Major research activities include generation of major and trace element data (50 samples), radiogenic isotope data (12 samples) and ages by the 40Ar/39Ar method for samples (24 samples) from the Campanian Ignimbrite, which is located near Naples, Italy. A computer code to simulate open-system magma chamber processes was also developed.

Major research findings include: 1) The age of the Campanian Ignimbrite, which is located near Naples Italy, is ~39,000 years. This result is based on ~ 15 high precision Ar dates. (2) Previously unidentified older ignimbrites are located near the Campanian Ignimbrite. These older deposits may represent 3 distinct eruptive episodes that occurred at 206, 190, and 158 thousand years ago. The origin and extent of these deposits is currently under investigation, but they may represent previously unidentified ignimbrites in the Naples area. This discovery may have impact on volcanic hazard assessment for the Naples area. (3) The chemistry of the Campanian Ignimbrite reveals that the magma chamber associated with this eruption underwent open-system magma chamber process. This result is revealed by detailed Th isotope analysis of 12 samples of the ignimbrite. Despite evidence of open-system process, a Th isochron is evident from the Th isotope data, suggesting that isochronic relationships can be maintained despite contamination.

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.

Collaborative Research: Role of Shear Heating in the Generation and Ascent of Granitic, Basaltic, and Komatiitic Magma

A model has been set up to study the geochemical evolution of magma bodies undergoing simultaneous assimilation, fractional crystallization and replenishment. This model explicitly incorporates energy conservation and provides information regarding the path magma follows as thermal equilibration is approached. The model itself is a set of 2+s+t coupled non-linear ordinary differential equations that solve explicitly for the temperature of the restite wall rock, fraction of melt in the magma body, concentration of s trace elements in the magma body and isotopic ratio of t radiogenic or stable isotopes as a function of magma temperature during the approach to equilibrium. This model, called EC-RAFC for Energetically Constrained Recharge, Assimilation and Fractional Crystallization is currently being prepared for submission to Journal of Petrology as two papers in a 4 part series.

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.

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.

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.

Natural marine hydrocarbon seeps near Coal Oil Point, Santa Barbara County, CA, pollute the coastal ocean and regional atmosphere. Chemical signatures from these seeps can be followed for 100s of kilometers away from the point. Potentially, the most serious consequence of seepage is its relationship with the formation of ozone. The seeps are significant sources of reactive organic gases (ROGs) which are precursors to ozone.

During the period 1994-1996, we estimate that the total emission of natural gas to the atmosphere from this seep field to be between 1 x 10⁵ and 2 x 10⁵ m³/day, using calibrated sonar surveys of the bubble plumes. The gas is 88% methane, 5% ethane, 4% heavier hydrocarbons (which are ROGs), and 3% non-reactive gases. We estimate total flux of ROGs from these seeps is about equal to the total anthropogenic source in Santa Barbara county. Because of the importance of these marine seeps as natural polluters to the local environment and because of uncertainties associated with the calibration of the sonar returns, we attempted to independently determine the emission rate using an atmospheric tracer technique. The technique requires releasing a gas tracer (sulfur hexafluoride) over the seeps and simultaneously measuring the tracer and seep gas (methane) down wind. Unfortunately, total emission rate of the seep gas to the atmosphere could not be determined using the tracer method because of the absence of a methane plume down wind of our field area.

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

INDEPTH III geologists discovered xenocrysts and xenoliths at three separate localities in the Qiangtang terrane. The most abundant xenoliths are granulite-facies metasedimentary rocks with garnet + enstatite + oligoclase + sanidine + quartz ± phlogopite ± sillimanite ± hercynite ± monazite ± zircon; mafic granulites include phlogopite-bearing clinopyroxenite, amphibolite, and clinopyroxene + orthopyroxene + plagioclase rocks. Mineral textures and compositions provide unambiguous evidence of three distinct phases of recrystallization. The primary recrystallization, characterized by the coexistence of almandine + enstatite + F-phlogopite + plagioclase + sanidine ± quartz ± cordierite ± sillimanite in metasedimentary rocks, occurred at 900-1200°C and 1.1-1.5 GPa. During secondary overprinting at 1000-1400°C, the phlogopite grains in some samples broke down to form a symplectic intergrowth of chiefly orthopyroxene + spinel. Subsequently, the xenoliths were entrained into the magma and carried to Earth's surface.

Monazite grains show age zoning, determined by in situ Pb/Th dating with an ion microprobe, compatible with (re)crystallization at c. 15 Ma, followed by another thermal event near 3.4 Ma. Sanidine xenocrysts gave an Ar/Ar eruption age of 3.2 ± 0.1 Ma. Numerous salient implications fall from these observations:

The rates at which basalt and gabbro transform to blueschist and eclogite influence numerous slab characteristics, including: 1) buoyancy–and thus how the slab sinks and bends; 2) mechanical behavior, as weak minerals are replaced by strong; 3) seismic wave speeds and the seismological interpretation of slab structure; 4) the distribution and rate of fluid release that carries volatiles into and fluxes melting in the overlying mantle wedge; and 5) stress state and seismicity. Kinetic models of phase transformation rates make predictions significantly different from conventional equilibrium models.

