coal Oil Point, California. We have estimated seep hydrocarbon discharge here by mapping gas bubble plumes emitting from the sea floor with 3.5 kHz and 50 kHz sonar. Our studies of this seep field suggest that discharge from parts of it has decreased up to 50% during the past two decades; this we attribute to a decrease in pressure in the reservoir feeding the seeps due to offshore oil production. This assumes that the discharge is not varying under natural influences. However, repeat surveys we have completed suggest a large variation in volume discharge over a few years in spite of little change in reservoir pressure over the same period. Our objective this year is to compare repeat sonar surveys of the Coal Oil Point field to detect time variation in gas discharge. New surveys will be compared against a baseline survey completed in August 1996. We will compare both the differences in sonar reflection signatures to measure relative change, and also the computed differences in volume discharge. Volume discharge will be estimated by calibrating sonar returns against gas captured at the ocean surface in the same location. We made a repeat survey in September 1998 that includes complimentary sonar and capture data, and we request funds to analyze that data. We also request funds to make repeat surveys in September 1999 and February 2000. This will give us two-year, one year, and seasonal comparisons. Other projects underway by our research group include estimates of seep discharge by atmospheric tracer experiments and by direct capture of seep gas with a drifting surface buoy (mentioned above). A parallel project is requesting funding for time series analysis of gas capture data from a large gas containment structure placed on the sea floor over one seep in this field by an oil company in 1982. The results from these integrated projects will identify discharge variation due to natural and industrial causes, estimate the magnitude of the variation, and predict future discharge amounts. Estimating the effects of oil production in decreasing seepage rates would permit attaching an environmental benefit to offshore oil production.

 

Bruce Luyendyk

7/1/00-6/30/01

University of California Energy Institute, UCSB 20000798

A New Device to Sample Marine Oil Slicks (ND-SOS)

Oil and tarballs continually wash up on the shores of Santa Barbara Channel (SBC) including both popular tourist beaches in Santa Barbara and those on the islands in the protected Channel Islands National Marine Sanctuary. The oil arises from both natural and anthropogenic sources and is transported by currents and winds to the beaches. The anthropogenic sources are related to oil production, boat transportation, and run-off of car motor oil. The environmental toll taken by an oil spill results from both the quantity of oil released and the changes it undergoes while being carried to shore. Although it is popularly assumed that oil on SBC beaches results from oil production, research suggests otherwise. However, convincing data linking marine slicks and oil on beaches to natural oil and gas seeps are lacking. Oil slicks evolve in time due to numerous processes. The knowledge of this process is imperfect because the source of the oil is often unknown and sampling has been spotty. Our goal is to provide detailed estimates of the quantity and composition of an oil slick as it varies in time and position. We will adapt a technique developed to sample the sea surface using a rotating glass drum suspended on a small catamaran. By towing the catamaran sampler through a slick a continuous sample record can be obtained. This record can then be later analyzed for oil composition and quantity. The sampler will be tested on a natural marine oil slick within the Coal Oil Point hydrocarbon seep field, with the goal of attempting to quantify the amount of oil released from a single oil seep. The proximity of the seeps to UCSB allows easy access to the sites. This UCEI-funded research will be used as matching funds in a companion project funded by the U.S. Minerals Management Service.

 

 

Bruce Luyendyk

8/15/97 - 7/31/01

National Science Foundation, OPP-9615281

Collaborative Research: Air-Ground Study of Tectonics at the Boundary
Between the Eastern Ross Embayment and Western Marie Byrd Land, Antarctica: Basement Geology and Structure, and Influences on West Antarctic Glaciation

During the 1998/99 season SOAR conducted an aerogeophysical survey in western Marie Byrd Land in the region of Edward VII Peninsula and the Ford Ranges. A total of 212 lines were flown amounting to about 36,000 km of gravity, magnetic, radar penetration and laser altimetry data over an area of 130,000 km2. All navigation was by precision differential GPS. During 1999/00 we received most of the data products from Lamont Doherty Earth Observatory (LDEO) and the University of Texas at Austin Institute of Geophysics (UTIG).

Efforts to date have focused on quality control and editing of the data. Processed and edited magnetics and gravity data were received from LDEO in February 2000. Because the technician doing the processing had not anticipated that we would want to verify his work, there was a delay until March and April, 2000, until complete raw data and corrections were obtained. Data quality and processing is on the whole very good to excellent. Our changes to the original processing will probably be limited to adding back data in areas of limited crossover coverage where conservative acceptance criteria employed by LDEO have produced significant data gaps.

Surface-elevation and ice-thickness data were received from UTIG in March and April, 2000, although the surface elevation data set is still not quite complete. Processing of these data involves fewer subjective decisions, so we do not anticipate significant changes to the editing, but gridding is more difficult because the ice and bedrock surfaces lack the inherent smoothness of potential fields that were measured at significant elevation. Current efforts are focused on choosing an optimal gridding strategy for the bedrock surface, which is needed for quantitative interpretation of the magnetic and gravity data.

Paleomagnetic samples taken during the complementary ground program have been processed at UCSB, UC Santa Cruz, and Caltech. In addition, AMS on all samples is complete. To aid in the interpretation of the aerogeophysical data we are compiling magnetic properties and densities of the rock samples. We are also in contact with Christine Smith Siddoway who is mapping brittle structure in the Ford Ranges. These features will guide our interpretation of potential fields’ data.

 

Very preliminary interpretation of the relation between the bedrock topography and the potential fields has begun. As expected, the gravity field shows high coherence with the bedrock surface over most of the area. The Fosdick Mountains area is one of few places where the magnetic fieldshows similar coherence, suggesting that the range has moderate strength of magnetization and uniform polarity and intensity. A few high-amplitude positive magnetic anomalies occur on the geographic southeast part of the survey area, suggesting young volcanic or shallow intrusive centers under the ice.

Paleomagnetic Characteristic directions from Cretaceous and Devonian intrusive rocks in the southern Ford Ranges are all normally magnetized and in a direction statistically identical to the synthetic mid-Cretaceous paleomagnetic pole for East Antarctica (EANT) of DiVenere, et al. (1994). The southern Ford Ranges pole is distinct from the northern Ford Ranges pole of Luyendyk et al. (1996). There are two possible interpretations: 1) There is an EANT/MBL (Marei Byrd Land) crustal boundary of Late Mesozoic or Cenozoic age located within the Ford Ranges beneath the Crevasse Valley glacier that separates the northern and southern Ford Ranges. Or 2), The relative motion of MBL with respect to EANT since the Cretaceous, as recorded in the northern and southern Ford Ranges, is purely the result of vertical-axis block rotations there and the southern Ford Ranges have not been rotated. The second interpretation is reinforced by the fact that none of the southern Ford Ranges VGPs have been tilted about a horizontal axis whereas tilting is seen at several locations in the northern Ford Ranges (Luyendyk et al., 196). Anisotropy of magnetic susceptibility (AMS) measurements have been made at UCSB and UC Santa Cruz on our samples. The measurements reveal a regional pattern of foliation and lineation that depends on the age of the intrusive rocks, as published by Adams (1987),suggesting the development of fabric during or shortly after initial intrusion.

 

Bruce Luyendyk

7/1/98-6/30/02

National Science Foundation, OPP-9725876

GPS Measurement of Isostatic Rebound and Tectonics
in Marie Byrd Land, West Antarctica

In the 1998/99 season three autonomous Global Positioning Satellite (GPS) receiver systems were installed in western Marie Byrd Land. The objective is to continually record positions at three bedrock points to monitor crustal deformation and postglacial rebound. Stations were installed in the Rockefeller Mountains on Edward VII Peninsula (MBL1; Mt. Patterson), the Phillips Mountains of the northern Ford Ranges (MBL2; Mt. Carbone), and the central Ford Ranges (MBL3; Clark Mountains). During this past season (99/00) these stations were visited twice. On the first visit in November 1999 it was found that all stations suffered wind damage to the wind generator systems. In addition, the disk drives in the data logging system had failed. Before revisiting the stations in January 2000 we designed a smaller wind generator modeled after a system used by Dr. R. Dibble on Mt. Erebus. We decided to switch from logging data on disc to solid state memory. During the January visit MBL2 and MBL3 were upgraded with flash Databridges and Multi-fan Generators. MBL2 was brought into operation, but we were unable to confirm it was logging due to weather and time constraints. MBL3 was operating fine after upgrading. MBL1 was removed except for the tower and antenna but not upgraded due to weather and time constraints. It is now set up and operating at McMurdo Station. We recovered data for sites MBL1, MBL2, and MBL3 during the 1998-1999 season and some data from the site MBL1 in November 1999, providing a time baseline of just under one year. While the velocity results are inconclusive they do suggest that spreading is occurring across the Ross Embayment. To strengthen our solution we used data available from the GPS stations Mount Coats and Mount Cocks from February and November 1999 (from Dr. C. A. Raymond). Our results show that MCM4 (McMurdo) is moving south 7+- 13 mm/yr, west 15+-24 mm/yr, and up 3+-44 mm/yr relative to MBL1, located in the Rockefeller mountains. The errors are scaled one-sigma errors or scaled by a factor of 1.8 such that the chi-squared per degree-of-freedom is one in the velocity solution. Examined in a different way, the baseline between MCM4 and MBL1 is lengthening at a rate of 17+- 22 mm/yr and moving transverse to each other at a negligible rate of 2+-16 mm/yr. The low vertical rate between the two stations lends confidence that the solution is fairly robust. Additional years of data will improve the velocity estimate while reducing the errors.

