Ralph Archuleta

Jamison Steidl

Alexei Tumarkin

Scott Swain

Alla Tumarkin

Nuclear Regulatory Commission,

11/25/93-05/31/96 NRC-04-92-050 $687,437

11/30/93-12/29/95 NRC-04-93-053 $332,956

12/20/95-12/19/97 NRC-04-96-046 $175,000

Institute for Nuclear Protection and Safety,

10/21/93-11/25/95 CEA OSSN 93993MGVB $378,995

11/26/95-11/25/96 CEA OSSN 95866MGVB $102,707

Garner Valley Downhole Seismographic Array (GVDSA)

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), is designed to improve understanding of the effects of a shallow soil column on the recorded ground motion at the surface of the column. 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 motions scale to strong motions. 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 which is expected to experience a large earthquake of magnitude 6.5 or greater.

At Garner Valley they 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 addition to these instruments, in the past year they have been recording ground motion at a portable station located less than 2 km from the GVDSA main station on a rock (granite) outcrop.

On May 25th, 1995 they began recording data from the deepest instrument (500 meters) and dynamic pressure transducers located within sealed off fracture zones within the borehole. The pressure transducers are designed to look for correlations between fluid pressure and seismic activity. In the past year they have had some small earthquakes which have been analyzed to determine that the new deep instrumentation is functioning correctly.

In March of 1996 Archuleta hosted the second annual BVDA/GVDA meeting, which brings together researchers from France, Japan, and the US, interested in site effect studies and borehole data. There is a data exchange agreement with Japan to exchange earthquakes from the Garner Valley experiment with their Borrego Valley downhole array (BVDA) experiment. Each year the parties get together to discuss the past years activities and current results.

Seismic risk studies are based on empirical attenuation relations for ground motion parameters. In the past year Archuleta and colleagues established attenuation relations for small earthquakes recorded at GVDSA. Extrapolation of these relations produces reasonable predictions up to a magnitude 6.5 level, for larger earthquakes non-linearity is expected to reduce the ground response at high frequencies.

In the next year they are planning to deploy a liquefaction array at Garner Valley to examine how the very near-surface water saturated alluvium will respond to strong ground motion from nearby large earthquakes. They also plan to install a permanent rock outcrop station and a 30 meter borehole station in the rock very near the GVDSA main station.

Ralph Archuleta

Jamison Steidl

Alexei Tumarkin

12/01/94-11/30/96 $105,407

US Geological Survey, USGS 1434-94-G2410

LA Basin Microzonation

The project objective is to collect seismic data from sites throughout the Los Angeles metropolitan region for seismic hazard analysis. It has long been known that each soil type responds differently when subjected to ground motion from earthquakes. Usually the younger softer soils amplify ground motion relative to older more competent soils or bedrock. The goal has been to instrument different sites in the Los Angeles area to quantitatively measure this amplification of ground motion and produce a data base of amplification factors. These amplification factors are then used to help distinguish regions where the seismic hazard is greatest due to amplification from the surface geology and sub-surface structure.

In the 95/96 year they removed the last 4 sites from the Los Angeles area and have begun the final data analysis on the events recorded over the two year life of this project. The focus of the data analysis in the past year has been on the Northridge aftershock data which resulted in presentations and publications in the proceedings of two international conferences, Nice, France (10/95) and Acapulco, Mexico (6/96), and a submission to the Bulletin of the Seismological Society of America.

They plan to bring this project to a close this fall with some final analysis on some of the stations not used in the Northridge study. In particular they will focus on the stations in the greater Los Angeles basin, south of the Northridge study area. They are also planning a new study which will involve data collection from borehole and surface instruments at strong-motion sites in the Los Angeles area.

Ralph Archuleta

04/01/91-01/31/96 $1,878,491

University of Southern California, 572726

UCSB Participation in the Southern California Earthquake Center

See individual research summaries for grant USC 572726.

Ralph Archuleta

Robert Pizzi, Bishop Diego Garcia High School

Alan Hoffman, San Marcos High School

Malcolm Campbell, Dos Pueblos High School

02/01/95-01/31/96 $20,000

University of Southern California, 572726

Seismology Curriculum Using CUBE for Santa Barbara County Schools

The objective of the project, to incorporate seismology into the curricula of the Santa Barbara City high schools utilizing the CUBE (Caltech-USGS Broadcast of Earthquakes) system, has been achieved. A 10-day seismology curriculum was developed that included numerous lab exercises with basic information about earthquakes and the shaking caused by them. The basic premise is that the curriculum should be flexible so that basic information can be taught at different levels (e.g., senior physics or freshman environmental science) in a high-school environment and that instructors can emphasize different aspects of the curriculum. There is a menu of activities, from which the instructor can choose depending on the level of the class, to reinforce the basic concepts in seismology.