In the fluid-limited kinetic model (#4), much lower densities and P-wave velocities are sustained to great depth, particularly in the gabbroic layer. Specifically, the transition to characteristic blueschist-facies velocities (7.6 km/s) is suppressed from 40 km down to >180 km in the lower 4 km of the oceanic crust.

Understanding the exhumation of ultrahigh-pressure rocks continues to be one of the outstanding tectonic questions of our time. Considerable progress has been made in areas such as the Dabie Shan of eastern China, but a strong tectonic overprint there casts a heavy veil over the ultrahigh-pressure history. In contrast, the impressively coherent nappes of the Scandinavian Caledonides have almost no post-orogenic overprint and contain a much more complete record of pre-, syn-, and post-ultrahigh-pressure sedimentation, metamorphism, magmatism, and deformation. We are evaluating existing and our own tectonic models for the exhumation of ultrahigh-pressure rocks in the Scandinavian Caledonides, by constructing pressure-temperature-deformation-time histories of the key nappes and their bounding faults. The strength of our approach that we bring to understanding the exhumation of ultrahigh-pressure rocks lies in our familiarity with other ultrahigh-pressure orogens, combined with our aggregate expertise in structural geology (field and laboratory-based kinematic analysis), metamorphic petrology (thermobarometry and microstructural analysis), and geochronology (U/Pb thermal ionization mass spectrometry and sensitive high-resolution ion microprobe, Sm/Nd thermal ionization mass spectrometry, and Sm/Nd plus Lu/Hf inductively coupled plasma mass spectrometry).

The goals of this project are to characterize the Quaternary faulting and earthquake history of prominent faults that make up the eastern California shear zone in the Bishop, California area. Trenching studies along the Owens Valley fault (OVF), eastern California were completed. The 10.5 m long x 3.3 m deep trench revealed two strands of the OVF cutting a sequence of fluvial sand and pebble deposits which interfinger with alkali mud and clay. The youngest strand cuts and offsets all of the deposits except a 13 cm thick section of unconsolidated fine-grained sand exposed at the ground surface. These relations suggest that this strand formed during the 1872 earthquake. Although the OVF is dominantly a right lateral strike-slip fault, here the 1872 rupture exhibits vertical offset of ~10 cm. The older strand is located ~2.5 meters west of the 1872 rupture and exhibits one rupture event prior to the 1872; vertical offset is ~40 cm. Several samples were collected from coarse sand layers that have been cut and offset by the older fault strand and were deposited across the fault trace. These samples will be used for optically stimulated luminescence (OSL) dating. Ages from these samples should constrain the age of the older event and provide an estimate of the recurrence interval along the OVF.

A manuscript, entitled "Quaternary faulting history along the Deep Springs fault, California" was submitted to the Geological Society of America Bulletin. The manuscript includes 27 pages of text, figure captions, and references, 3 tables and 10 figures. The Deep Springs fault is a normal fault that connects two subparallel, right-lateral, strike-slip faults, the Owens Valley fault and the Fish Lake Valley fault zone. Our manuscript describes the results of the first detailed geologic mapping, structural, geomorphic, geochronologic, and diffusion erosion modeling investigations along the Deep Springs fault to assess its Quaternary faulting history. We estimate that slip along the fault began 1.6-2.4 Ma at a horizontal extension rate of 0.81-1.26 mm/yr. The fault is characterized by multiple fault planes and fault scarps; the youngest set of fault scarps cut across Holocene alluvial fan deposits and yield an average vertical offset of 2.65 m. Diffusion erosion modeling of these fault scarps and radiocarbon analyses on detrital charcoal found in the footwall of one of these scarps indicates that the most recent surface rupture occurred ~1900 years ago. This earthquake ruptured a ~20 km section of the fault implying a MW of ~6.9. This normal fault is one of several displacement transfer normal faults within a broad zone of diffuse deformation that accommodates ~24% of Pacific-North American relative plate motion. The young age and recent earthquake activity along this fault is consistent with a model proposed for the kinematic evolution of this part of the eastern California shear zone.

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. This project is being completed with the collaboration of geologists from the Institute of Geology, State Seismological Bureau, Beijing. Field, structural, metamorphic, and thermochronologic data from the Kangmar Dome were presented in three back to back talks at the Fall, 1998 Geological Society of America National meeting held in Toronto. The tectonic implications of our studies in the Kangmar dome were presented in invited talks to the departments of Geological Sciences at the University of Vermont and New Mexico State University during the Winter and Spring, 1999, respectively.

