Personnel: R. Archuleta*, F. Bonilla, A. Byers, J. Cadkin, M. Campbell, P. Dioquino, G. Ely, G. Gurrola, D. Harris, J. Hartsfield, T. Hayes, A. Hoffman, E. Keller, J. Lees, G. Lindley, B. Locke, R. Lucas, A. Martin, J. McKenna, D. Oglesby, K. Olsen, R. Pizzi, M. Robertson, P. Rodgers, B. Seely, J. Steidl, S. Swain, A. Tumarkin, A. Tumarkina, M. Watkins (*Agenda coordinator)

CUBE in Santa Barbara High Schools (SCEC USC 572726) (Ralph Archuleta, Robert Pizzi, Alan Hoffman, Malcomb Campbell)

The purpose of this project is to incorporate the CUBE (Caltech, USGS Broadcast of Earthquakes) system into the curricula of the Santa Barbara City high schools. The effort is to stimulate the interest of students at each school in the study of earthquakes and the methods of research. This project will develop a seismology curriculum for a two week module to be integrated into the Physical Science, Earth Science, or Physics course of study at the high school level. The project consists of: (1) the installation of three CUBE systems at the participating high schools, (2) the training of these participants in its operation, usage, and maintenance, and (3) the incorporation of the CUBE data into the various curricula at each school. The participating schools and personnel are as follows: Robert Pizzi, Bishop Diego High School;Alan Hoffman, San Marcos High School; Malcolm Campbell, Dos Pueblos High School. The CUBE project has been established at each of the schools. The curriculum is in its final stages of completion.

The Portable Broadband Instrument Center (PBIC) (SCEC USC 572726) (Ralph Archuleta, Aaron Martin, Peter Rodgers)

Peter Rodgers and Aaron Martin, along with engineers from other California institutions, developed a procedure for calibrating electromagnetic sensors. The method has been used to calibrate large numbers of sensors quickly for experiments such as LARSE as well as network installations (LABNet) and portable deployments (LA Microzonation) . Their paper describing the procedure was recently published in the Bulletin of the Seismological Society of America, Vol. 85, No. 3, pp 845-850, June 1995. Reprints are available on request from the PBIC.

The PBIC WWW page underwent some development this past year. The updated page includes more information about the PBIC as well as equipment status in a timeline format. This allows users to see where the equipment is utilized and by which projects.

Southern California undergraduates got some practical field experience during the SCEC Los Angeles Regional Seismic Experiment (LARSE) 94 project. The PBIC organized the UCSB contingent of student volunteers for this large cooperative project aimed at imaging the LA Basin. Student volunteers Jennifer Cadkin, Priscilla Dioquino, Geoff Ely, Todd Hayes, Brian Locke, Braden Seely and Robert Lucas deployed and retrieved seismic stations under the direction of ICS researchers.

The PBIC participated in several outreach presentations this past year. Presentations were given at several Santa Barbara Elementary schools as well as at UCSB as part of the Earthquake Fair. Several other presentations involved use of PBIC equipment by other SCEC personnel.

Garner Valley Downhole Array (GVDSA) (NRC-04-92-050) (Ralph Archuleta, Jamison Steidl, Alexei Tumarkin, Scott Swain)

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 our 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 collection makes it possible to advance two major areas of engineering seismology. 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.

We have found that the ground motion of small earthquakes can be scaled to match that of large earthquakes. However, the necessary scaling implies that the stress change from the small earthquakes must be multiplied by a factor of 10 to account for the differences in the level of ground motion. If this difference in stress is borne out by future investigations it has critical implications about the self-similarity of earthquakes.

The common assumption that the nearby rock site represents the "reference" motion to a soil site has to be questioned. This assumption does not seem to hold in the case of Keenwild (KNW), Pinon Flat Observatory (PFO) and Borrego Valley Downhole Array (BVDA). Spectral ratio estimates of amplification which use these surface rock sites underestimate the amplification at frequencies above 2 to 5 Hz. This is because the surface rock sites have a site response at these frequencies, instead of the assumed flat response behavior associated with a "reference" site. The rock site response is most likely due to the weathered and fractured nature of the near surface which causes the velocity to drop in the near surface. Even sites located on what appears to be competent crystalline rock show this high frequency amplification when compared to borehole data at the same site. These results suggest that one must be very careful in the choice of a "reference" site for site-specific hazard analysis and that the non-reference site techniques be examined in more detail.