 

Craig Nicholson

Marc Kamerling

2/1/99-1/31/00

US Geological Survey, 99HQGR0079

Continuation of Acquisition of 3-D Subsurface Well Data & 3-D GIS
for the Ventura Basin, California

In the Ventura Basin, faults and folds accommodate high rates of oblique crustal strain and uplift rates exceed 10 mm/yr. These faults represent a significant seismic hazard to a large urban population, yet little is understood about these active tectonic structures or about the folding associated with these oblique faults, because little has been done to document the nature or subsurface geometry of these structures in 3D. For this reason, we are evaluating a unique 3D data set provided by the Ventura Basin Study Group (VBSG). The VBSG study consists of 17 structure contour maps and 84 interlocking cross section data panels based on nearly 1200 correlated deep-penetration (1-5 km) wells. Many of these wells drill active fault and fold structures associated with major fault systems, including the San Cayetano, Oak Ridge, and Santa Susana faults. This integrated 3D study is based on wire-line logs, mud logs, paleontological reports, core analyses, and surface maps. Each data panel typically ties in 4 directions to define the sides of a 3D data volume or cell. The result is a 3D presentation of an enormous quantity of high-quality subsurface data that have been reconciled into a coherent geological interpretation. Any 2D or 3D kinematic model of the basin and its associated fault and fold geometry must incorporate these data, if it is to be successful.

After 1) acquiring the VBSG subsurface study, and 2) developing an on-line digital database of these and other data for the Ventura Basin, this continuation project consists mostly of creation of three-dimensional models of the structures within the Ventura Basin, and linking these onshore structures with similar structures observed offshore in the Santa Barbara Channel. To date, we have identified and mapped several stratigraphic surfaces, including Top Lower Pico, Top Repetto, Top Monterey, Top Vaqueros, and an approximately 1 Ma surface originally identified by Yeats [1991]. Fault surfaces, including the Oak Ridge, Red Mountain, Pitas Point and North Channel, have also been produced. These 3D surfaces then provide a basis for understanding the geometry and tectonic evolution of these structures in both space and time. The VBSG maps and cross sections have been digitally scanned and are now available to the public from our website at http://www.crustal.ucsb.edu/hopps. Over the last year, this website typically sustains on average over 2000 hits per month from approximately 200 different users.

 

Craig Nicholson

Marc Kamerling

7/1/98-6/30/01

National Science Foundation, EAR-9805200

Faulting and Folding in Oblique Convergence: A Test of Fault-Related Fold
Models for the Western Transverse Ranges, California

The western Transverse Ranges are one of the most active tectonic regions of the world. In the Ventura Basin, faults and folds accommodate high rates of oblique crustal strain and uplift rates exceed 10 mm/yr. The 1994 M6.7 Northridge earthquake occurred on a blind, south-dipping fault beneath the San Fernando Valley that is considered part of the same fault and fold system that extends westward into the Ventura Basin and eastern Santa Barbara Channel. These active fault structures represent a significant seismic hazard, yet much of what is 'known' about these faults and folds has been inferred from 2D balanced fault-related fold models. To test the reliability of these 2D models to predict 3D subsurface structure within the western Transverse Ranges, we are evaluating a unique integrated 3D data set. This data set includes the Ventura Basin Study Group structure maps and cross sections, combined with detailed subsurface information provided by seismicity, seismic reflection profiles and deep penetration well log data both onshore and offshore in the Ventura basin and eastern Santa Barbara Channel. Together, this integrated data set is being used to identify and map in 3D several distinctive subsurface fault and fold structures, stratigraphic horizons, and active fault surfaces. The resulting 3D images and structure contour maps are then used to test predictions made by different fault-related fold models and to evaluate the evolution of the subsurface deformation with time using 3D map restoration techniques. Preliminary results of this 3D analysis are available from our ICS website at http://www.crustal.ucsb.edu/vbmrp.

Our results indicate that the high rates of oblique crustal strain observed in the Ventura basin are accommodated by a wide variety of deformational processes. Most models used to describe this deformation, infer subsurface fault geometry, or resolve geodetic strain or geologic data into fault slip typically presume rigid footwall blocks, non-deforming planar fault surfaces, and deformation that is largely restricted to the hanging-wall. In California, however, several deep, subsiding basins–including the Ventura basin–are often bounded by oblique reverse faults that thrust early-Cenozoic and older rocks over young unconsolidated sediments. This suggests that footwall deformation, subsidence and compaction may play an important role in accommodating regional tectonic strain. Even in the absence of active crustal shortening, differential subsidence, pressure solution, and 3D compaction of footwall sediments relative to hanging-wall basement rocks can produce surficial motions that mimic deep fault slip. Such effects could then contribute to net horizontal and vertical motions in both geologic and geodetic data, resulting in overestimates of the inferred seismic hazard. In this project, we are attempting to identify and quantify the net influence that these largely aseismic processes like subsidence and compaction may have.

In addition to quantitative analysis of the subsurface deformation with time, another aspect of this project is the 3D visualization of these active subsurface fault and fold structures. Thus, as part of this project we are developing a 3D visualization and analysis laboratory at ICS. Currently, this facility consists of two SGI workstations with state-of-the-art interactive 3D software (gOcad). Silicon Graphics, Inc. donated the workstations, and the software was provided as part of a cooperative effort between ICS, the gOcad university consortium, and Oxy Petroleum.

 

Nicholas Pinter

Chris Sorlien

7/1/97 — 12/31/99

US Geological Survey, 1434HQ7GR03173

Slip on the Channel Islands-Santa Monica Mountains Thrust: Testing Models
of Fault-Related Folding

During the past year, Chris Sorlien, my students, and myself have continued work on our NEHRP-funded research project on the Northern Channel Islands. As discussed in last year's ICS Annual Report, the "thrust" of this research is using precisely-measured deformation of coastal terraces and interpretation of seismic-reflection profiles around the islands to infer the geometry and rates of slip on the reverse-fault structure that is believed to core the Northern Channel Islands trend.

A new development on this project in the last year was that work was extended from Anacapa Island and Santa Cruz Island, to the two western islands, Santa Rosa and San Miguel, through a supplementary award from NEHRP. In particular, work during this past year took advantage of an arrangement with UNAVCO, the Universities Navstar Consortium based at University of Colorado, by which three dual-frequency GPS receivers and a field technician were provided in order to acquire high-precision position measurements for terrace sites on San Miguel and Santa Rosa islands. Work in previous years had utilized ground-based surveying equipment (laser total station) to measure site positions, which was a slow and labor-intensive technique. The GPS equipment was highly mobile, and each point was established independent of all others (except the base station). The result was that site positions were measured with accuracies of 5 cm or better, with just 10-15 minutes of satellite data collection per location.

Analysis of these measurements is now underway, and preliminary results suggest that the dense network of high-precision terrace measurements on all four islands, combined with the offshore data, will provide an unprecedented history of Late Quaternary deformation of the Northern Channel Islands.

 

Christopher Sorlien

2/1/99-1/31/02

University of Southern California

Southern California Earthquake Center, USC 572726

Rapid Subsidence and South Propagation of the Active Santa Monica
Mountains-Channel Islands Thrust

Funding for this project arrived recently and work on this project is just getting underway. Preliminary work shows that the offshore Santa Monica fault, known as the Dume fault, has a 700 m-high sea floor escarpment west of its intersection with the Palos Verdes fault, and about 2 km of vertical separation of the youngest pre-folding horizon (not yet identified). North-dipping reflections beneath and south of this escarpment are interpreted to be fault-plane reflections. The Malibu Coast fault is identified in the hanging-wall north of the Dume fault, and mapped to be continuous with the Santa Cruz Island fault. The hanging-wall block of the Dume fault shows little net vertical motion, as there is little erosion of the mainland continental shelf southwest of Point Dume. Lack of uplift with respect to sea level is nearly irrelevant to activity on the Dume fault and underlying blind Santa Monica Mountains thrust; it is the structural relief that is important. Vertical motion is the sum of tectonic uplift, sediment compaction (can be ignored on shelf), and isostatic subsidence.