A manuscript, entitled "Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints", was submitted to Tectonics for review. The manuscript includes 41 pages of text and figure captions, 4 tables (total of 23 pages), and 16 figures. The Kangmar Dome is a gneiss dome, one of several exposed within the Tethys Himalaya. Prior to our investigations, reconnaissance studies established the general structural framework within the Kangmar Dome. On the basis of these studies, previous workers suggested that the dome formed as a result of metamorphic core complex extension, diapirism, or duplex formation. Clearly, each of these mechanisms has dramatically different implications for the evolution of the Tibetan plateau. Our detailed studies indicate the following structural history: NS-contraction followed by thermal re-equilibration leading to moderate pressure/high temperature metamorphism, subsequent vertical thinning and horizontal extension, and finally exhumation by thrust faulting and erosion. Our data do not corroborate metamorphic core complex extension, diapirism, or duplex formation as the mechanism by which this dome formed. Rather, we propose that vertical thinning and horizontal extension occurred as consequence of maintaining a stable orogenic wedge geometry and subsequent movement of the Kangmar region up and over a N-dipping thrust fault ramp resulted in the domal form. Mica and potassium feldspar, and apatite fission track thermochronology indicate that exhumation of the Kangmar Dome by thrust faulting and erosion occurred during the middle to late Miocene. Exhumation of the Kangmar Dome is synchronous with normal slip along the Southern Tibetan detachment system and thrust faulting along the Renbu-Zedong thrust fault, raising the possibility that there is a kinematic link between these structures.

Field and structural relations and their implications from the Mabja Dome, southern Tibet (located ~150 km west of the Kangmar Dome) were presented at the American Geophysical Union meeting held December, 1998 in San Francisco. Mineral separates from samples collected have been completed for Ar/Ar thermochronology; the samples are currently being irradiated.

We conducted a marine study of the Cenozoic glacial and tectonic history of western Marie Byrd Land (MBL), West Antarctica, in the region of the eastern Ross Sea, Edward VII Peninsula, and the Ford Ranges. The study region is located at the eastern edge of the Ross Sea and Ross Ice Shelf. The approach was an offshore geology and geophysics study of the continental margin here. The offshore work used the icebreaker N.B. Palmer in the first year of the project. No marine geophysical data were previously available from this large region.

We are concerned with these main problems:

o What is the history of the West Antarctic ice sheet in this region?

o What is the tectonic history in the eastern Ross Sea?

o How are the tectonic history and the glacial history related?

Our study was the first of its kind on the eastern side of the Ross Sea rift. We identified a new Ross Sea unconformity, RSU7 and a new syn-rifting unit, RSS1-lower. Our work demonstrates that deformation here is largely restricted to pre-RSU6 time, which is mid-Tertiary in age. It also shows that detachment faulting likely extended throughout the Ross Sea prior to Cenozoic time. The results of our work mesh nicely with the onshore work in the Ford Ranges where we determined Late Cretaceous cooling ages that suggest crustal deformation here at that time, synchronous with the Ross Sea.

We are investigating the eastern shoulder of the Ross Sea Rift in Antarctica using aerogeophysical surveys. Structure at the boundary between the Ross Sea Rift and western Marie Byrd Land (wMBL) may be a result of 1) Cenozoic extension on the eastern shoulder of the Ross Sea rift, or 2) uplift and crustal extension related to Neogene mantle plume activity in wMBL; or a combination of the two.  Faulting and volcanism, mountain uplift, and glacier downcutting appear to now be active in wMBL, where generally E-to-W-flowing outlet glaciers incise Paleozoic and Mesozoic bedrock, and deglaciated summits indicate a previous N-S glacial flow direction.

Our study includes aerogeophysics within in a 400 x 400 km area using the NSF/OPP SOAR facility aircraft (Support Office for Aerogeophyscial Research) combined with on-ground geology. Our rationale for a combined program is the complete coverage of crustal-scale structure from the airborne geophysics, and mountain-range and outcrop scales from the geologic field work.

This past 98/99 season the SOAR group finished flying the project area in western Marie Byrd Land.  The survey area covers the Edward VII Peninsula, the Ford Ranges and a large portion of the West Antarctic Ice Sheet.  Within this region are included 12 survey blocks about 112 km on a side. During December and January the SOAR Twin Otter aircraft acquired airborne magnetic, gravity, laser elevation, and radar ice thickness data during flights over the survey area. A UCSB graduate student, Rob Meyer, joined a ground team in the Ford Ranges led by Christine Siddoway who is the Collaborating PI from Colorado College.  He spent 3 weeks in the field collecting paleomagnetic samples. The organization phase for the airborne data will be complete soon. Paleomagnetic samples are currently being processed at UCSB and Caltech.  Doug Wilson will be involved in the data analyses.

In December of 1998 we installed three continuously operating autonomous Global Positioning System (GPS) stations in Marie Byrd Land, West Antarctica to measure glacio-isostatic rebound and tectonic deformation. The instruments were designed and constructed by the project collaborator Andrea Donnellan at the Jet Propulsion Lab and installed by her team. An important, but poorly understood piece in the global tectonics puzzle is found in Antarctica. It is widely accepted that active deformation is ongoing in western Marie Byrd Land (wMBL) and the Ross Embayment, which are both part of the West Antarctic Rift System, but rates and causes of the deformation are unknown. Two possible causes of the deformation observed in this region include tectonic extension in the Ross Embayment as West and East Antarctica separate, and crustal uplift caused by isostatic rebound following the last glacial maximum. The type and magnitude of deformation in this region has serious implications both for global tectonic interpretations and ice sheet models. If active extension is occurring in the Embayment, it could affect interpretations of the global plate tectonic model, depending on the magnitude, which may be as high as 10 mm/yr. Knowing the rate could also aid in improving our understanding of the extension occurring in the Embayment as well as the uplift history of the Transantarctic Mountains. Determining the uplift rates due to post glacial rebound will help to determine the timing and configuration of the ice sheet during the Last Glacial Maximum, and may help to determine whether the ice sheet collapsed during the mid-Holocene, about 6000 years ago. The GPS stations will gather data for at least four years, and have been installed in concert with a series of autonomous stations in the Transantarctic Mountains, resulting in an unprecedented long baseline array across the Ross Embayment.