Bedrock borehole recordings of ground motion can provide good reference motion for soil sites even at distances greater than 20 km from the soil site. When using the bedrock borehole as a reference the effect of the down-going wavefield (surface reflection) and the resulting destructive interference must be considered. This destructive interference may produce pseudo-resonances in the spectral amplification estimates. If one is careful, the bedrock borehole ground motion can be considered a good "reference" site for seismic hazard analysis even at distances as large as 30 km from the soil site.

500 Meter GVDA (NRC-04-92-050) (Ralph Archuleta, Jamison Steidl, Alexei Tumarkin, Scott Swain, Jim Hartsfield)

A two-year development effort culminated in the installation of several instruments into a 520 meter deep borehole located in the Garner Valley, California area. The string of instruments was lowered into the borehole and set in place during the months of April and May, 1995. The new equipment began operations on May 24, 1995.

The instrument string consists of two accelerometers, located at a depth of 500 meters; two dynamic pressure transducers, located at depths of 335 meters and 419 meters; and six inflatable packers, which couple the accelerometers to the borehole wall, and seal off zones in the borehole for pressure measurements. Stainless steel tubing extends from the surface into the six zones (defined by the top five of the six packers and the wellhead seal at the top of the borehole) to monitor the quasi-static pressure and to provide sampling lines for the water in the different zones.

This experiment is designed to look for correlations between fluid pressure and seismic activity. 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. This site is the location of the separately-funded Garner Valley Downhole Seismic Array, which has been in operation since late 1989.

Site Amplification of the San Fernando Valley, CA: A site effects study using weak- and strong-motion data (SCEC USC 572726) (Fabian Bonilla, Jamison Steidl, Alexei Tumarkin, Ralph Archuleta)

During the months that followed the 17 January 1994 M6.7 Northridge, California earthquake, portable digital seismic stations were deployed in the San Fernando, Los Angeles metropolitan region to recover aftershock data. One of the goals of this deployment was to examine the seismic hazard in this urban environment by way of site-specific amplification factors. About 1365 waveforms from 39 aftershocks ranging from M3.0 to M5.1, and depths from 0.2 to 19 km were recorded and analyzed at 35 three-component stations. The instrument response was removed from all the waveforms. We compared site response estimates which use the two horizontal components of ground motion as a complex signal with estimates which use only a single component or the geometrical mean of the site response estimate from each component. We compared whole record estimates of the site response with estimates which window the s-wave and coda-wave portions of the data. We also compared horizontal coda-wave with vertical coda-wave site response estimates. In general, the coda method overestimates the site amplification factor with respect to the factor obtained from the S-waves for all analyzed frequencies. We found that the vertical coda-wave site response estimates tend to give larger amplification factors than horizontal coda-wave estimates. We also found that the vertical and horizontal coda-wave estimates produce larger amplification factors by 2.0 and 1.6 respectively when compared to the horizontal S-wave estimates.

In addition to the results presented in France, results were presented at the 1995 annual SCEC meeting, and the 1995 Fall AGU meeting. These results showing the comparisons between different methods for estimating site-specific seismic hazard have important implications for researchers attempting to mitigate the hazard from future earthquakes. ICS researchers will also be providing engineers with estimates of site response which use the response spectrum instead of the Fourier spectrum, in order to better the communication between seismologists and engineering communities. Another promising technique developed here at ICS by Dr. Alexei Tumarkin estimates the site amplification in the time domain. This new technique will also prove useful to engineers who are not just interested in the frequency content of seismic energy but also the duration of shaking. With a number of important publications expected in the next few months based on the data analysis and work done by the PI's and graduate student Fabian Bonilla, we hope to appropriate additional funding to continue this important task of defining the seismic hazard in the Los Angeles and surrounding Southern California region.

Site-Specific Strong-motion Amplification Factors for the Southern California Region (SCEC USC 572726) (Andrew S. Byers-1995 Intern)

We 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. We 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 southern California and assist in further study of seismic events.

Earthquake Hazards in the City of Santa Barbara (SCEC USC 572726) (Larry Gurrola, Ed Keller)

Research conducted by L.D. Gurrola and professor E.A. Keller has yielded significant results providing a better understanding of the earthquake hazards in the city of Santa Barbara and surrounding areas. The notable results include:

The Ellwood marine abrasion platform and terrace surface are folded as a result of deformation along the More Ranch fault. It is a gentle, asymmetric fold and both flanks are observed in the Ellwood platform and terrace surface.

The More Mesa marine abrasion platform and terrace are back-tilted and probably represent the north flank of a fold in which the south limb and associated axial surface are offshore.