 

Chris Sorlien

Marc Kamerling

2/01/99 — 1/31/00

US Geological Survey, 1434HQ97GR0080

Fault Displacement and Fold Contraction Estimated by Unfolding of
Quaternary Strata, Onshore and Offshore Ventura Basin, California

We created digital maps that show the geometry of deformed layers along faults bounding onshore and offshore Ventura basin. The mapping of a 1.8 million year-old horizon includes a complex stack of thrust faults and folds along the northern margin of offshore Ventura basin. The cities of Carpinteria, Santa Barbara, Goleta, and the UCSB campus all lie in vulnerable positions in the hanging-walls of these north-dipping faults. Unfolding of the digital maps and restoration of the flattened fault block outlines result in quantitative estimates of displacement and shortening. North-south shortening ranges from less than 1 km south of the UCSB campus to 3 km between Santa Barbara and Carpinteria. Most of the folding south of the UCSB campus post-dates 1 million years ago, and young folding is suggested but not yet proved south of Santa Barbara and Carpinteria. Our mapping and restoration extend to the north Branch of the Red Mountain fault. Additional deformation has occurred across this fault and in the folds and faults along the coastal plain.

We created digital subsurface maps on Quaternary horizons through onshore and offshore Ventura basin and the faults and folds that make up the northern margin of that basin. The software UNFOLD was used to restore the digital maps to an initial horizontal state. The unfolded surfaces in each fault and fold block were fit together and compared to the present deformed state to result in a model for horizontal displacement. The original project focused on the Oak Ridge fault, while this renewal project focuses on the Pitas Point fault, North Channel fault, and south branch of the Red Mountain fault. Reflections from these faults are imaged on the seismic reflection profiles used for mapping, and faults were also mapped from the sea floor geology and detected by repeated sections and abrupt dip changes in wells. The renewal project resulted in 34 digital structure-contour maps of the top Lower Pico Formation (~1.8 Ma) on fault-bounded blocks. The onshore maps were constructed in depth from numerous cross sections, while the offshore maps were constructed in travel time from industry seismic reflection data tied to 45 wells and converted to depth using velocity models derived from wells.

The N-dipping Pitas Point fault and the North Channel fault are distinct from each other in cross section but are stacked to map along the same E-W trend in northern Santa Barbara Channel. Both faults intersect in the upper 1-km, with the Pitas Point fault dipping more steeply to diverge down-dip. The south branch of the offshore Red Mountain fault is a separate parallel structure 6-km to the north. The Red Mountain fault splays into two main branches near the Carpinteria coast. The northern branch decreases in displacement to the west and continues west of the UCSB campus where it may die into folding. The south branch also decreases in displacement to the west and dies out in a syncline south of Santa Barbara. The North Channel and Red Mountain faults probably intersect with depth, detaching the intervening upper crustal block. Displacement is transferred from one fault to another via NE-SW cross faults and probably by vertical-axis local block rotations. Major cross faults are mapped or inferred near Santa Paula, along the Ventura River, southwest of Rincon Point, and from Fernald Point to south of Santa Barbara. Abrupt changes in shortening and fault-fold style along the strike of the Pitas Point and North Channel faults occur across these NE-SW faults. Unfolding and restoration of the 34 digital maps on the 1.8 Ma horizon indicates shortening varying from 1 km south of the UCSB campus to 3 km between Santa Barbara and Carpinteria. This average less than1-2 mm/yr. rate is much less than the GPS rate of 5-6 mm/yr. across this area. The difference is probably due to shortening in the hanging-wall north of the Red Mountain fault, acceleration of the shortening rate since 1.8 Ma, and because sediment compaction within offshore Ventura basin causes an apparent shortening in the GPS data. Most of the folding south of the UCSB campus post-dates 1 million years ago, and young folding is suggested but not yet proved south of Santa Barbara and Carpinteria. In addition, folding rates appear to be higher in the last half million years than before, and the fold and thrust belt in northern Santa Barbara Channel may now accommodate shortening at rates approaching the GPS-derived motions.

Earthquakes

 

Ralph Archuleta

4/1/91 - 1/31/02

University of Southern California

Southern California Earthquake Center, USC 572726

See individual research summaries below for projects included in this grant.

Ralph Archuleta

2/1/97-1/31/02

University of Southern California

Southern California Earthquake Center, USC 572726

The Portable Broadband Instrument Center (PBIC)

The Portable Broadband Instrument Center (PBIC) provides seismic instrumentation to SCEC investigators for seismic research in southern California. Having control of instruments allows for rapid redeployment of the equipment in the event of a significant southern California earthquake. Past aftershock deployments have used PBIC equipment to supplement Southern California Seismic Network (SCSN) coverage and to obtain digital records at existing strong ground motion sites. PBIC instrumentation is compatible with Incorporated Research Institutes for Seismology (IRIS) PASSCAL equipment and has been used in several cooperative projects. In addition, the PBIC develops calibration and other quality control methods for use with the recording equipment and performs routine maintenance and repairs on seismic instrumentation for other SCEC institutions.

The PBIC provided equipment to nine different research experiments over this past year. Substantial field assistance was provided on several of the larger projects including the active phase of LARSE II and the Hector Mine deployment. The PBIC has completed the basic timing corrections and preliminary event associations of the Hector Mine data. Further processing steps are already underway and new data sets will be available in the next few months. This preliminary data set is available via the web at http://www.crustal.ucsb.edu/scec/hectormines. The Santa Barbara Array, delayed by the Hector Mine deployment and subsequent experiments, now has nine stations operational. The equipment was used heavily over the past year with the exception of a several month period following the completion of the Hector Mine deployment. This time was used to perform further maintenance on DASs and to upgrade all the GPS units to correct a bug introduced by the WNRO and Y2K upgrades performed in mid 1999. ARGOS satellite telemetry systems from PBIC and PASSCAL were used to monitor equipment status of the Hector Mine deployment and allowed researchers to better gauge battery replacement intervals and the rate of data collection.

The PBIC web page, http://www.crustal.ucsb.edu/scec/pbic/, registered over 21000 hits from over 2000 different users and is being redesigned to provide easier and expanded access to user documentation and legacy information. Summaries and repair histories for all major pieces of PBIC equipment are now available online. Response information as well as the equipment information is being migrated to a web accessible database form.

Outreach programs continue to play an important role in the PBIC. ICS researchers and the PBIC participated in seismological demonstrations at local schools and hosted a booth at the Earth Day 2000 celebration in Santa Barbara at Santa Barbara Community College.

Ralph Archuleta

Alexei Tumarkin

2/1/99 - 1/31/02

University of Southern California

Southern California Earthquake Center, USC 572726

Strong Motion Database (SMDB)

SMDB continues to see significant use from both the seismological and earthquake engineering communities. In 1999, more than 139,000 Web pages were accessed from the site; 3.7 GB of strong motion data were downloaded from SMDB; and 9,760 files were downloaded from off-site FTP and Web sites maintained by other agencies. http://smdb.crustal.ucsb.edu

These numbers translate to an average daily access of 380 Web pages, download of 10 MB of strong-motion data from SMDB, and download of 27 files from external sites, representing increases of 83%, 23%, and 63% percent, respectively, over 1998 numbers (Figure 2).

Improvements to SMDB in 1999 include:

Data from thirty-four new earthquakes have been added with 440 station records. This is an increase of 32% for the number of station records over the beginning of the year. The new data includes records from the 10/16/99 Hector Mine earthquake (Figure 3), as well as from earthquakes in Alaska, Chile, Mexico, and Japan. Links to external FTP sites have been added from the University of Nevada Reno (for Guerrero Gap records) and from K-NET (for Japanese records). The database now contains 143 earthquakes and 1,796 site records.

Quality assessment has been conducted to improve the accuracy of important parameters in the database. All database values for epicentral distance, hypocentral distance, peak ground acceleration, response spectral amplitudes, and filter frequencies have been compared to values determined from the data and updated where discrepancies were found.

Query times have been reduced by up to 50% by optimizing code. For example, a query returning all data from the 1994 Northridge CA earthquake has been reduced from thirteen to seven seconds.

A new map access method has been added to the database (http://smdb.crustal.ucsb.edu/ows-bin/mapcode.cgi?title=World+Map). This access method produce a map based on a user query showing all earthquakes and stations that resulted from that query. Clicking on an earthquake or station on the map produces a page showing a summary of data for that earthquake or station. The user can also zoom in or out on the map. An example is shown in Figure 3.

Navigation of the Web site has been improved by adding the major Web site links to the top of each page produced by the site.