The pilot program started in the summer of 1994. The first CUBE display was placed in a small enclosure within an administration office at Bishop Garcia Diego High School. The enclosure was remodeled to house the pen and ink seismograph and the CUBE system hardware. The display is located in the main corridor where it can be viewed by the largest number of people. A remote keypad (four keys only) allows the inquiring person to interact with the CUBE monitor. The first draft of the curriculum was developed and used in both senior and freshman level classes.

Phase II of the project, completed during the summer and fall of 1995, extended the project to San Marcos High School and Dos Pueblos High School. CUBE systems were installed and personnel were trained in the operation, usage and maintenance of the systems. The curriculum was refined and introduced into the curricula at each school.

Ralph Archuleta

Craig Nicholson

Jamison Steidl

Alexei Tumarkin

07/01/95-06/30/98 $54,763

UC Office of the President, UCSB 08950868

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

This three-year project is part of the Campus Laboratory Collaborative (CLC) initiative to foster more collaborative research between the UC campuses and the national laboratories. The purpose of this project is to estimate the anticipated seismic shaking at four UC campuses--Santa Barbara, San Diego, Los Angeles and Riverside--from future earthquakes. Each of the four campuses plus Lawrence Livermore National Laboratory (LLNL) will take part in this project. The seismic hazard for each campus, i.e., the location of the faults, the activity of the faults, the style of faulting, will be estimated and detailed in a written report. At each campus one building will be targeted for more detailed study. The subsurface geology will be determined from logs of boreholes and seismic profiling. Once the subsurface geology is known, boreholes will be drilled so that downhole seismometers and accelerometers can be placed in competent rock below the building. A matching set of sensors will also be placed at the ground surface. Geotechnical logs of the elastic parameters of the material between the surface and the deepest part of the borehole will provide parameters to theoretically predict the amplification. By comparing the borehole recordings with the surface recordings of earthquakes, the amplification of the seismic waves will empirically be determined. Assuming that the theoretical and empirical measurements are in agreement the measured subsurface structure over the entire campus can be used to extrapolate the amplification to other parts of the campus. The ground motion expected from large earthquakes will be estimated by summing the recordings of small earthquakes as well as using theoretical 3D simulation methods. The project started in the Spring of 1996. The seismic profiling will take place in the summer of 1996 with the drilling and instrument installation to follow in early Fall.

In a second related project that is within the CLC, Dr. Lawrence Hutchings of LLNL will collaborate with Archuleta on methods of using recordings of small earthquakes to infer ground motion from future large earthquakes. The emphasis will be on earthquakes in the San Francisco area.

Ralph Archuleta

11/01/95-10/31/96 $4,000

UC Multi-Campus Research Incentive Fund, MRIF

Workshop: Earthquake Hazards at the University of California

Using funds from the UC Multicampus Research Incentive Fund, researchers from most of the UC campuses and Lawrence Livermore National Laboratory met at UCSB in late April to discuss the type of research necessary to define and possibly reduce the seismic hazards at UC campuses. Experience from past earthquakes has shown that the ground motion is highly variable over small distances. One of the primary issues was how to measure that variability. One possibility is to put instruments in three closely spaced (<= 1 km) boreholes that reach the basement rock to define the variability in the incident wavefield with a dense array of about 100 three-component seismometers at the earth's surface to measure the ground motion at a spacing of tens to hundreds of meters.

Ralph Archuleta

Aaron Martin

Pete Rodgers

02/01/95-01/31/96 $125,000

University of Southern California, 572726

The Portable Broadband Instrument Center (PBIC)

The Portable Broadband Instrument Center (PBIC) provides seismic instrumentation to SCEC investigators for specialized Center 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

Demand for the PBIC equipment for research projects decreased this past year allowing the PBIC to thoroughly test and calibrate the equipment. The PBIC did repairs, preventive maintenance and made upgrades to the equipment. Ongoing research projects formed the bulk of the PBIC equipment usage. Dr. Yong-Gang Li (USC) continued his trapped wave studies on the San Jacinto and Landers fault zones. Dr. Jamison Steidl (UCSB) and James Chin (USC) continued to examine site effects in Los Angeles using PBIC equipment in the LA Basin microzonation project. The LABNet calibration project run last year by SCEC intern Ryan Smith (USC) was supplemented this year by additional testing involving overall response characteristics of the LABNet recording system. Another ongoing project is the Garner Valley rock site run by Dr. Jamison Steidl (UCSB) where he compares surface rock motion with motion recorded in boreholes.