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, yet much of what is 'known' about these faults and folds has been inferred from 2D balanced cross section models. To test the reliability of these 2D models to predict 3D subsurface structure, we are evaluating a unique 3D dataset 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.

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 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.

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 Indigo II workstations with state-of-the-art interactive 3D software (gOcad). The workstations were donated by Silicon Graphics and the software was provided as part of a cooperative effort between ICS, the gOcad university consortium and Oxy Petroleum.

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.

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.

Collaborative Research: Testing Models of Fault-Related Folding, Northern Channel Islands, California

Christopher Sorlien's part of this collaborative project with Nicholas Pinter of Southern Illinois University has been to interpret the structure and stratigraphy of the shelf and slope between Santa Cruz-Anacapa islands and deep Santa Barbara basin. High-resolution U.S. Geological Survey seismic reflection data was used for the sequence stratigraphic interpretation, and industry multichannel seismic reflection data was used to map structure. The sequence stratigraphy of the shelf has been interpreted north of Santa Cruz and Anacapa Islands. A NE-prograding delta is sourced from the Prisoner's Harbor drainage on Santa Cruz Island. An older more voluminous sequence of this delta has been correlated from the shelf into the deep basin north of Anacapa Island, where it is located above the 1 million year old horizon of Yeats (1981). We interpret that the tops of prograding sequences are controlled by sea level, and that the older sequence has subsided between 100m and 200m. Another, probably older sequence has subsided as much as 400 m. At least north of Santa Cruz Island, older sequences are more tilted than younger ones. Uplift of the islands and subsidence of the shelf is a progressive north tilt that is not consistent with the published structural model. Uplift of the islands should be evaluated from a subsiding base-level, not from sea level, when used for slip estimates on buried faults. The late Quaternary growth of the fold that makes up Santa Cruz and Anacapa Islands decreases to the east. The lack of continuity of the late Quaternary fold with the Santa Monica Mountains may be related to a sharp bend in the strike of the offshore Santa Monica (Dume) fault and not to a lack of kinematic continuity.

We acquired GPS data for 250 bench marks in 21 of 25 previously established UCSB leveling arrays across faults in southern California to turn those arrays into leveled alignment arrays. The surveys involved kinematic recording between a fixed base station for each array and a rover on the individual bench marks. The precision of each measurement is centimeter level, rather than the desired sub-centimeter level, because we occupied the bench marks for only 8 minutes each, upon poor advice from other investigators. The field season terminated before we could resurvey all 250 bench marks to rectify the mistake.

Establishing the marine terrace chronology is the most crucial aspect of calculating uplift rates, which aid in earthquake hazard assessment. Although fragments of faulted, folded, and otherwise deformed marine terraces are abundant along the California coastline, few terrace deposits contain the solitary corals necessary for absolute dating by U-series methods. Furthermore, Kaufman et. al. (1971) showed that mollusks remain open systems with regard to Uranium after death, and therefore may not yield accurate U-series ages. Accordingly, a number of methods for correlating undated terraces to those dated by U-series methods have been developed. They include amino-acid racemization, 14C dating of mollusks and charcoal, faunal assemblage analysis, and oxygen isotope stratigraphy. This research is part of a project to determine the utility of oxygen isotope stratigraphy for assigning ages to marine terraces where no corals are found.

Research on Santa Cruz Island is part of an ongoing study to determine the utility of oxygen isotope stratigraphy for correlating marine terraces of unknown age to those dated by U-series methods. Santa Cruz Island provides an important opportunity for this study, because the oxygen isotope stage 5e (125 ka) marine terrace on the island has been identified and dated by U-series methods on solitary corals (Pinter et al., 1998). Presumably, fossil mollusks collected from 125 ka marine terraces should preserve a distinct isotopic signature, easily differentiated from those of mollusks of other ages, based on the extreme sea level highstand at this time. Determining the isotopic signature of the 125 ka marine terrace is extremely important, as positively identifying this terrace is a major stumbling block to resolving marine terrace chronologies.

Oxygen isotope analysis of fossil shells from Forney Cove reveal surprisingly cold isotopic signatures, in direct contrast with the bulk of evidence indicating Eemian ocean temperatures as warm as, or warmer than, present day ocean temperatures. Fossil shells from the Nestor terrace, another U-series dated 125 ka terrace, also preserve a cool isotopic signature. X-ray diffraction analysis of shells sampled shows no evidence of diagenetic calcite, (Olivella biplicata, the species used in the study, is composed wholly of aragonite) so the isotopic data are sound. This may indicate that the U-series dates on these terraces are wrong, or have been misinterpreted. Dates on both the Forney Cove and Nestor terrace are actually older than 125 ka, and the cool water signature suggests that these terraces may in fact correlate to low stands of sea level (stage 6?). If this is true, uplift rates for these areas need to be recalculated, and our understanding of the dynamics of marine terrace formation must be rethought. The study is ongoing.