Following requests from users, data summary pages now show sensor locations and component angles for each trace. These parameters are shown along with the station name, station owner, peak ground acceleration, and epicentral distance.

A new page has been added to the site listing all agencies that have contributed data to the database: (http://smdb.crustal.ucsb.edu/www/sources.html). This new page makes it easier for the user to identify the agency from which data has been collected and gives additional credit to the agencies involved.

A program to convert strong-motion data to the Seismic Analysis Code (SAC) format has been written. This program reads fourteen different ASCII strong-motion formats and converts them to one single format. The program is available from http://smdb.crustal.ucsb.edu/www/sm2sac.pl

To increase outreach to the engineering community, the database was presented at the EERI annual meeting in San Diego on February 5-7, 1999.

 

Edward Keller

2/1/98-1/31/02

University of Southern California

Southern California Earthquake Center USC 572726

Seismic Sources and Earthquake Hazards
of the Santa Barbara Fold Belt, California

We determine that folding is the principal style of late Pleistocene deformation in the Santa Barbara Fold Belt and is the result of blind, reverse and/or thrust faulting. Geomorphic mapping of active anticlines and uplifted, marine terraces in conjunction with U-series age dating of sampled, terrace corals establish Quaternary chronology and determine local, rates of uplift that range from approximately 1.0 to 2.0 m/ka. We identify the oxygen isotope substage 3a (45 ka) marine terrace as the first emergent terraces at Ellwood Beach, at Isla Vista and the campus of UC Santa Barbara, and at More Mesa. We identify geometric, geomorphic, and structural fault segments of the Mission Ridge fault system as well as tear faults that may control the nucleation or termination of moderate to large earthquakes.

 

 

Daniel Lavallée

2/1/99-1/31/02

University of Southern California

Southern California Earthquake Center, USC 572726

Modeling of Nonlinear Strong Ground Motion During the 1994 Northridge
Earthquake at the Van Norman Dam Complex

SCEC sponsored studies and strong motion data recorded at the Van Norman Dam Complex (VNDC) during the 17 January 1994 Northridge earthquake (Mw 6.7) have been used to determine the presence of nonlinear effects in strong ground motion. The data available included borehole velocity profiles, weak to strong motion records, and dynamic soil laboratory tests. Measurements of ground motion observed at the Jensen Generator Building (JGB) site located on bedrock, have been coupled with the nonlinear model to generate scenarios of ground shaking at the Jensen Main Building (JMB) site. The nonlinear model formulation includes effects such as anelasticity, hysteretic behavior –also known as the memory effect– and cyclic degradation due to pore water pressure. In this study, two situations have been investigated; one that includes the effect of pore pressure and the second without it.

The results of the numerical experiments conducted with this site support the assumption that pore pressure cyclic mobility did contribute significantly to the ground shaking observed at the surface. A characteristic features of simulations including pore pressure effects for this site is the depletion of the high frequency in the signal in good agreement with the recorded acceleration. None of the scenarios generated without pore pressure were able to reproduce this feature. Also spiky waveforms in the tail can be detected in both synthetic and observed accelerograms. Similar conclusions applied to both horizontal components of the JMB site. We have to stress that in this study, conclusive finding of nonlinearity is based on direct simulations of nonlinear soil dynamics and the quantification of the nonlinearity in terms of the model parameters.

The model formulation includes nonlinear effects such as anelasticity, hysteretic behavior –also known as the memory effect– and cyclic degradation due to pore water pressure. The effect of pore pressure has been incorporated using a model developed by Towhata and Ishihara, 1985; and Iai, 1990a, b. The constitutive equation corresponds to a plane strain multiple mechanism model —or multi inelastic spring model— used to simulate cyclic mobility of sands under undrained conditions. In this model, the pore pressure development is correlated to the shear work.

Several hysteresis models have been developed and discussed in the literature (e.g., Pyke, 1979, Li and Liao, 1993; McCall, 1994; Muravskii and Frydman, 1998; Yoshida et al., 1998, Xu et al., 1998). Masing’s original formulation is inadequate to describe the hysteretic behavior of material under noncyclic loadings (see Pyke, 1979 and references therein; also Li and Liao, 1993). For instance, numerical simulations with noncyclic signals suggest that application of the Masing rules leads to an unphysical situation–such as the computed stress exceeding the strength of the material (see Figure 1).

To understand the behavior of the soil during strong shaking we have developed a general formulation of hysteresis based on the Masing rules (see Kramer, 1996). The generalized Masing rules provide a framework for understanding the nonuniform dilation and translation of stress-strain loops for a material subject to non-periodic stresses (or strains). This new hysteresis formulation has several interesting features. It has a functional representation and it includes the Cundall-Pyke hypothesis (Pyke, 1979) and Masing original formulation as special cases. In its most elementary implementation, the generalized Masing rule is even simpler than the Masing and extended Masing rules (Kramer, 1996). The model depends only on one free parameter named the fiducial point. This parameter controls the size of the loop in the stress-strain space and therefore can be related to the amount of energy dissipated through the nonlinear property of the material. We have derived a relationship between the anelastic damping of a stress-strain loop and the fiducial point for cyclic loadings (Archuleta et al., 1999). In other words, the generalized Masing rules provide a mean to introduce the effect of the damping ratio into nonlinear modeling independently of the other soil parameters. The theoretical formulation of the generalized Masing rules is presented in Archuleta et al., 1999; 2000. Incorporation of the pore pressure model with the Generalized Masing rule is discussed in Archuleta et al, 1999.

 

Peng-Cheng Liu

2/1/99-1/31/02

University of Southern California

Southern California Earthquake Center, USC 572726

Source Parameter Inversion for 1999 M 7.6 Chi-Chi Taiwan Earthquake
Using 3D Green's Functions

We will invert the near-field ground motion from the magnitude 7.6 Chi-Chi, Taiwan, earthquake on September 21, 1999 to determine the source parameters of finite fault by using 3D Green's functions. Because the 3D Green's functions play key role in this kind inversion, we should have the 3D velocity structure of the region around the 1999 Chi-Chi earthquake, as well as we should have technique to simulate wave propagation in 3D structure. Our study will base on the known 3D velocity structure for this region, instead of construct a new one, although, that is certain to be improved in the course of this study. We developed an efficient 3D viscoelastic finite difference (FD) algorithm in which we incorporated the improved modeling of absorbing boundaries and constant Q. In additional the 4th order, staggered-grid, 3D-FD algorithm is designed with discontinuous grid spacing so that computational and memory requirements can be significantly reduced. Having the 3D structure and numerical technique in hand, we can compute 3D synthetic Green's functions. A global hybrid search algorithm will then be employed to solve for the two components of fault slip, rise time, and rupture time for each subfault. For each time history we limit the time window to the direct P and S waves to mitigate the influence of the uncertainties in the 3D-velocity structure on later arrivals.

 

 

Kim Olsen

2/1/97 - 1/31/02

University of Southern California

Southern California Earthquake Center, USC 572726

Three-Dimensional Elastic Finite-Difference
Simulation of a Dynamic Rupture

Professor Raul Madariaga, Ecole Normale Superieure, and Dr. Eiichi Fukuyama, National Research Institute for Earth Science and Disaster Prevention, Japan, visited ICS during the summer of 2000, funded through OKP03.

In collaboration with Prof. Madariaga I have studied the conditions for rupture initiation and propagation along a fault surface subject to a heterogeneous stress distribution and/or inhomogeneous frictional parameters using a fourth-order finite-difference program. We have identified a non-dimensional parameter that controls rupture. This parameter measures the ratio of local available strain energy to fracture energy derived from the friction law. A bifurcation occurs when this parameter has values greater than a certain critical value that depends mildly on the geometry of the stress distribution on the fault. We have shown that the non-dimensional number can be used to describe dynamic rupture in the presence of a highly heterogeneous stress field for the 1992 Landers earthquake.

Together with Dr. Fukuyama and Prof. Madariaga I initiated the development of a hybrid method for flexible and efficient modeling of dynamic rupture propagation on curved fault geometry and its radiation in a heterogeneous three-dimensional medium. The dynamic rupture propagation is computed using the boundary integral equation (BIE) method. The computation of radiated waves away from the fault is carried out by an efficient fourth-order staggered-grid finite-difference (FD) method. The hybrid method enables dynamic modeling of rupture propagation on curved or multi-segmented faults in laterally and vertically heterogeneous earth models with an accurate free surface. In addition to the model of dynamic rupture on a single fault, the efficiency of the hybrid method can be used to model the statistics of recurrent ruptures on multiple, curved faults. We demonstrated the promise of the method by simulating spontaneous rupture with radiation on a curved fault.