Outreach programs have played an increasingly important role in the PBIC. There has been continued development of the PBIC World Wide Web (WWW) page. Sections on response processing, vendor and product information and the outreach program itself have been added. The File Transfer Protocol (FTP) server on the ICS fileserver was updated to provide increased security and advanced features. Seismological demonstrations were presented at several local schools including Bishop Diego High, Isla Vista Elementary and San Marcos High. ICS graduate students participated in Earth Day 96 by setting up PBIC equipment in downtown Santa Barbara and giving presentations and demonstrations throughout the day. Another aspect of the outreach program was the PBIC's involvement in the National Science Foundation's Young Scholars Program. A Santa Ynez high school student, Elizabeth Cochran, studied site amplification in the Santa Ynez valley by deploying two seismic monitoring stations, one in Solvang and the other on Figueroa Mountain. The study results were presented to NSF judges at Loyola Marymount University. Because of its exceptional low-noise characteristics, the Figueroa Mountain site is being maintained indefinitely by the PBIC.

Ralph Archuleta

David Oglesby

05/01/93-12/31/96 $96,338

National Science Foundation, NSF EAR 92-19721

How Do Earthquakes Generate Extreme Ground Acceleration: A Combined High- and Low- Frequency Faulting Model for the 1992 Petrolia Earthquake

The 1992 Petrolia Earthquake (M = 7.1) was quite remarkable in that an accelerometer at Cape Mendocino (CAP) recorded a high-frequency pulse of almost 2 g, whereas the nearby Petrolia station (PET) recorded a maximum of only 0.6 g. In their model for this event, Archuleta and Oglesby have investigated the possibility that extreme ground acceleration with high spatial variability can be caused by a source effect, specifically the rupture of a barrier-like asperity. This type of rupture feature can produce extreme ground acceleration provided that certain geometrical constraints are met. In the present study, they derive a combined high- and low-frequency faulting model for the 1992 Petrolia earthquake, that is consistent with the slip distribution on the fault, and that produces the high-frequency pulse at CAP.

The model consists of four parts: 1) A low-frequency (0 - 0.6 Hz) inversion to determine the slip, rupture time, and rise time distributions on the fault. 2) An interpolation of the above distributions to produce synthetic seismograms up to 3.0 Hz. 3) Random white noise scaled to the spectral amplitude level of the data up to 12.5 Hz. 4) The contribution from the rupture of a barrier-like asperity, which produces greatly amplified radiation at CAP and not at other stations.

The results emphasize the interaction between fault geometry and rupture propagation to produce ground motion, as well as the hazard associated with locations above the hanging walls of dipping faults, where the geometry permits the production of extreme ground acceleration.

Ralph Archuleta

Alla Tumarkin

Alexei Tumarkin

02/01/95-01/31/96 $50,000

University of Southern California, 572726

Strong-motion Database SMDB and Empirical Green's Functions Library

SCEC databases are developed at ICS by Alla Tumarkin, Ralph Archuleta and Alexei Tumarkin. They provide an easy access for both the seismological and engineering communities to an extensive collection of seismological observations. Their common features include: menu driven user-friendly interface, World Wide Web access allowing to use any computer with a WWW browser (work currently under progress, to be completed by the end of 1996), possibility of performing user-defined complex queries, plotting geographical maps with chosen earthquake and station locations and visualizing recorded seismic signals. They include:

1. Strong Motion Database SMDB. Contains parameters and waveforms of the strongest ground shaking recorded in Southern California since 1933. It is extensively used by scientists and engineers to predict effects of future large earthquakes and for seismic structural design purposes.

2. Empirical Green's Function Library EGFL. Contains parameters and waveforms of smaller earthquakes (currently M>=3.0) recorded in Southern California since 1981. It provides means to calibrate numerical models of wave propagation against observations. This is especially important for ground motion prediction and studies of the crustal structure, in particular imaging of faults. Also weak motion data are used to estimate site-specific ground motion amplification.

Andrew Byers

Jamison Steidl

University of Southern California, 1995 Summer Intern

Site-Specific Strong-motion Amplification Factors for the Southern California Region

Byers updated the existing SCEC/ICS strong-motion database to include response spectral ordinates and site-specific amplification factors for sites recording events since the 1933 Long Beach earthquake up to and including the 1994 Northridge quake. Response spectral acceleration, velocity, and displacements were first calculated for each event at each site in the database. The attenuation relation of Sadigh et al (1994) was used as a reference model for a site-specific comparison of the site's peak ground acceleration and specific response spectral values. Strong-motion data is compared with this attenuation relationship in order to find site amplification or de-amplification factors. He also calculated amplification factors using the average rock response spectral values as a reference. The results of these comparisons will be available through the strong motion database. The records will consist of four amplification factors at each site for all available events within the southern California region. The four amplification factors are at periods of 0.1, 0.3, 1.0, and 3.0 seconds. The sites were chosen because of their structure type and geographical position. Sites were limited to include only those located in the southern California region (with a maximum latitude of 36.0 and a minimum latitude of 32.0) and those that could be classified as free field. These results will help to produce a more complete understanding of seismic hazards for the southern California and assist in the further study of seismic events.