Impacts of Oil Production on Natural Hydrocarbon Seepage in the Santa Barbara Channel Determined by Fluid Dynamic Modeling of Dissolved Hydrocarbon Plumes

Natural oil and gas seeps offshore of Coal Oil Point in the Santa Barbara Channel release a significant amount of hydrocarbons (in the form of gas, oil and tar) into the environment. Because the seeps are in relatively shallow water only part of the gas dissolves in the ocean while the rest is released into the atmosphere where it contributes to ozone formation. Sonar surveys by the Institute for Crustal Studies determined the size and location of the seep field and have shown that the amount of gas (primarily methane) being released into the atmosphere is 10⁵ m³ per day. Analyses of the water during the 1995 and 1996 cruises found peak dissolved methane concentrations of the order 1000 nmol/l. In order to verify the magnitude of the gas flux, I have used the Princeton Ocean Model to simulate the water flow in the Santa Barbara Channel. The computational domain extends from shore to the Channel Islands and from Anacapa Island in the east to Point Conception in the west. The numerical grid consists of 105 columns by 43 rows of control volumes. There are 10 control volumes from sea surface to bottom scaled in size based on the water depth. Turbulence, realistic bathymetry, temperature and salinity variations, tidal forces, and wind stresses are included in the simulations. Dissolved hydrocarbons are represented by a passive tracer. The simulations are driven by both wind stresses (constant northwesterly wind over the entire domain) and tidal fluctuations in the water depth. The simulations produce a predominantly westward flow close to shore in agreement with available oceanographic data. In order to obtain dissolved methane concentrations consistent with our analyses, the amount of methane going into solution must be around 5 x 10⁴ m³ per day.

Hydrocarbons emitted from natural seeps on the sea floor are an important source of air pollution for Santa Barbara County. Of particular concern are reactive organic gases (ROG’s) which are precursors in forming ozone, a significant health hazard. In addition to contributing to air pollution, the seeps are an important source of marine pollution in the area because they emit liquid oil that collects in large oil slicks. The seeps also produced a plume of dissolved hydrocarbons that extends westward along the northern coast of the Santa Barbara Channel (Clark et al., 1999 and Cynar and Yayanos, 1992).

The seepage off Coal Oil Point occurs in liquid and gaseous phases. It is the gaseous phase that is the focus of this research. Bubbles consisting of methane, ethane, propane, butane, and longer chain hydrocarbons rise from seeps and ascend through the water column as bubble plumes. Upon reaching the sea surface the bubbles burst and inject seep gases into the atmosphere. Most of the gas in the bubbles is methane (C1,~88%) with ethane (C2), propane (C3), and butane (C4) making up most of the remainder. The bubbles are often lined with thin membranes of oil. After the bubbles burst at the sea surface, this oil is left behind to form oil slicks. The slicks drift away from the seepage area under the influence of winds and currents (Clester et al., 1996).

The gaseous hydrocarbons entering the atmosphere due to bubble plumes are a significant fraction of the total ROG’s released daily into the atmosphere over Santa Barbara County. The air pollution Control District of Santa Barbara County estimates the gaseous seepage rate offshore of Coal Oil Point to be 8 tons per day (Santa Barbara Air Pollution Control District, 1994) out of a total 143 tons per day. In a recent study Quigley (1997) estimates the rate to be higher, about 24 tons per day, an amount somewhat larger than the 22 tons per day of ROG’s contributed by all mobile sources (mostly automobiles) in the county.

The focus of this research was to develop an instrument to make direct estimates of the flux of gaseous hydrocarbons from the Coal Oil Point seep field. Other estimates, such as those of Quigley (1997) , are based on indirect measurement techniques such as sonar surveys or tracer releases. These other approaches are subject to significant calibration errors. Our approach was to design and construct a gas collection device, called a "flux buoy", that would repeatedly drift through the seep field while continuously making direct flux measurements.

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 à 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.

In the past year, surface array measurements of both ambient noise and a vibroseis truck were made at GVDA to complement the already large collection of site characterization data. As Garner Valley research comes more closely aligned with the geotechnical and earthquake engineering communities, more outside collaborators are recognizing GVDA as an excellent engineering seismology test site to develop and calibrate methods for site characterization. A number of papers regarding the GVDA and HEO projects were presented at the International Symposium on the Effects of Surface Geology on Ground Motion, Yokohama, Japan and at the American Geophysical Union Fall Annual Meeting, San Francisco, California, in December of 1998.

The Hollister Earthquake Observatory (HEO), which was donated to ICS in 1998, and is now maintained under the NRC and IPSN funding agreements, immediately began to produce excellent data for ICS researchers. An earthquake sequence which included a Mw 5.1 mainshock in August of 1998, has been the catalyst for modeling the effects of the shallow soil column in the low strain regime at HEO. The surface observations are reproduced up to a frequency of 10 Hz. These results will be presented at the 12^th World Conference on Earthquake Engineering in Auckland, New Zealand in early 2000. It is data such as this that provides calibration for the extensive site characterization efforts of our geotechnical engineering colleagues, and continues to demonstrate to the earthquake engineering community the first class nature of engineering seismology research conducted at ICS.