A shallow, south-dipping reverse fault that juxtaposes Santa Barbara Formation against Quaternary gravels has been identified, and probably is the south-east extension of the San Pedro fault. This fault coincides with a linear ridge and truncates an asymmetric, hanging wall fold. Paleomagnetic analyses of the deformed sediments have yielded normal remanent magnetization and suggests along with stratigraphic relationships, that they are mid-Pleistocene or younger (<790 ka). Therefore, the fault is determined to be a fault-propagation fold and is potentially active.

An east-west trending structure that exhibits reverse displacement of fluvial sediments in Maria Ygnacio Creek has been identified.

A potentially active fault-propagation fold and associated anticline hill with wind and water gaps has been identified in the hills west of downtown Santa Barbara. The footwall anticline is truncated by the San Jose fault and associated fault scarps several meters high have been identified in 1928 aerial photos. Paleomagnetic results yield normal magnetization of the sediments that are mid-Pleistocene in age.

Fossil deposits have been identified in the More Mesa and La Mesa marine terrace deposits and will potentially yield solitary corals for dating of these terraces.

Analysis of Source Spectra, Attenuation, and Site Effects using Broadband Digital Recordings from the National Seismograph Network (NRC-04-94-079) (Grant Lindley)

The first year of a three-year project has just been completed to analyze data from the U. S. National Seismograph Network (NSN), a network of state-of-the-art seismic instruments that have been installed in the last several years. The purpose of the project is to study seismic hazard in the central and eastern United States. A data base of NSN earthquake recordings has been collected and is stored on-line at the Institute for Crustal Studies. The data set includes seismograms from 185 earthquakes recorded at 22 stations in the central and eastern United States.

In order to study issues relevant to seismic hazard, Fourier spectra of the data have been computed using a Fast Fourier Transform algorithm. The observed spectra are modeled as a combination of site, path, and source effects, where the source effect is the effect due to the earthquake source, the path effect is the effect of the propagation of the seismic waves through the crust of the earth, and the site effect is the effect on the seismic waves due to propagation in the near-surface of the earth. The results of this modeling will enhance our understanding of the ground shaking caused by earthquakes and will allow us to better predict ground motion from future earthquakes in the central and eastern United States.

3-D Dynamic Rupture Models of Interacting Fault Segments (EAR-9218652) (Grant Lindley)

The purpose of this project is to produce a realistic computer model for earthquake ruptures. Many theoretical models for earthquakes have been proposed and developed; however, most of these models are based on very simplified assumptions of the earthquake source. For example, the earthquake fault is generally assumed to be a flat, planar surface. In reality, geological observations show fault zones to have geometrical complexities and non-planar features at many length scales. One of the commonly observed geometrical complexities is a fault jog or fault step, where two fault traces at the surface of the earth come to within a few kilometers of each other, but do not intersect.

This project has modeled the affect of a fault step on the earthquake rupture. The model includes two faults and includes the dynamic interaction between the two faults during the earthquake rupture. Where two interacting faults are present, the ground motion caused by the earthquake is observed to have an additional pulse of energy corresponding to the earthquake rupture slowing down when it encounters a fault step, then accelerating as it jumps from one fault segment to the next. This research shows how geometrical complexities can control earthquake ruptures and the ground motion that is caused.

Santa Barbara Channel Seismic Hazard Assessment (LLNL B291439) (Bruce P. Luyendyk)

ICS personnel assisted LLNL personnel in the formulation of a draft statement of seismic hazard in the Santa Barbara Channel for presentation to the U.S. Minerals Management Service. The role of ICS was to contribute towards a report that was co-authored by LLNL personnel; "Technical Issues Relevant to Seismic Hazard Analysis of the Eastern Santa Barbara Channel". The draft of a co-authored report was filed April 27, 1995. Nine ICS scientists participated in the writing of this document. We contributed sections on local geology, structure, seismic activity, tsunami and slide hazard, and strong ground motion. One feature we emphasized was the major unanswered questions regarding seismic hazard in our region. These include lack of knowledge of the controlling structures; whether thin or thick-skinned tectonics predominates in the Channel. Blind faults have been proposed for the Channel but not generally mapped. The maximum credible earthquake for our area is not constrained. Another important issue is lack of offshore data for strong ground motion of the Channel floor.

Removal Of Instrument Responses From Selected After-Shocks Of The Northridge Data Set (SCEC USC 572726) (Jason R. McKenna-1995 Intern),

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.

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. (SCEC USC 572726) (Craig Nicholson, Jonathan M. Lees)

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

We 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 [e.g., Seeber and Armbruster, 1995], 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, make 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.