Grant Lindley

08/05/94-08/04/97 $213,370

Nuclear Regulatory Commission, NRC 04-94-079

Analysis of Source Spectra, Attenuation, and Site Effects using Broadband Digital Recordings from the National Seismograph Network

Research is being conducted to study seismograms recorded by the U. S. National Seismograph Network (USNSN). The USNSN is a network of broadband seismographs that have been deployed since 1992. This is the first nationwide deployment of permanent broadband seismograph stations. As such, this data set will provide invaluable information about the structure of the crust and upper mantle in the United States and about earthquake processes.

One of the important goals of the USNSN deployment is to allow more accurate prediction of ground motion from future earthquakes. In order to do this, more accurate models of the earthquake source, seismic wave propagation within the earth's crust, and near-surface effects must be produced. Recently completed work using this data set has examined the attenuation of seismic waves within the contiguous United States, especially how attenuation varies depending on region. Large differences were found in the attenuation of seismic waves between the western and eastern United States. However, no statistically significant differences were found in seismic wave attenuation within the western or within the eastern United States. Future work using this data set will focus on the source properties of the earthquakes, to examine if there are also variations in the source properties of the earthquakes with region.

Ed Keller

Larry Gurrola

02/01/95-01/31/96 $24,000

University of Southern California, 572726

Earthquake Hazard of the Santa Barbara Fold Belt, California

The first emergent marine terraces at Isla Vista, University of California, Santa Barbara (UCSB) campus, and Santa Barbara have been age-dated with collaborative efforts from Jim Chen (California Institute of Technology). U-Th series analysis of a fossil coral ("Balanophyllia elegans") obtained from the Isla Vista terrace deposits, yields an age of 47,000 +/- 0.5 ka for the Isla Vista/UCSB marine terrace. The associated paleoshoreline is expressed west of the Isla Vista marine terrace as a buried south-facing scarp at an elevation of 17.2 +/- 1.6 m. The local, vertical rate of uplift for the western Santa Barbara Fold Belt (SBFB) is 1.22 +/- 0.13 mm/yr. A fossil coral obtained from the seacliff of the first emergent marine terrace at Santa Barbara Point, yields a U-Th series age of 70 +/- 2.0 ka. The associated paleoshoreline is expressed at an elevation of 30.5 +/- 3.1 meters, therefore a local, vertical uplift rate of 1.58 +/- .09 mm/yr is calculated for the central SBFB.

Detailed paleoseismic study of the Loon Point fault-propagation fold in the eastern SBFB provides evidence that indicate the first emergent marine terrace is extensively folded. The asymmetric, open fold is formed on the hanging-wall of an east-west-trending reverse fault and is characteristic of structures in the SBFB. Fault trenches excavated across the Loon Point fault scarp document the Loon Point marine terrace is displaced a total of 6 meters by the Loon Point fault.

Geomorphic mapping of the More Ranch, Mission Ridge, and Arroyo Parida segments of the Mission Ridge Fault System in the Santa Barbara Fold Belt provide additional evidence for Quaternary faulting and folding. In Montecito, an uplift along the Mission Ridge segment has been recently discovered (central SBFB) as well as characterization of a fault-propagation fold in the Montecito seacliffs. Additionally, further understanding of the segmentation of the MRFS is documented which may provide understanding of the rupture style for this 70 km long fault system.

Alexei Tumarkin

Ralph Archuleta

David Oglesby

09/15/94-12/31/96 $64,896

National Science Foundation, NSF EAR 94-16214

Northridge Source Inversions

Observations of small earthquakes at a site can be successfully used to explain recorded ground motions as well as predict time histories from a future large earthquake. If these events are co-located and have focal mechanisms consistent with the large earthquake's anticipated rupture, then approaches by Tumarkin and Archuleta (1994) or Hutchings (1994) can be applied. In most practical cases, however, high-quality seismic data are limited to a few events. Archuleta et al proposed a way of improving ground motion simulations obtained by the kinematic modeling of an earthquake rupture with 1-D synthetic Green's functions (SGFs). The common SGF approach has two major problems: strong dependence of predictions on assumed slip distribution (which is a priori unknown), and great uncertainty in accounting for site response except for the rare cases when a complete geotechnical description is available. After fixing the ratio of the slip and the rise time everywhere on the fault, the resulting synthetics are weakly dependent on the slip distribution even in the near-field. By comparing any observation with the corresponding synthetics one gets an empirical site-specific transfer function which represents the inaccuracy of the theoretical path and site models (as well as the small event's source model). After applying this transfer function to SGF predictions of the large earthquake a considerable improvement in both the amplitude and duration of predictions can be achieved. Variability of transfer functions obtained from different small earthquakes recordings at a site represents the uncertainty of modeling small earthquakes using the SGFs, and thus can be used to estimate the uncertainty of forward predictions.