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 year. While simple linear models of soil response are well suited for modeling the low-strain observations we have seen to date at GVDA and HEO, 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 lack of observed large-strain data from borehole sites in California forces us to calibrate our nonlinear models with data from other regions. Using the Port Island vertical array observations from the Hyogo-ken Nambu (Kobe) earthquake and the vertical array data from the Kushiro-Oki earthquake, we are testing and calibrating these newly developed nonlinear models.

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.

Southern California Earthquake Center, USC 572726

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

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 equipment was well utilized the entire year. The field deployment for the passive phase of LARSE98, managed by Dr. Monica Kohler of UCLA, used almost all the equipment for the first part of the year. This deployment had stations spread out from Malibu to Lancaster collecting data for over six months. Dr. Jamison Steidl of the Institute for Crustal Studies (UCSB) continues his portable borehole study this year by installing additional sites in the Van Norman Dam complex in the northern San Fernando Valley. He is using the Argos satellite telemetry system on one of the sites to check feasibility of telemetry for all 18 PBIC recorders. The PBIC assisted Dr. Steidl in the installations of the permanent sites at Wonderland School, Mira Catalina School and Griffith Park Observatory as part of the SCEC borehole initiative. The information from these sites is telemetered to Caltech and is then available for remote retrieval.

We have received three-component recordings at 40+ sites in the South Bay area for the August 12, 1998 magnitude 5.4 earthquake near San Juan Bautista from Dr. Allan Lindh. We have corrected the data for instrument response and computed 2-6 hz peak velocity amplification, relative to a reference site. The amplification pattern, which is not corrected for distance, shows a general correlation with the basin areas, with larger amplification above deeper sediments within the basin to the northeast, while this correlation is less significant in the basin to the southwest. The amplification tends to increase towards the northeast.

Recently we have finished adding data from the Kyoshin Net in Japan. We included all records that had at least one trace greater than 50 gals with at least one trace in an earthquake greater than 350 gals. That resulted in 15 earthquakes, 194 stations, and 633 new traces. Also we have added three Alaskan earthquakes, the 1985 Valparaiso, Chile, earthquake, and 11 earthquakes recorded by the Guerrero Array in Mexico.

During the first year of the project we have studied three folds to evaluate fold growth and development: 1) Mission Ridge, which is apparently propagating west through the Santa Barbara urban corridor; 2) Rincon Creek anticline, which is propagating laterally into the City of Carpinteria; 3) and South Mountain, near Ventura, CA, which we hypothesize is propagating laterally to the west at a rate of several cm per year. We are studying processes of active fold growth above buried reverse faults with a set of known and new geomorphic indicators of active tectonics.

Our seismic hazard assessment of the onshore Santa Barbara Fold Belt (SBFB) identifies the south-dipping, reverse Mission Ridge Fault System as the one of the principal seismic sources in the SBFB. This fault system as well as other previously unrecognized, seismic sources, many of which are blind, form topographically well-expressed anticlinal hills. Emergent marine terraces are preserved on the flanks and crests of active, uplifting anticlines and rates of uplift range from approximately 1.0 to 2.0 m/ka. Paleoseismic investigations of the Loon Point fault-propagation fold and the More Ranch segment of the MRFS determine two, possibly three prehistoric ruptures on the More Ranch fault. Based on amount of reverse slip per event in the More Ranch fault trench, we estimate a single segment (15 to 17 km long) rupture to produce a Mw (moment magnitude) 6.5 earthquake. If multiple segments were to rupture, then a Mw 6.8 to 7.0 earthquake is possible. The city of Santa Barbara is subject to amplified ground shaking during an earthquake on the Mission Ridge segment as the result of free surface effects of the hanging wall and possible directivity of seismic waves. Also, the downtown area is susceptible to liquefaction where the historic estero (salt marsh) has been filled, in part from debris from the 1925 earthquake.

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.

The 17 January 1994 Northridge earthquake (M 6.7) has greatly increased the available set of strong ground motion records for the Los Angeles area. Taking advantage of the strong motion data, several studies have undertaken to determine the presence of nonlinear effects in the recorded strong ground motion (Field et al., 1997; Hartzell, 1998; and Su et al, 1998 among others). All three studies, based on the comparison of the transfer function of the weak and strong motion, concur in finding nonlinear wave dissipation for the sedimentary sites. Other studies have investigated the contribution of nonlinear soil effects to the mainshock at specific sites (e.g. see Archuleta et al., 1998; and Cultrera et al., 1998). Now that there are more data available since the Northridge event, namely borehole velocity profiles, weak to strong motion records, and dynamic soil laboratory tests, we can study these particular sites in more detail. The object of this proposal is to go a step further. The perspective adopted is that fully nonlinear soil dynamics are essential to understand and reproduce the complex behavior observed in the strong motion data. The tasks of this project are two-fold. First we will validate and calibrate the nonlinear soil model using SCEC sponsored studies and strong motion data. More specifically, we will use data recorded at the Van Norman Dam Complex (VNDC). When this task is achieved, the computed model will be used with the data to interpret and quantify the nonlinear features in earthquake strong ground. With these results in hand, we may be in position to confirm the presence of nonlinear effects in the mainshocks at the VNDC and to infer the amount of nonlinearity. Another issue will be to explain the large variation of the peak ground acceleration over a small area observed at the VNDC sites (Archuleta et al., 1998) and to identify the physical mechanism and soil properties at the root of this behavior.