Sophie Peyrat, a Ph.D. candidate at Ecole Normale Superieure in Paris visited ICS for a month during November and December 1999, funded through this project. During this time Sophie and I continued ongoing work together with Prof. Madariaga on simulated dynamic rupture for the 28 June 1992, {\bf M} 7.3 Landers earthquake. Our model used a single, vertical fault plane, combining the three segments of the Landers earthquake, a simple slip-weakening friction law, and a heterogeneous initial stress deduced from the slip inversion results of Wald and Heaton. While Olsen et al. (published in Science) obtained a fairly good fit between the dynamic radiation and strong motion records at four sites at various azimuths to the fault, we carried the work a step further by modifying the initial stress field derived from Wald and Heaton in a trial-and-error procedure, thereby obtaining a much improved fit between the dynamic radiation and strong motion records. In fact, we found that our dynamic radiation provided a much-improved fit to the strong motion data compared to that obtained by the Wald and Heaton kinematic model with a single fault plane involved.

During a visit at the National Research Institute for Earth Science and Disaster Prevention, Japan, during October-November 1999 I initiated a study on the variation of dynamic rupture velocity in homogeneous and heterogeneous media with Dr. Eiichi Fukuyama. This collaboration was continued during Dr. Fukuyama's visit at ICS July-August 2000. We use a long rectangular fault with primarily in-plane dynamics and the boundary integral and finite-difference methods for our study. On the homogeneous fault three regimes of rupture propagation appear. In regime I (large rupture resistance) the velocity increases slowly toward the Rayleigh wave speed. In regime II (intermediate rupture resistance) the velocity repeatedly jumps to super-shear values for a short distance but drops back to the Rayleigh wave speed. In regime III (low rupture resistance) velocity jumps to super-shear values and obtains values between the S- and P-velocity. Very narrow (critical) ranges of rupture resistance characterize the transitions between the three regimes. Such critical boundary is also observed in the presence of an asperity at the middle of the fault where the initial stress is raised by a certain amount. Here, the boundary includes two corners between which the critical boundary is located where the product of the width and height of the asperity becomes constant.

 

Kim Olsen

9/1/99-9/30/00

University of Southern California (PG&E), 050186

3D Modeling of the Santa Clara Basin

I have participated in the ground motion simulation code verification study in southern California. Four difference groups have compared synthetics for simulations in uniform halfspace and layered models with point and extended sources. The comparisons show excellent agreements between the synthetics from the different groups. Comparisons to be made include synthetics computed in the SCEC 3D-velocity model version 2.

 

Kim Olsen

2/1/99 - 1/31/02

University of Southern California

Southern California Earthquake Center, USC 572726

Ground Motion Modeling in Los Angeles

Ellen Gottschaemmer, a Ph.D. candidate at Karlsruhe University in Germany, visited ICS for a month during November and December 1999, funded through this project. During this time Ellen and I compared the accuracy of two implementations of the explicit planar free-surface boundary condition for 3-D fourth-order velocity-stress staggered-grid finite-differences, 1/2 grid apart vertically, in a uniform halfspace. Due to the staggered grid, the closest distance between the free surface and some wavefield components for both implementations is 1/2 grid spacing. When compared to a reflectivity solution computed at the staggered positions closest to the surface, the total misfit for all three components of the wavefield is generally found to be larger for the free surface co-located with the normal stresses, compared to that for the free-surface co-located with the xz and yz stresses. However, this trend is reversed when compared to the reflectivity solution exactly at the free surface. When the wavefield is averaged across the free surface, thereby centering the staggered wavefield exactly on the free surface, the free surface condition at the xz and yz stresses generates the smallest total misfit for increasing epicentral distance.

Sophie Peyrat, a Ph.D. candidate at Ecole Normale Superieure in Paris visited ICS for a month during November and December 1999. During this time Sophie and I simulated dynamic rupture for the 28 June 1992, M 7.3 Landers earthquake. Our model used a single, vertical fault plane, combining the three segments of the Landers earthquake, a simple slip-weakening friction law, and a heterogeneous initial stress deduced from the slip inversion results of Wald and Heaton. While Olsen et al. (published in Science) obtained a fairly good fit between the dynamic radiation and strong motion records at four sites at various azimuths to the fault, we carried the work a step further by obtaining a much improved fit between the dynamic radiation and strong motion records by modifying the initial stress field derived from Wald and Heaton in a trial-and-error procedure. In fact, Peyrat et al. found that their dynamic radiation provided a much-improved fit to the strong motion data compared to that obtained by the Wald and Heaton kinematic model with a single fault plane involved.

 

Jamison Steidl

2/1/98 - 1/31/02

University of Southern California

Southern California Earthquake Center, USC 572726

SCEC Borehole Instrumentation Initiative

and

Application of Site Characterization Studies to Ground Motion Prediction: Uphole/Downhole Modeling at the Van Norman Dam Complex

The 1997 Uniform Building Code (UBC) used in the design of structures by the engineering community places a great deal of emphasis on the average shear wave velocity in the upper 30 meters to classify sites and to assign site response correction factors. The emphasis on characterization of the near-surface properties in California, especially at sites with strong motion instrumentation, provides a wealth of new information for site response studies. Borehole geotechnical data coupled with downhole instrumentation allow for the direct estimation of the effects of surface geology on seismic ground motions and the ability to calibrate and improve our physical models of soil response for different levels of ground motion.

In March of 1997, a workshop was held to discuss the initiation of a borehole instrumentation program within SCEC to be coordinated with other ongoing drilling programs in Southern California. Shortly after the workshop the first year of the program was approved with three borehole sites planned for the first year, and three per year proposed for the following three years. The long term scientific objectives of this program are: to estimate the degree of nonlinearity for strong ground shaking on typical Southern California soils; to improve our capabilities in predicting the effects of near-surface soil conditions on ground motions; and to examine the details of the earthquake source process.

This marks the third year of the borehole instrumentation program. To date, seven boreholes have been drilled, logged and sampled for near-surface soil properties, and cased for deployment of the permanent borehole instrumentation. Four of the seven boreholes have been instrumented and are providing data real-time to the Caltech/USGS Southern California Seismic Network (SCSN). This data is available via the SCEC data center archive. These four boreholes are located on the exterior of the Los Angeles basin, three to the north within the Santa Monica Mtns. in a linear array with about 7 km spacing, and the fourth to the south-west on the Palos Verdes Peninsula. A fifth borehole located at the CDMG California strong motion instrumentation program (CSMIP) Obregon Park site in the LA Basin should be instrumented this fall and data will be streamed real-time to both the SCSN and CDMG in Sacramento. A 350 meter deep borehole has been drilled and cased in the LA Basin at the Long Beach water reclamation plant, and along with a shallow 30 meter borehole at the same location, will be instrumented this fall/winter. This vertical array will provide critical data on the soil behavior of a "typical" LA Basin site in both the small and large strain regimes as the waves propagate up through the soil column.

In addition to the Borehole Instrumentation Initiative, a second SCEC funded project uses portable borehole instrumentation at the Van Norman Dam site in the San Fernando Valley to help calibrate the current numerical wave-propagation methods for predicting soil response. This data can also be used for development of new nonlinear modeling techniques for the behavior of soils under large strains, as the instrumentation allows for ground motion up to 0.5-g downhole.

Using the borehole data from both SCEC projects as the input to our linear models, and the geotechnical site characterization data provided under collaboration with the ROSRINE project, we are able to reproduce the surface observations in the time, frequency, and response spectral domains for frequencies up to 10 Hz. The degree of non-linear behavior in California soils at large strain levels is a critical issue for determining the maximum plausible ground motions from large earthquakes. However, the lack of observed large-strain data from these new borehole sites in Southern California forces us to calibrate our nonlinear models with data from other regions. We use the Port Island vertical array observations from the Hyogo-ken Nambu (Kobe) earthquake and vertical array data from the Kushiro-Oki earthquake to test newly developed nonlinear soil behavior models.

 

 

Ralph Archuleta

7/1/95 — 6/30/01

UCOP

Campus Laboratory Collaboration, UCSB 08950868

(now referred to as the UC/CLC Campus Earthquake Program (CEP)

Estimation of the Ground Motion Exposure from Large Earthquakes at Four UC Campuses in Southern California

The purpose of this research is to estimate the site-specific ground motion that could be expected at UC campuses from earthquakes on nearby faults. The principal institutions involved are UCSB, UCR, UCSD and LLNL. In the past year we have completed our simulations of ground motion from hypothetical M 6.8 earthquakes from the Red Mountain-Pitas Point-North Channel fault system. This fault system is made up of thrust faults that would break the surface in the Santa Barbara Channel but whose fault area dips beneath the UCSB campus. Using a small M 3.2 earthquake recorded in March 1998 by the CEP borehole and surface accelerometers, we have computed the ground motion from 120 scenario earthquakes, M 6.8. The simulated ground motion is based on a stochastic procedure developed at UCSB. Using geotechnical information (from earlier CEP studies) about the soil on which the campus is built we have included a newly developed nonlinear analysis to refine our estimates of the expected ground motion should such an earthquake occur. Our final estimates of ground motion are compared with those of consulting engineers for the retrofit and design of buildings on the UCSB campus, and with the USGS national hazard mapping programs probabilistic hazard analysis for the Santa Barbara campus. These results were incorporated into the UCSB phase II report that is presented to the UCSB chancellor and the UC office of the president.