Alexei Tumarkin

Ralph Archuleta

02/01/95-01/31/96 $25,000

University of Southern California, 572726

Empirical Time-Series Simulation

Strong ground motion time-histories of large earthquakes can be successfully simulated using recordings of small earthquakes which are often referred to as Empirical Green's Functions (EGF) in seismology. EGF methods were introduced by Hartzell (1978) and extensively used for predicting effects of large earthquakes (Joyner and Boore, 1988; Aki and Irikura, 1991). The major advantage of using EGFs is that they depict realistic path and site effects when they are located within the rupture area and have the same focal mechanism as the simulated large earthquake (Hutchings, 1994). After assuming source time functions (e.g., the Aki-Brune model) a small earthquake's waveforms can be appropriately scaled and lagged to predict motions of any size. Tumarkin and Archuleta established methods applicable to multiple observed EGFs based on a constant and a variable rupture velocity (Tumarkin and Archuleta, 1995).

Alexei Tumarkin

Jamison Steidl

Ralph Archuleta

02/01/95-01/31/96 $20,000

University of Southern California, 572726

Strong-Motion Site Amplification Factors

In the 95/96 year Tumarkin et al expanded on the work of last years SCEC summer Intern Andy Byers, and began looking at correlations between amplification factors at sites which have recorded strong motion data, and site geological and geophysical classification schemes. They utilized the data from the SCEC strong-motion database. The idea was to see if they could assign an average amplification factor to be used in hazard analysis for individual geological and geophysical site classifications. They also compared results from weak-motion studies with the results from the strong-motion data to examine the effect of non-linear soil response. Preliminary results suggest that they can see this nonlinear effect in the data sets and they have begun to compare it with analytical models which include non-linear soil response. Tumarkin et al are currently using these new results in writing the site response chapter of the SCEC phase III report, scheduled to come out later in 1996.

Jason McKenna

Fabian Bonilla

Jamison Steidl

University of Southern California, 1995 Summer Intern

Removal Of Instrument Responses From Selected After-Shocks Of The Northridge Data Set

Aftershocks of the 17 January 1994 Northridge (M 6.7) mainshock recorded by the Southern California Earthquake Center (SCEC) portable deployment, TERRAscope and Southern California Seismic Network (SCSN) stations were retrieved from the SCEC database and selected according to the following criteria: (1)completeness (east-west, north-south, and vertical components) of two stations: LA00 and SSAP of the Northridge portable deployment, (2) proximity to the San Fernando Basin, and (3) quality of the waveforms. The instrument response was then deconvolved from the data by dividing the Fourier spectrum of the waveform by a transfer function constructed with pole, zero, gain information specific to that instrument.

Craig Nicholson

02/01/95-01/31/96 $30,000

University of Southern California, 572726

Seismicity Studies of the Santa Barbara-Ventura Area

A continuing ICS project is the geophysical study of the velocity structure and seismicity within the Santa Barbara Channel and extending eastward into the Ventura Basin. Except for the recent seismicity of the 1994 Northridge sequence, few of the earthquakes in this area have ever been located with a velocity model that accounts for the known 3-D velocity structure of the region. It was hoped that a reliable 3-D velocity structure could be estimated by inverting earthquake phase data available from the regional Southern California network. However, preliminary analysis of the waveforms archived at the SCEC Data Center indicates that much of the telemetered data at certain critical sites from the western part of the network near the Santa Barbara Channel suffer from cross-talk problems, and apparent phase arrival times are subject to large systematic errors. These problems inhibit the ability to invert the data for 3-D velocity structure at this time, but the data can be used to solve for improved 1-D velocity models using progressive inversion and either exponential (L1) and Gaussian (L2) norm residual minimizations. Additional independent control of velocity structure will be provided by including information from check-shot surveys in onshore and offshore test wells, and from regional refraction surveys (such as LARSE and wide-angle refraction studies conducted along and across the Santa Barbara Channel). Check-shot survey information is particularly useful, since this information covers the upper 3 to 4 km where earthquake phase data have poor resolution and where some of the largest heterogeneities in velocity structure are present. Arrival-time information, including recently discovered 1984 phase data for earthquake swarms in the Santa Barbara Channel recorded using OBS stations operated by USC, will be used to relocate earthquakes within this improved velocity structure, and the results of the relocated earthquakes would be compared with recently proposed fault and fold models for the Santa Barbara Channel [e.g., Shaw and Suppe, 1994] to test the validity of these proposed subsurface faults, to provide improved resolution of subsurface fault geometry, and to assess the seismic behavior of these active offshore and onshore structures in the Santa Barbara -Ventura area.