Waveform inversions have been successfully applied to calculate kinematic parameters of earthquake rupture histories from strong ground motion records. One of the primary limitations of these studies is that they were restricted to laterally homogeneous (1-D) velocity models. While 1-D approximations can be justified in certain areas, the rupture process determined from such inversions may be significantly biased due to the omission of 3-D geological and structural effects in the inverted wavefield. It is likely that 3-D wave propagation effects are mapped to source complexity in the process of fitting the synthetic seismograms to data.

Three-dimensional finite difference simulations of seismic wave propagation indicate that long-period wave propagation is still strongly influenced by the geometry and structure of the laterally heterogeneous basins. In general, amplifications above deepest parts of the basin or near the steepest edges of basin are relatively large compared to its surroundings; the basin structure increases the duration significantly and produces complex waveforms. The amplitudes of the synthetic ground motion from 3-D simulations were two to three times as large as those from 1D models (Figure 1, Olsen, 1998, written communication). Wald and Graves (1998) analyzed how variations in 3-D earth structure affected simulated waveform amplitudes. They compared the simulations with the observations for three different 3-D velocity models for Los Angeles. They showed that there can be large disparities between the simulations depending on the earth structure used.

This proposal focuses on including 3-D wave propagation effects in the determination of earthquake rupture process on a finite fault by inverting seismic waveforms. It is now possible to arrive at this target. First the 3-D structure of the earth is becoming better known through detailed investigations separate from this proposal. Second, 3-D finite differences FD (or finite element FE) methods can be used to calculate the Green’s functions for complex earth structure. Including these 3-D Green’s functions in inversions for the kinematic source parameters on a finite fault is a natural extension of current methods for determining the parameters of an earthquake rupture. We plan to develop an efficient approach by combining the finite element and finite difference methods for the calculation of Green’s functions. Using bilinear interpolation of source parameters and Green’s functions as suggested by Spudich and Archuleta (1987), we have improved the hybrid global inversion algorithm of Liu et al. (1995) that we will use to determine the parameters of the rupture process. Using these techniques and the 3-D geological model of the Los Angeles area (Magistrale et al., 1998), we will re-analyze ground motion data from 1994 M6.7 Northridge earthquake. The new inversion results for the source parameters will be compared with those determined previously using 1-D synthetic Green’s functions.

For the Los Angeles basin I simulated the 1994 Northridge earthquake using a viscoelastic finite-difference method and the combined slip inversion result by Wald et al. The recently developed SCEC 3D basin model version 1 was used in the simulation. Waveforms for data and synthetics were compared in a common bandwidth of 0.1-0.5 hz at 11 stations representing various azimuths and distances from the causative fault. The fit among the phases was generally good but varied considerably. The best results were obtained for stations located near the source, such as SSA (Santa Susanna) and JFP (Jensen Filtration Plant). The fits were degraded near the Santa Monica Mountains and at large distances from the fault. The duration and amplitude of data and synthetics agree relatively well. I also compared average horizontal 0.1-0.5 Hz peak velocities for the Northridge simulation at 59 sites in the Los Angeles basin to those from data. Almost all of the peak velocities were fit within a factor of 2 within this frequency range. The residuals were generally small above the basin, suggesting that the strength of anelastic attenuation used in the simulation was appropriate. The log standard deviation of the residuals for the 59 sites was 0.37, a reduction of 45% compared to the overall regression value for pseudoacceleration response spectra reported by Boore et al. This result suggests that long-period ground motion estimation can be improved considerably by including the 3D basin structure.

and

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.

Atmospheric Excitation of Planetary Normal Modes

It is well known that the Earth undergoes oscillations after large earthquakes. But two years ago, we discovered that our Earth is quivering constantly even when earthquakes are not occurring. Fundamental spheroidal modes between 2 and 7 milli-Hertz are seen continuously, irrespective of earthquake occurrence. We have proposed to analyze this phenomenon and delineate its cause. This work has been supported by Caltech President's Fund, California Space Agency and NASA. Also NSF has notified us recently that their support for this work will be forthcoming later this year.

We first established the basic fact of oscillations by publishing a paper in Geophysical Research Letters. This work was featured in articles in Science News and Physics Today. Then we examined its cause and have recently narrowed its cause to be outside of the solid Earth, possibly in the atmosphere. We did this by discovering seasonal variations in amplitudes of these oscillations. The article summarizing this work has just been accepted by the Journal of Geophysical Research and will be published shortly. We are proposing that these oscillations are excited by random pressure fluctuations in atmospheric pressure. Since this is a new phenomenon, we also developed a basic theory of such a mechanism and published theoretical results in Geophysical Journal International (1999).