A Combined High- and Low-Frequency Faulting Model for the 1992 Petrolia Earthquake (NSF EAR 9219721) (David Oglesby, Ralph Archuleta)

An unusual effect of the 1992 Petrolia earthquake was that an accelerometer at Cape Mendocino recorded a high frequency pulse of 1.47g, whereas the nearby Petrolia station recorded a maximum of only 0.6g. Archuleta (1992) and Oglesby and Archuleta (1993) have shown that extreme ground acceleration with high spatial variability can be caused by source effects provided certain conditions are met. If an accelerometer is located at a point of high symmetry with respect to a rupture pattern on a dipping fault, seismic radiation can interfere constructively at the station and greatly amplify ground acceleration. In this model the geometry constrains the location of the asperity on the fault plane, but the rupture of the asperity must also have a degree of symmetry. In the present study, we derive a faulting model that is consistent with both the high frequency pulse at Cape Mendocino and the overall distribution of slip. First, we have inverted the strong motion data (f<1.06 Hz) for the slip and rupture evolution of the earthquake using the method of Cotton and Campillo (1995). The inversion shows an area of high slip and high isochron acceleration on the fault where the asperity is geometrically constrained to lie. We add to the slip model the rupture of a circular asperity. By this procedure we arrive at a slip model that is consistent with our low-frequency inversion results and correctly produces the high-frequency pulse at Cape Mendocino. The results emphasize the hazard associated with locations above the hanging walls of dipping faults, where the geometry permits the production of extreme ground acceleration.

Simulation of 3-D Elastic Wave Propagation in the LA Basin (USGS 1434-94-G2410) (Kim Olsen, Ralph Archuleta)

The project is in progress and preliminary results include simulations of 3-D ground motion in the LA Basin from hypothetical M 6.75 constant-slip ruptures on three faults on the Los Angeles fault system. These simulations are a part of the SCEC ground motion scenario predictions. The three scenario simulations are for the Palos Verdes and Elysian Park faults, and the January 17 Northridge event. The ground velocity is computed on a grid of points with 400 m spacing throughout a volume that is 155 km x 134 km x 34 km (11.1 million grid points) covering the entire greater Los Angeles area. The maximum frequency is 0.4 Hz. The simulations show significant 3-D basin effects, including edge-generated waves and prolonged durations above the basin. Compared to more typical earth models where the velocity varies only in 1-D (depth), the ground motion is amplified by factors of 2 to 6 throughout the Los Angeles basin.

Signal Coil Calibration of Electro-magnetic Seismometers (SCEC USC 572726) (Pete Rodgers, Aaron Martin)

Together with Aaron Martin, Michelle Robertson and Mary Hsu of USC, and Dave Harris of LLNL, we developed a new method by which electro-magnetic seismometers may be easily calibrated, and with unprecedented accuracy. This is now the standard method by which all SCEC electromagnetic seismometers are calibrated. Unlike previous methods, the only information required from the manufacturer is the mass of the inertial element. The method involves removing a step of current from the signal coil of the seismometer, and simultaneously switching the signal coil to a recorder to capture the response. No calibration coil is required. A theory was developed which obtains the damped generator constant, resonant frequency, and damping ratio of the seismometer from the output of a system identifier used to analyze the response. Only the seismometer mass (from the manufacturer) and the applied current (measured) need be known for a complete calibration. The coil and damping resistances are not required. The method was confirmed by comparing this signal coil method with weight lift and calibration coil calibrations. For a GS-13 V seismometer, these results were within 1.3% of each other. The undamped generator constant obtained by the signal coil method matched the generator constant given by the manufacturer to better than 1%. Calibration of nine new L-4C components resulted in undamped generator constants all within 3% of the values given by the manufacturer.

LA Basin Microzonation (USGS 1434-94-G2410) (Jamison Steidl, Ralph Archuleta, Alexei Tumarkin)

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. Our 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 94/95 year we received 10 instruments from the PASSCAL instrument center and deployed these in addition to the 3 SCEC and 2 Caltrans instruments that we have been operating since March 1993, for a total of 15 portable stations in the Los Angeles metropolitan region. Undergraduates Mike Watkins, Robert Lucas, Priscilla Dioquino, and Geoff Ely were involved in the instrument calibration, the site deployment and maintenance. The 15 sites were operating from December of 1994 to July of 1995, when the PASSCAL instruments were returned. Currently we are still maintaining the 3 SCEC and 2 Caltrans stations.