Craig Nicholson

Marc Kamerling

07/15/94-12/31/95 $107,146

National Science Foundation, NSF EAR 94-16194

Integrated Study of Seismic and Well Data in the Ventura Basin and the San Fernando Valley for Active Subsurface Faults

The 17 January 1994 M6.7 Northridge earthquake occurred on a blind, south-dipping fault beneath the San Fernando Valley. This structure is believed to be associated with the same Oak Ridge fault trend that extends farther west into the Ventura Basin and out into the Santa Barbara Channel. Similar active blind faults represent a significant seismic hazard to many other communities in California, as indicated by the devastating earthquakes near Santa Barbara in 1925, near Coalinga in 1983, and near Whittier in 1987, yet this seismic hazard is still not well resolved. The use of high-quality seismic reflection data, however, can provide tremendous insight into the nature of such subsurface fault systems through the analysis and interpretation of near-surface deformation that occurs in response to deep crustal faulting. The major problem is that such high-quality seismic data is extremely expensive to acquire, and, in many cases, areas of particular interest are situated in regions where it is physically impossible to collect such data today, at no matter what the cost. This is because these areas are now either highly populated or because new environmental regulations would preclude such data acquisition activities today.

To provide university access to some of the available subsurface seismic data, this project funded the purchase of the California dataset from GTS Corporation. The dataset consists of seismic reflection lines shot with dynamite in areas before many of the current restrictions were put in place, and includes lines in the Ventura Basin, the San Fernando Valley, and the Los Angeles Basin, as well as additional profiles throughout the Great Valley and other parts of the Transverse Ranges. Although these lines were limited to regions thought to have hydrocarbon potential, because folds above active faults have proven to be excellent hydrocarbon traps, many of these seismic lines were shot directly above active subsurface faults in those areas. In addition, because of the amount of data coverage, this dataset will be invaluable to the generic problem of investigating blind faults in other areas of California and to assessing their seismic hazard potential.

ICS has successfully acquired the GTS data in both analog and digital form, and is now conducting an integrated investigation of those seismic lines in the Ventura Basin. Preliminary analysis reveals that the GTS data need to be carefully tied to subsurface structure and stratigraphy by using available drill-hole data. They are currently negotiating the acquisition of such data files that can provide the necessary subsurface control. These studies will be combined with on-going investigations of faulting in the offshore Santa Barbara Channel, that tie directly to the onshore structures they will investigate using the GTS data. This will help provide a basis for understanding the geometry, tectonic development, and seismic hazard potential along strike of such major fault and fold systems as the Oak Ridge trend.

Craig Nicholson

Jonathan Lees

03/1/93-04/13/95 US Geologic Survey, 1434-93-G-2294

02/1/94-01/31/96 University of Southern California, 572726

Analysis of the 1992 Joshua Tree Earthquake Sequence and its Relation to the southern San Andreas Fault; 3-D Analysis of Seismicity, Focal Mechanisms and Stress Using the 1992 Landers-Big Bear-Joshua Tree Earthquake Sequences, Southern California

1. Xmap8 -- Interactive Color Graphics for Analysis of GIS and 3-D Geophysical Data, Part I: Demonstration and Program Capabilities.

Nicholson and Lees developed a new program to interactively manipulate a broad range of geophysical and geological data for exploratory analysis and presentation [Nicholson and Lees, 1994]. The program is similar to GIS systems that allow ascii data-bases stored in memory to be accessed through a user-friendly graphical interface, but differs in that it allows users to interact with data in a third dimension. Data can be viewed in either map or cross section, and at any strike or dip. Xmap8 was primarily designed to handle large sets of earthquake related data. Color-coded geological and geophysical maps (or cross sections) can be overlain with earthquake hypocenters, focal mechanisms and station arrays. Special attention has been put into dynamic plotting of earthquakes as time sequences, connecting related events derived from different velocity models or phase data, and plotting earthquakes with a variety of options related to hypocentral parameters. Several different views of earthquake focal mechanisms are available including traditional beach-ball plots, P- and T-axes, or single nodal planes with slip vectors, color-coded as a function of rake. Users are allowed to select individual nodal planes from suites of focal mechanisms, that align with seismicity trends in space and time, as a means of identifying structural details of subsurface fault geometry. A contouring package is included for plotting 2-D surface or subsurface field data in map view, or for projecting contour slices in cross section. 3-D projection of deviated wells, dipmeter logs, and well stratigraphy color-coded by lithology, in both map and cross section, makes visual correlation of many diverse data sets intuitive. Hard-copy output of graphic displays is all done in PostScript. Examples are presented involving fluid injection and seismicity at the Coso Geothermal Field, organizing focal mechanisms from the 1992 Joshua Tree sequence, delineation of the magma chamber and seismicity at Mt. St. Helens, and relating structural subsurface features in the Santa Barbara Channel.