We are now proposing that this mechanism will be useful for the study of Mars' interior structure. According to our preliminary estimate, Mars’ atmospheric pressure, even though it is much smaller than that of the Earth, may be exciting oscillations of Mars as well. Then we have a new tool for Mars seismology, because all we have to do is to install seismometers on the surface of the planet to detect the oscillations excited by its atmosphere. If this is successful, this will tell us about the mantle structure of Mars! We are hoping that this idea will be examined in the planned mission to Mars in the near future.

Our recent studies were 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 degree of discretization of the seismic fault. We have studied the temporal evolution of the apparent stress for a number of observed earthquakes as well as for dynamic models. We have found a common feature that manifests itself in a sharp increase of the apparent stress to about 3-4 times the final value in a rather short initial phase of the rupture. That indicates that earthquakes start as a high stress drop subevent 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 Purdue University, Indiana.

We have continued developing efficient and robust procedures for ground motion prediction. Our new implementation of the stochastic approach uses kinematic extended source modeling with moment and energy constraints. That allowed us to produce maps of expected ground motions in the LA basin from scenario earthquakes on the Elysian and Newport-Inglewood faults.

Dr. Tumarkin and Dr. Olsen were working on combining 3D numeric modeling with stochastic high-frequency simulations to arrive at a self-consistent method of broadband ground motion prediction. In collaboration with LANL they applied this approach to modeling non-linear site response during the Northridge mainshock and to estimation of seismic risk from scenario earthquakes for the Greater Los Angeles Basin. Results were presented at the 1999 SSA Annual Meeting, the paper is in preparation.

Petroleum Hydrocarbon Demonstration Project

In June 1994, the State Water Resource Control Board (SWRCB) contracted with a consortium of campuses of the University of California to study the cleanup of Leaking Underground Fuel Tanks (LUFT) in California. ICS Researchers Dr. Stephen J. Cullen, Dr. Lorne G. Everett, and Dr. Joel Michaelsen played key roles on the consortium team. The study consisted of data collection and analysis from LUFT cases throughout the state and a review of other studies on LUFT cleanups. Two final reports were submitted to the SWRCB in October and November of 1995. The reports were entitled: Recommendations To Improve the Cleanup Process for California’s Leaking Underground Fuel Tanks (LUFTs) and California Leaking Underground Fuel Tank (LUFT) Historical Case Analysis.

One of the important recommendations of this study was to identify a series of LUFT demonstration sites and to form a panel of experts made up of scientific professionals from universities, private industry, and Federal and State regulatory agencies. This panel would evaluate and interpret data from LUFT demonstration sites for the purpose of applying the recommendations derived from the previous studies mentioned above. Again, ICS researchers Dr. Stephen J. Cullen and Dr. Lorne G. Everett, because of the experience and demonstrated knowledge of the field, were invited to participate on the expert panel.

Sites selected for inclusion in the study were:

Army Presidio at San Francisco

Barstow Marine Corps Logistic Center

Camp Pendleton Marine Corps Base

Castle Air Force Base

El Toro Marine Corps Air Station

George Air Force Base

China Lake Naval Weapons Center

Travis Air Force Base

Vandenberg Air Force Base

The project consisted of meeting with site staff, regulators, and expert panel liaisons to discuss and develop a site conceptual model and to identify site contaminant migration pathways and environmental receptors of concern. A site visit was made to each demonstration site to view the site physical layout and to identify gaps in the site hydrogeologic database. The expert panel then analyzed and evaluated the data to determine the potential for natural attenuation mechanisms to exist at the site and to determine the applicability of implementing a remediation approach which relies on intrinsic remediation at the site. Further, at each site, the panel evaluated contaminant sources, pathways, and receptors and subsequently developed an appropriate risk management strategy.

Based on the results of the findings at the nine demonstration sites, the expert panel then evaluated the results of the investigations and demonstrations across the combined sites. The findings indicated that the general recommendations and conclusions derived from the 1995 reports cited above could be successfully applied and used to develop effective remediation strategies at specific sites throughout the state of California.

Initiative to Improve VOC Cleanup Process by Using Historical Case Analysis

There are currently several national initiatives to reevaluate the volatile organic compound (VOC) cleanup process. The VOC initiative proposes to do an historical case evaluation that uses a large number of cases which can identify VOC release conditions that pose low risks that can be managed with minimal effort and cost versus release conditions that pose higher risks and warrant the large expenditure of money often applied to all VOC releases. Dr. Everett participated in a national consortium made up of agencies including the EPA, Department of Energy and the Department of Defense. This group was actively involved in evaluating 400 chlorinated volatile organic compound (CVOC) sites across America.

United States Navy National Test Site Fuel Hydrocarbon Remediation Program

Lorne G. Everett participated as a member of the United States Navy Science Advisory Panel in support of the Navy National Test Site Program. The program focuses on developing hydrocarbon remediation technologies that will be used by the United States Navy throughout the world. Contributions from the Vadose Zone Monitoring Laboratory focused on selecting monitoring strategies to optimize the remediation activity. Remediation technologies such as heap biopile programs, hot air vapor extraction, and the German UVB recycling system were demonstrated.