Many of the sites chosen for deployment in this project are co-located with permanent strong motion stations. In addition to collecting weak motion data at these sites, we will also be able to compare amplification factors derived from weak motion data to those derived from strong motion records. An important question to engineering seismologists is how significant is the non-linear effect on strong ground motion, and at what amplitude level can we expect to see it? The Northridge mainshock and aftershock data recorded by this study, along with the permanent strong motion stations will provide ground motion data with the amplitude range to address these questions. In addition to the busy year of field work collecting data in the Los Angeles basin, we also have studied the data previously collected from the January 1994 M6.7 Northridge aftershock sequence.

Prediction of Ground Motions For Large Earthquakes Using Observations of Small Earthquakes (SCEC USC 572726) (Alexei Tumarkin, Ralph Archuleta)

New methods of site-specific ground motion prediction in the time and frequency domains were developed. A large earthquake is simulated as a composite (linear combination) of observed small earthquakes (subevents) assuming various functional models of the source time functions (spectra). Source models incorporate basic scaling relations between source and spectral parameters. Ground motion predictions are consistent with the entire observed seismic spectrum from the lowest to the highest frequencies avoiding deficiency in the vicinity of the target corner frequency. These methods are designed to use all the available empirical Green's functions (or any subset of observations) at a site. Thus a prediction is not biased by a single record, and different seismic wave propagation paths are taken into account. Any procedure of adding subevents in the time domain requires knowledge (or determination) of rupture times of subevents. Joyner and Boore [1988] recognized a major problem with using a uniform distribution of rupture times: the natural assumption of a constant rupture velocity leads to a significant underestimation of the main event's spectrum in the vicinity of the target corner frequency (by producing a local minimum of energy instead of a global maximum). As the spectral corner frequency acts as a source resonant frequency, any misfit to the spectral amplitudes near the corner frequency significantly affects the total energy in the computed time- series. This problem can not be overcome by allowing for different subevent sizes, but only by imposing a specific variation of the rupture velocity or of the stress drop. Our time-series prediction algorithm is based on determination of a specific distribution of rupture times of subevents. This approach is an extension of the method proposed by Wennerberg [1990]. The method is completely empirical. It requires only four input parameters for the simulated large event: i) seismic moment; ii) size of the rupture area; iii) location of the hypocenter; and iv) direction of rupture propagation. There are no other free parameters. We applied this method to predict ground motions in the Los Angeles Basin from scenario earthquakes on San Andreas and Elysian Park faults.

Rupture complexity of the Northridge earthquake (SCEC USC 572726) (Alexei Tumarkin, Ralph Archuleta, David Olgesby)

We study the source complexity of the Northridge mainshock by a dual approach to inversion for the slip and rupture histories using both theoretical and empirical Green's functions. One of the most promising ways to incorporate both the high-frequencies and the site response into the inversion scheme is to utilize empirical Green's functions (EGF) - recordings of aftershocks. The problem here is how to correct these recordings to obtain true Green's functions, i.e. what source time-functions should be deconvolved from the data. Without doing this deconvolution we should use EGFs only below corner frequencies of the aftershocks. Another well-known feature of aftershock distribution is that aftershocks tend to avoid the area of the major slip on the fault. We propose a way how to use a single aftershock record to improve quality of inversion with synthetic Green's functions. In this hybrid approach we first simulate an observed aftershock using a plane layered structure and then deconvolve synthetic ground motions from the recording at a site. This transfer function accounts for a possible mismatch between the model and real propagation and site effects. In performing the inversion we apply this empirical correction for each site. Having the digital data from the instruments co-located with permanent CDMG and USGS sites which recorded the mainshock we are able to perform inversions in different frequency ranges and compare results from different methods.

SCEC Strong-motion Database SMDB and Empirical Green's Functions Library EGFL (SCEC USC 572726) (Alla Tumarkina, Alexei G. Tumarkin, Ralph J. Archuleta)

SMDB is used by scientists and engineers from 33 institutions in the US, Canada, France, Germany, Italy, Japan and UK. During the last year we added Northridge data from USGS, CDMG and USC as well as response spectral ordinates at 0.3, 1 and 3s. The Web page for SMDB is at:


We are now putting data into EGFL. EGFL allows to search through parameters of seismic records, access unclipped and low-noise records and process them with SAC, plot selected earthquakes and stations and interactively obtain additional information from the maps. We have already processed all TERRAscope data and started working on SCSN data.

Performance of EGFL and SMDB was demonstrated on a stand-alone SUN workstation during the Annual SCEC Meeting (September 1995, Ojai).

Two-dimensional Simulation of Northridge Aftershocks (SCEC USC 572726) (Mike Watkins -1995 Intern, Kim Olsen)

We 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. We 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.