2. Xmap8 -- Interactive Color Graphics for Analysis of GIS and 3-D Geophysical Data, Part II: Application to the 1992 M6.1 Joshua Tree Earthquake Sequence.

The 1992 M6.1 Joshua Tree earthquake occurred about two months prior to the M7.4 Landers earthquake and was followed by nearly 6,000 M>1 aftershocks recorded by the permanent regional network and an 11-element portable array deployed by the Southern California Earthquake Center [Nicholson and Hauksson, 1992]. This sequence defined a complex set of subsurface faults that included secondary structures that strike either sub-parallel to the Joshua Tree mainshock rupture or on relatively short, left-lateral cross faults that strike at high angles to the mainshock plane. Seismicity on this fracture network ceased in the hours prior to the Landers event and did not resume. Instead, the Landers mainshock appears to have caused the activation of a new fracture network located farther west, that intersects the previous Joshua Tree activity in the area of the Joshua Tree mainshock. Much of this later activity coincides with a first-order discontinuity in 3-D velocity structure imaged by tomographic inversion of P-wave arrival times [Lees and Nicholson, 1993]. The Joshua Tree data thus provide important information on the pattern of subsurface stress and strain, and how it changed with time, before and after the Landers mainshock.

The large numbers of earthquakes, the wide variation in focal mechanisms observed, and the complex pattern of subsurface faults involved, makes the Joshua Tree earthquakes an ideal data set to examine in more detail using the SCEC portable digital data and a new enhanced, interactive 3-D color graphic program - Xmap8 [Lees, 1994, 1995]. Comparison of arrival times hand-picked from the portable data at Scripps (UCSD), ICS (UCSB) and Yale indicates that ~10% of the data are still susceptible to large timing errors. To remove large systematic errors from the portable phase data, a cluster analysis was performed. Any individual arrival times that moved an event epicenter beyond the cluster radius of 0.7 km or increased the RMS error by more than 0.04 s were removed from the relocation procedure. Relocated hypocenters are typically deeper and located farther south and west, if both portable and permanent network data are used. Polarities of the vertical components at the portable stations were also found to be reversed. Revised focal mechanisms with 15 or more first-motions were determined for 1,484 Joshua Tree events (23 April to 28 June). Relocated hypocenters and revised focal mechanisms were then used to assess the pattern of subsurface active faults in the Joshua Tree area. Faults were identified by alignment of nodal planes and hypocenters in both space and time using Xmap8.


* Analysis of the SCEC portable digital data indicates that timing problems can be largely overcome and that the data are extremely useful for increasing model and structure resolution.

* Xmap8 proved to be an effective analytical tool in helping to organize large numbers of hypocenters and focal mechanisms (that occurred in a relatively small geographical area) in to recognizable patterns of subsurface faults.

* The Joshua Tree sequence is largely composed of predominantly strike-slip events that typically strike either subparallel or at high-angles to the Joshua Tree mainshock rupture plane.

* The main north-south structure responsible for the Joshua Tree mainshock is composed of en echelon fault segments that typically strike slightly west of north.

* Several of these strike-slip fault segments are actually curved when viewed in cross section; this includes the Joshua Tree mainshock fault plane.

* In addition, second-order faults that dip at moderate-angles exist within this fracture network; this includes structures with significant normal, reverse, or oblique components of slip.

* Preliminary results indicate that rotations of local stress fields may have occurred within the Joshua Tree area as a function of time prior to the M7.4 Landers mainshock.

Kim Olsen

University of Southern California, Postdoctoral Fellowship

Simulation of 3-D wave propagation in the Los Angeles basin

By means of generous donations of CPU-time on the SGI Powerchallenge supercomputer at the University of Utah from Dr. Schuster, Olsen and Archuleta simulated hypothetical M 6.75 earthquakes on the Palos Verdes, Santa Monica, Elysian Park faults, and as a reality check, the January 17 1994 Northridge event (Olsen and Archuleta, 1996). Though limited to periods longer than 2.5 sec due to computer limitations, the results showed strong amplification effects from the 3-D basin structure. The peak velocities (up to 67 cm/sec above the basin) were up to an order of magnitude stronger above the basin compared to rock sites.