In order to understand the response of Engineering I the CEP installed seismometers throughout the building and outside. A rotating elliptical weight was installed on the fifth floor that shook the building so that its response could be measured as well as the effect of shaking on the ground near the building. This information was used to determine the fundamental and higher overtone frequencies of Engineering I. http://www.crustal.ucsb.edu/clc/

In addition to the work on the UCSB campus, we also started working on simulations of ground motions for the UC Riverside campus. Using a similar approach, we use a small M3.8 earthquake recorded in March 22, 1999 by the UCR CEP surface and borehole accelerometers to compute the ground motions from 100 realizations of a M7.0 scenario earthquake. Similar to the UCSB campus, the UCR geotechnical data from in-situ measurements and dynamic laboratory testing are used to calculate the nonlinear effect of the near-surface soil structure on the predicted ground motions. The results from this work will be incorporated into the UCR phase II report.

 

 

Ralph Archuleta

11/26/95 - 11/25/99

Institute for Protection and Nuclear Safety, 4060-00001217

Institute for Protection and Nuclear Safety, 4060-9B016120/SH

Institute for Protection and Nuclear Safety, 4060-0A523570/SH

12/20/95 - 12/19/99

Nuclear Regulatory Commission, 04-96-046

04/01/00 — 03/31/03

Nuclear Regulatory Commission, 04-00-038

Garner Valley Downhole Seismographic Array (GVDSA) Project

Project Background:

The Garner Valley downhole seismographic array (GVDSA) project, installed under the U.S. Nuclear Regulatory Commission contract NRC-04-87-108 in cooperation with the French Commissariat a l'Energie Atomique (CEA), has two main scientific objectives. The first is to understand the effects of the near-surface soil conditions on seismic ground motion to improve the ground motion prediction capabilities for design, seismic hazard assessment, and hazard mitigation. The second is to understand the effect of earthquake ground motion on the hydraulic conductivity of ground water systems for the deep storage of nuclear waste.

The near-surface geological site conditions have been shown to be the dominant factor in controlling the amplitude and variation of strong ground motion, and the damage patterns that result from large earthquakes. A unique set of data collected from the Garner Valley project makes it possible to advance two major areas of engineering seismology. The first problem is how weak motion scales to strong motion. The second one is how the recordings at different soil types scale to each other, especially with respect to a competent rock ("reference") site. The understanding of competing effects of amplification and attenuation (including non-linearity) is of a vital importance for seismic design studies. The site is located near the Anza segment of the seismically active San Jacinto fault in Southern California, which has a relatively high probability for a large earthquake of magnitude 6.5 or greater in the near future.

At Garner Valley we measure the ground motion during earthquakes in the bedrock 500 and 220 meters below the surface, at 50 meters below the surface in a zone of weathered granite, and at 22, 15, and 6 meters below the surface in a layer of soft alluvium. The ground motion is also measured at the surface above these borehole instruments by 5 stations in a linear array, one of which is directly above the borehole instruments.

In the deepest borehole (500 meter), downhole pressure transducers are located within sealed off fracture zones. The effects of earthquake ground motion and the rock-mass hydraulic response to ground motion are important factors in the short- and long-term performance of a high-level nuclear waste repository. This part of the project was designed and undertaken to provide fundamental data regarding the influence of earthquake ground motion on dynamic and static changes to the pore pressure in the rock mass. In addition to the downhole pressure transducers, static and dynamic changes in the pore pressure at different levels within the bedrock borehole are also measured via tubes (sampling lines) which are connected to pressure transducers at the surface and extend into the borehole to various depths.

Located 3 km from the GVDA main station is a remote rock station, which consists of surface and 30 meter borehole accelerometers connected to a Kinemetrics K2 data recorder. The data are stored on the K2 and transmitted via radio modem to the Garner Valley main station. The 30 meter borehole instrument and surface instrument directly above provide critical data on the effects of the weathered rock layer on the earthquake ground motion and help to define the "reference" or input motion for use in site-specific hazard analysis.

For more information on the Garner Valley Downhole Array project please see our website at http://quake.crustal.ucsb.edu/gvda

Current Research:

One of the highlights of the past year has been another significant donation to ICS. The operation and maintenance of a third vertical array observatory has been added to the engineering seismology test site program at ICS. The Borrego Valley Downhole Array (BVDA), donated to ICS in Fall of1999, is another state-of-the-art engineering seismology test site installed by Agbabian Associates for Kajima Corporation in 1993. This donation, while not officially valued yet by the Development Office at UCSB, is an even larger donation than the Hollister Earthquake Observatory (HEO), which was donated to ICS in 1998 and valued at 1.25 million dollars. It is the data from vertical arrays like GVDA, BVDA, and HEO that provide the calibration data for the extensive site characterization efforts of our geotechnical engineering colleagues, and continue to demonstrate to the earthquake engineering community the first class nature of engineering seismology research conducted at ICS. BVDA and HEO, along with GVDA, are all maintained under the NRC and IPSN funding agreements.

Another highlight of the last year’s research has been the data recorded from the M7.1 Hector Mine Earthquake. The vertical array data from GVDA and BVDA are sure to be the subject of future publications on engineering seismology research by ICS personnel. While the earthquake was located just over 100 km from the GVDA site, it was the first event which demonstrated clear evidence of pore pressure build up in the near-surface soil layers, even at relatively low strain levels. This kind of data is critical to our understanding of nonlinear soil behavior and the processes, which lead to soil liquefaction. In addition to the pore pressure build up in the near-surface soils, changes in the deep pore pressure were observed within the 500-meter borehole, which penetrates 400 meters into the country rock. These pore pressure changes are only just being analyzed and will also no doubt be the subject of future publications.

Extensive work on the development of new methods for incorporating nonlinear behavior into the dynamics of wave propagation through near-surface soils has taken place in the past years and have culminated in the successful defense of Fabian Bonilla’s Ph.D. Thesis. While simple linear models of soil response are well suited for modeling the low-strain observations we have seen to date at GVDA. HEO and BVDA, the degree of non-linear behavior in California soils at large strain levels is still a critical unresolved issue for determining the maximum plausible ground motions from large earthquakes. The data from the Hector Mine earthquake at the GVDA site which include pore pressure observations will be critical in validation and calibration of the nonlinear effective stress modeling of the GVDA site at the onset of the nonlinear soil behavior. The lack of observed large-strain data from borehole sites in California forces us to continue to calibrate our nonlinear models at large strain levels with data from other regions, such as Japan.. http://quake.crustal.ucsb.edu/gvda

 

Ralph Archuleta

1/15/98-12/31/00

National Science Foundation, EAR 97-25709

Dynamic Earthquake Rupture Simulation on Dipping Faults

Our main goal in this project was to investigate the dynamics of dip-slip faulting, and specifically to characterize the effect of asymmetric geometry on the rupture propagation, fault slip, and resultant ground motion. For compressive tectonic regimes such as the Los Angeles area, Japan, and Central and South America, and in extensional regimes such as the Mediterranean and the Great Basin of Nevada, Utah, and Idaho, most seismic hazard lies in such non-vertical (dipping) faults. A general goal was to show that an important difference between a vertical and a non-vertical fault is the breakdown of symmetry with respect to the free surface. Because of this geometrical asymmetry in the non-vertical case, seismic waves radiated by an earthquake rupture can bounce off the free surface and hit the fault again and thus modify the stress field on the fault. This interaction causes variations in the normal stress on the fault, and these variations can affect the friction and hence the rupture and slip dynamics of the earthquake. The net result is that the time dependent normal stress produces asymmetric ground motion in the proximity of the fault. Using a numerical simulation technique, we sought to document this effect and explain its physical origin. We performed a number of dynamic simulations of earthquakes on non-vertical faults, using both two and three-dimensional finite element techniques. In the process, we developed a fault boundary condition for implementation in the finite element codes. These research activities were the bulk of David Oglesby’s dissertation research in the Department of Geological Sciences at UC Santa Barbara. The results have been presented in a variety of professional and education/outreach presentations, as well as in an article in Science magazine. Additional scientific journal publications will appear soon.