The simple approximation to the Northridge earthquake reproduced the overall spatial pattern of the long-period particle velocities, successfully predicted the timing of late arriving waves and matched peak velocities with discrepancies generally less than a factor of two. While the Northridge earthquake was incredibly damaging to the Los Angeles area, the 3-D simulations show that earthquakes with the same magnitude on the Palos Verdes or Elysian Park faults produce more severe ground shaking in the Los Angeles Basin. It appeared that 3-D modeling was necessary to accurately predict ground motion from earthquakes in the Los Angeles basin.

During a supercomputing workshop in San Diego Olsen was offered to use the nCUBE2 parallel supercomputer at the Earth Resources Lab at MIT. This computer, with its 512 processors and 2 Gigabytes of RAM, opened up the possibility to carry out a scenario that Southern California seismologists had wanted to do for years: simulation of a large earthquake on the San Andreas fault.

Olsen spent 3 months adapting his finite-difference code to the parallel topology of the nCUBE2 computer. This work was carried out in collaboration with Dr. Joe Matarese, formerly of MIT; the work included a trip to MIT to work on the nCUBE2 and lecture in a parallel programming class. They completed 2 minutes of 3-D wave propagation in the Los Angeles basin model for a magnitude 7.75 earthquake on the San Andreas fault in Southern California.

The most significant part of the results was the strong shaking obtained above the Los Angeles basin: the simulation showed peak velocities up to 1.4 m/s, up to 10 times as large as those for rock sites outside the basin at similar distances from the fault. The simulation consolidated the previous results from the simulations of earthquakes on the Los Angeles Fault System, namely that the 3-D basin structure significantly amplifies the ground motion, and that the conventional 1-D analysis is insufficient to accurately predict the ground motion. The results were presented at the annual American Geophysical Union meeting in San Francisco on Dec 11. 1995 and attracted an quite a bit of attention. The results were published in Science (Olsen et al., 1995).

Kim Olsen

University of Southern California, Postdoctoral Fellowship

Site amplification in the Los Angeles basin

Encouraged by the results from the 3-D Los Angeles basin simulations Olsen simulated the same earthquake scenarios in a 1-D (flat-layered) model of Southern California as he had carried using the 3-D model. By taking the 3D/1D ratios of key amplification parameters (such as response spectra and durations). Olsen generated maps of the amplification for each earthquake scenario. In order to construct an overall amplification for the basin he averaged the ratios obtained from each earthquake scenario. The mean amplification map showed 3D/1D ratios up to about 5 but varied considerably from event to event. This is the first attempt to establish a single map of the 3-D amplification effects from the most plausible earthquake scenarios. There is a large variation of amplification from scenario to scenario, but the maps for the individual events agree on one point: the largest amplification occurs above the deepest parts of the basin or above the steepest slopes of the basin boundaries. Despite the promising results, more scenario simulations are required to decrease the uncertainties within the average amplification pattern.

Kim Olsen

University of Southern California, Postdoctoral Fellowship

3-D Visualization

Olsen implemented OpenGL 3-D graphics software, that he brought from the University of Utah, on the SGI Indigo2 workstation at ICS. This software, as well as the Explorer 3-D graphics software and Matlab turned out to be extremely useful to visualize the 3-D Los Angeles basin model as well as making animations of the simulations described. A dual deck VCR allowed him to put the 3-D visuals on a video tape which could be distributed to the media and brought along for presentations.

Another application of the 3-D graphics on the SGI was visualization of aftershocks from the January 17 1994 M 6.7 Northridge, California, earthquake. By plotting the earthquakes as color-coded spheres centered at their hypocenters and superimposing the mountain topography and fault plane solution from the main shock, he was able to correlate aftershock activity with the supposed fault plane. The Explorer software allowed rotation of the 3-D perspective in any direction which was useful to analyze clusters and subruptures of the event.

Mike Watkins

Kim Olsen

University of Southern California, 1995 Summer Intern

Two-dimensional Simulation of Northridge Aftershocks

Watkins and Olsen have used 2-D elastic finite-difference methods to simulate 0-3 Hz P, SV and SH waves for three Northridge aftershocks with epicentral locations near the northern edge of the San Fernando Valley. They used a vertical, approximately N-S striking, cross section (Vs > 0.5 km/s) taken from a geological model assembled by Magistrale and others which includes the structure of the Los Angeles and San Fernando basins. The simulations show multiply reflected phases and dispersive surface waves propagating southward, generated at the northern edges of the San Fernando and Los Angeles basins. The aftershock simulations mostly underpredict the durations observed, and the fit between observed and synthetic peak velocities varies significantly. However, these comparisons may be useful to improve the models of the 2-D basin structure.