The asymmetry of the fault with respect to the free surface manifests itself chiefly by causing the normal stress on the fault to change with time. Ahead of the crack tip, due to the shear stress increase, the normal stress change is tensional for a normal fault and compressional for a thrust fault. This effect makes it more difficult for a thrust fault to break through to the surface, but this effect also causes a much higher energy release if the fault does break through. Behind the crack tip, in the slipping region of the fault, the stress changes are of opposite sign due to the drop from static to sliding friction on the fault. Therefore, the effect of the free surface on normal stress also changes sign: In the slipping region near the free surface, the normal stress on a normal fault is increased, while it is decreased for a thrust fault. After starting to slip, a normal fault will have a stronger frictional force holding it back, and have decreased particle motion. Conversely, a thrust fault will have lower friction, a greater stress drop, and increased particle motion. Additionally, for both thrust and normal faults, the hanging wall has higher ground motion than the footwall. This effect is due to the fact that the hanging wall has less volume and thus less mass than the footwall near the free surface.

The results of our simulation show that 1) in all cases the thrust fault produces higher ground motion than the normal fault on the free surface above the fault and 2) there is a large discontinuity in particle velocity and displacement as one crosses from the footwall to the hanging wall. The results of our simulations may explain some observations in the vicinity of non-vertical dip-slip faults, such as increased ground motion in the hanging wall compared to the footwall and larger ground motion for thrust faults than normal faults given the same site and source geometry. An important caveat is that the effects shown above exist only for faults that intersect the free surface of the Earth, not for buried faults.

We have also simulated the dynamics of dip-slip faults that have an abrupt change in dip with depth (a "bend" in the fault). We found that the pure dynamic effects of this geometry are small compared to the effects of the free surface mentioned before. The implications are that the deep structure of faults may not have a great effect on the near-fault strong ground motion.

http://quake.crustal.ucsb.edu/~oglesby/dipfault.html

 

Ralph Archuleta

Kim Olsen

2/1/99-1/31/01

US Geological Survey, 99HQGR0077

Earthquake Simulations and Ground Motion Prediction
for the San Francisco Bay Region

Dr. Ralph Archuleta, Dr. Peng-Cheng Liu and Dr. Kim Olsen have initiated a study to examine the effects of 3D crustal structure on kinematic slip inversion. They computed the ground motion using a finite-difference method and the reciprocity principle in 1D and 3D earth models of the 1979 Coyote Lake earthquake and inverted the tractions on the fault plane. Good comparisons of finite-difference and reflectivity synthetics validated the use of the finite-difference method. The inversion using the 3D Greens functions showed a better fit to data than obtained from the 1D Greens functions, suggesting that the forward and inverse modeling methodologies were appropriate, and that the accuracy of kinematic slip inversion may be improved by including the 3D earth structure. http://www.crustal.ucsb.edu/~kbolsen/BAY.html

 

Edward Keller

12/1/96 — 1/31/00

US Geological Survey 99HQGR0081

Earthquake Hazard of the Santa Barbara Fold Belt, California

Earthquake hazard assessment of the city of Santa Barbara, California identified a Quaternary fold and thrust belt on the coastal piedmont. Several of these folds are formed as the result of active blind faulting and are potential seismic sources. Detailed geomorphic and geologic mapping along with site-specific paleoseismic investigations determine uplift, folding and faulting rates for these potential sources. Faults in the Santa Barbara fold belt may produce Mw 6.0 to 6.5 earthquakes, however ruptures of Mw 7.0 are possible. Also, parts of downtown Santa Barbara may experience extensive, localized damage due to fault geometry and position as well as focusing and amplification of seismic waves.

The Santa Barbara area exhibits characteristic hills that are formed as the result of faulting. These hills along the coastal area have numerous ancient, raised beaches preserved on them. These beaches contain shells that are used to determine the age of the beach deposit. The estimated ages are used to determine the rate of uplift of the Santa Barbara coastal area.

The Santa Barbara area has ancient beaches that occur above the modern beach that range in age from 45,000 to 105,000 years. The uplift rate of the Santa Barbara area is approximately 1 to 2 mm/yr. This rate is ten times greater than previously estimated.

The Santa Barbara urban corridor is in an area of known seismicity with an earthquake hazard similar to that of the cities of Ventura and Los Angeles. Faults on the onshore portion of the Santa Barbara Fold Belt are capable of producing earthquakes with magnitudes 6 to 6.5 and some faults offshore may produce earthquakes with magnitudes in excess of 7. Damages from an earthquake similar to the 1925 event, which devastated downtown Santa Barbara would today cause several million to billion or more dollars of property damage and potentially loss of human life.

 

Edward Keller

James Kennett

7/15/98-6/30/00

National Science Foundation, EAR-9803115

Investigation of a Very Rapid Tectonic Process: Direction and Rates of Lateral Propagation of Reverse Faulting and Folding

Major objectives during the past year included: 1) development of geomorphic criteria to recognize the process of lateral propagation of active folds in the Santa Barbara Fold Belt; and 2) initiation of work on the mid-channel trend associated with the Oak Ridge and mid-channel faults. Preliminary evidence suggests that the fold ,which has the largest topographic expression of any anticlinal fold in the Santa Barbara Channel is propagating laterally to the west..

 

Daniel Lavallée

7/15/99-6/30/01

National Science Foundation, 08990536

Modeling of Hysteretc and Anelastic Soils: From Laboratory Data of
Earthquake Strong Ground Motion

The main objective of this research project is to study nonlinear effects in earthquake strong ground motion. The premise is that nonlinear soil dynamics are essential to a full understanding of earthquake shaking. The model includes nonlinear effects such as anelasticity, hysteretic behavior (also known as the memory effect), and lost of stiffness due to pore water pressure.

To understand and reproduce the behavior of the soil during strong shaking we have developed a new formulation of hysteresis based on the Masing rules. The generalized Masing rules provide a framework for understanding the nonuniform dilation and translation of stress-strain loops for a material subject to non-periodic stresses (or strains). This new hysteresis formulation has several interesting features. It has a functional representation and in limiting cases it reduces to standard models of hysteresis. In its most elementary implementation, the generalized Masing rule is even simpler than the Masing and extended Masing rules. The model depends only on one free parameter named the fiducial point. This parameter controls the size of the loop in the stress-strain space and therefore can be related to the amount of energy dissipated through the nonlinear property of the material. We have derived a relationship between the anelastic damping of a stress-strain loop and the fiducial point for cyclic loadings. In other words, the generalized Masing rules provide a mean to introduce the effect of the damping ratio into nonlinear modeling independently of the other soil parameters.

Using the in situ observations from the Garner Valley downhole seismographic array, we have modeled scenarios of ground motions at the surface for this site. The simulations show amplitude reduction as well as the shift of the fundamental frequency to lower frequencies as observed on vertical arrays. The synthetic accelerograms show the development of intermittent behavior–high frequency peaks riding on low frequency carrier–as observed in the acceleration records mentioned above. Inclusion of pore pressure effects produces large strains in the soil with large amplification in the low-frequency band of the ground motion. Manifestation of spiky waveforms in the synthetic accelerogram is enhanced by the degradation of the soil parameters that is synchronous with a build up of pore pressure. Comparisons between the nonlinear model predictions with those computed with the equivalent linear model for the identical physical situation demonstrate that the latter model does not capture some essential elements of nonlinear soil response.

 

 

Kim Olsen

2/3/97-9/30/99

Los Alamos National Laboratory, F42200017-3Z

General Support on Computations on Near-Surface Wave Amplification in
the LA Basin

During 1997-1999, Dr. Eric Jones, Los Alamos Natl. Laboratories, and I have developed a hybrid method to model non-linear soil amplification using finite-fault ground motion. We compute low-frequency source-time functions at a datum plane below the area of interest using a 3-D elastic finite-difference method, and high-frequency source-time functions by a stochastic approach developed by Alexei Tumarkin. The deterministic and stochastic source-time functions are then merged and propagated up to the surface through a 1D-soil column using a full non-linear method. We have tested the method on the site NWH (Newhall Firestation) in the San Fernando Valley, where borehole velocity logs and laboratory strength data are available. We find linear/nonlinear ratios similar to those obtained from ratios of weak and strong motion recordings from the Northridge earthquake, supporting the proposed pervasive nonlinearity in the San Fernando Valley.

 

Kim Olsen

10/1/99-9/30/00

UC Institute of Geophysics & Planetary Physics, IGPP-99-048

Fault Rupture, Nonlinear Ground Motion
and Whole Basin Attenuation Modeling

Dr. Chris Bradley, Los Alamos Natl. Laboratories and Prof. Steve Day, San Diego state University has implemented the coarse-grained visco-elasticity approach in the 3D staggered-grid finite-difference code developed by Olsen (1994). We are in the process of estimating the improvement of the ground motion computation with a minimum S-wave velocity of 500 m/s compared to previous results using a minimum S-wave velocity of 1000 m/s in the SCEC Phase 3 report. The coarse-grained visco-elastic finite-difference code is being used for this analysis. Preliminary results show that synthetics with the lower minimum velocity provide an improved fit to the strong motion waveforms. The near-surface sediments (Vs of 0.5-1.0 km/s) clearly affect the ground motion in the Los Angeles region and should be included in the simulations. However, the relation Qs = 0.1 Vs (m/s) and Qp = 1.5 Qs seems to break down for velocities less than about 1 km/s. In order to estimate accurate ground motions, an improved estimate of the distribution of Qs and Qp must be obtained.

 

 

Kim Olsen

8/15/96 - 7/31/00

National Science Foundation, EAR -9628682

Simulation of Three-Dimensional Ground Motion
in Los Angeles from Large Earthquakes in Southern California

Dr. Paul Spudich of the USGS in Menlo Park and I have studied the effects of a 1-2 km wide low velocity zone in which the Calaveras fault is embedded. Strong motion accelerographs G06, located in the low velocity zone 1.2 km from the Calaveras fault, and G07, 4 km from G06, recorded both the M 6.2 Morgan Hill and the M 6.9 1989 Loma Prieta earthquakes. Comparison of the ground motions show that a large 0.6-1.0 Hz velocity pulse observed at G06 during the Morgan Hill event my be amplified by focusing caused by the low velocity zone. Such amplified waves might be a mappable seismic hazard, and the zone of increased hazard can extend as much as 1.2 km from the surface trace of the fault.

In collaboration with Dr. Nicolas Shapiro, now at University of Colorado at Boulder, who visited ICS in the spring of 2000, I have simulated seismic wave propagation in models of the subduction zone off the Pacific coast of Mexico using a three-dimensional finite difference method. The results show that a significant part of the energy generated by earthquakes in the subduction zone can be trapped inside a wave-guide generated by the low-velocity material of the accretionary prism. The trapped waves in the simulations appear as long-duration wave-trains that follow the onset of the surface waves, similar to the signature of broadband seismograms recorded by stations along the Pacific coast of Mexico for subduction earthquakes.

Dr. Shapiro and I have also used finite-difference simulations in simple two-dimensional (2D) models of the lithosphere to model the duration of long-period (<0.5 Hz) ground motion incident onto the Valley of Mexico for subduction zone earthquakes. Our simulations suggest that two heterogeneous structures extend the duration of the ground motion between the subduction zone and Mexico City by more than one minute: (1) the Mexican Volcanic Belt and (2) two low-velocity layers in the coastal region: the accretionary prism and the water layer. The duration generated by a crustal model including these structures is similar to that for earthquake records observed in between the coast and Mexico City. In the Valley of Mexico, our models including only regional-scale heterogeneity reproduce approximately one half of the observed duration. The results suggest that both the regional- and the local-scale low-velocity structures must be taken into account in order to explain the observed extended signal duration in the Valley of Mexico.

 

 

Toshiro Tanimoto

7/1/98-6/30/00

California Institute of Technology, PF-448

Atmospheric Excitation of Planetary Normal Modes

The atmosphere most likely excites the background oscillations discovered in 1997. The atmospheric turbulence seems to be exerting a random force on the solid Earth which then lead to excitation of resonant vibrations of the solid Earth. It is similar to a case when we hit a bell in a random way; even though the mechanism of excitation is not efficient, the bell will eventually start to ring at its own resonant frequencies (tone). This phenomenon has a potential application to planetary seismology. Some planets are tectonically almost dead that it may not be a good idea to install seismometers waiting for quakes; quakes may not happen at all. But if the atmosphere is exciting the oscillations, the solid body still shakes by the atmosphere-solid Earth interaction. We will then get to know the resonant frequencies of its solid body from such observations, which will then lead to a new understanding of interior structure of the planet. This project sought the feasibility of such an approach for Mars, which will have some seismometers installed within the next ten years. Our estimate is marginal, however. It appears that the amplitude of excited modes is about half of those observed in Earth. Since it is hard to observe these oscillations in the Earth, it seems to be even more challenging to detect them in Mars, considering the technical difficulties involved. But it is too soon to conclude that it is impossible.

 

Toshiro Tanimoto

9/1/99-8/31/01

National Science Foundation, EAR-9972855

Nature & Cause of Long Period Background Oscillations

In 1997, we co-discovered the background oscillations along with some Japanese researchers. The whole Earth seems to be constantly shaking for frequencies between 2 and 7 millihertz, irrespective of earthquake occurrence; such a phenomenon was predicted by Benioff et al. (1959) before the earthquake-generated free oscillations were found in the records of the 1960 Chilean earthquake. But somehow it escaped detection by seismologists until 1997, almost 40 years later. This NSF grant supported the study of this interesting phenomenon. Through a series of papers, we have shown that these oscillations are caused by the atmosphere-solid Earth interaction; turbulent atmosphere seems to be hitting the surface of the Earth constantly, which results in ringing of the whole Earth body at its resonant frequencies. The critical evidence was seasonal variations, found in amplitudes of these excited oscillations (Tanimoto and Um, 1999). We developed a theory of atmospheric excitation (Tanimoto 1999, GJI paper), and analyzed data from global seismological network data (Tanimoto et al., 1999 in GRL and Tanimoto and Um 1999 in JGR). This new phenomenon was apparently intuitively appealing to the general public. It led to some extra-curricular activities such as quotes by a few general scientific journals, interview by a radio station and invitations to write articles for encyclopedia and journals for general public: they were (1) Science News (vol.154, July 1998) (2) NewScientist (September, 1999) (3) GEO (basically German version of Scientific American). (4) GRL paper (my first report on observation) was one of the four featured articles by Physics Today right after its publication (Physics Today. July, 1998) (4) Interviewed by National Public Radio in September, 1999. (5) An article for Encyclopedia (2001 McGraw-Hill). Toshiro Tanimoto

 

Toshiro Tanimoto

1/1/99-12/31/00

National Aeronautic & Science Administration, NAG5-7981

Atmospheric Excitation of Planetary Normal Modes
New Approach in Planetary Normal Mode Observation

This project expands the idea pursued in the Caltech grant and asks how the background oscillations are excited in other planets, in addition to Mars. If there is any other planet that has a large effect, a potentially new seismological approach to the planet may be planned in order to understand its interior structure.

Venus has been studied so far. Venus and Mars make an interesting contrast; in Mars, the atmosphere is thin (low density), but the wind is very strong. So the strong wind may play an important role in the excitation of background oscillations. In Venus, wind may be milder, but the atmosphere has high density and thus its force on the solid body has some inertial effects. In other words, the density may somewhat make up for the lack of winds. Our preliminary results indicate that background oscillations are excited as well as those in Mars.

We are extending this analysis further to other planets. We plan to come up with a comprehensive analysis of this effect for all planets.

 

Alexei Tumarkin

2/1/99-1/31/00

US Geological Survey, 99HQGR0078

Energy Constraints in Kinematic Models of Earthquake Sources

The study was focused on the problem of reducing the uncertainty of ground motion predictions by implementing the seismic energy constraint into the earthquake source models. The radiated energy is one of the two most important observational characteristics of the earthquake process. While the seismic moment constrains the level of radiation at the lowest frequencies, the energy is determined by the total spectral power, and effectively limits the acceptable combinations of such parameters as rise time, rupture velocity, and the degree of "roughness" of the seismic fault. Our two main results include a complete description of earthquake sources that possess given values of moment and energy and have: 1) the shortest possible total duration of the rupture; and 2) the highest peak moment rate. In particular, we found that the minimum total rupture duration is bounded by the square of the moment divided by the energy; and that the peak moment rate cannot exceed a constant times the cube root of the product of moment and energy. An investigation of the temporal evolution of the apparent stress for a number of observed earthquakes as well as for dynamic models has revealed a common feature that manifests itself in a sharp increase of the apparent stress to about 3-4 times the final value during a very short initial phase of the rupture lasting a fraction of a second. That indicates that earthquakes start as high stress drop sub-events and then continue at a more or less constant stress drop pace.

Results were presented at the 1998 AGU Fall Meeting, 1999 SSA Annual Meeting, 2nd International Conference on the Effects of Surface Geology on Seismic Motion, and the invited seminars at the Kyoto University in Japan and the Purdue University, Indiana.

 

Hazardous Waste

 

Lorne Everett

08/31/99-12/30/00

US Navy, N47408-96-A-7023

Support of the DOD National Environmental Technology Test Site for
Remediation Technologies

Lorne G. Everett provides commentary on technological approach and provides suggestions for increasing the technical quality of the proposed work in research, development and implementation of innovative remediation technologies at DOD sites.





Next Page || Previous Page || Table of Contents

Home Page || E-mail