Earthquakes

 

Ralph Archuleta

11/26/95 - 11/25/97 $219,022 12/20/95 - 12/19/98 $665,712

Institute for Protection and Nuclear Safety Nuclear Regulatory Commission

4060-00000470, 4060-00001217 NRC 04-96-046

 

 

Garner Valley Downhole Seismographic Array (GVDSA) Project

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, first; understanding 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: and second; understanding 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 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 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. Recent observations of dynamic pore-pressure changes from these sampling lines during small earthquakes may prove to be the first such measurements ever made in deep bedrock.

In the past year, research resulted in developing new approaches to empirical site response estimation. The idea is to study three-dimensional seismic radiation field instead of treating each component of ground shaking individually. A standard approach to estimating relative site response (Borcherdt, 1970) is to calculate the 1D transfer function between a rock and a soil sites as an average spectral ratio of individual components. We introduced a 3D transfer function between all three components of motions at two sites as a 3x3 matrix transformation. This matrix accounts for complex propagation effects such as conversions of different types of seismic waves, differences in incidence angles, etc. It can be determined from observations of at least three earthquakes at both sites. That suggests a possible explanation of some causes of an apparent event-to-event variability of standard 1D relative site response estimates as each single earthquake observation is capable of imposing only three constraints on nine a priori unknown components of the transfer matrix. When there are more than 3 observations, the transfer matrix is obtained as a maximum likelihood solution of an overdetermined system of equations. After combining two horizontal components into one complex time series we arrived at a new method of studying polarization properties of ground motion (Tumarkin and Archuleta, 1997). Our approach is different from the Takizawa's directional distribution of energy method (Takizawa, 1982; Kawase and Aki, 1990). For each frequency we are calculating the maximum amplitude and the corresponding azimuth of particle motion in the horizontal plane, as well as the eccentricity of this motion. That allows us to perform a detailed analysis of frequencies at which the ground motion is the largest and polarized the most, and how these frequencies vary from site to site. These new approaches are complementing the old ways of site response analysis and are designed for alerting to possible complex propagation effects (focusing, 3D structure, etc.), that are important in detailed microzonation studies for critical facilities.

 

Ralph Archuleta

7/1/95 - 2/28/99 $122,336

University of California, Office of the President

Campus Laboratory Collaboration, UCSB 08950868

 

CLC Seismic Hazard Study of the University of California

Campus at Santa Barbara

A multi-disciplinary seismic hazard study of the UC Santa Barbara campus is being conducted as part of a UC-LLNL Campus-Laboratory—Collaboration program. The primary objective of this project is to predict the ground motion for one critical structure on the UCSB campus based on potential seismic sources and local site conditions. Once completed, the CLC project will then provide a template to evaluate other buildings on campus, as well as provide a methodology for evaluating seismic hazards at other critical sites in California, including other UC locations at risk from large earthquakes. Another important objective of the CLC project is the education of students and other professionals in the application of this integrated, multi-disciplinary, state-of-the-art approach to the assessment of earthquake hazard. The CLC seismic hazard study will consist of four phases: Phase I — Initial Source and Site Characterization, Phase II — Drilling, Logging, Seismic Monitoring, and Laboratory Dynamic Soil Testing, Phase III — Modeling of Predicted Site-Specific Earthquake Ground Motions, and Phase IV — Calculations of 3D Building Response. Phase I has been completed and Phase II is nearly finished.

To date, the CLC study includes GIS-based mapping of active faults and folds, CPT soil studies, shallow P- and S-wave seismic refraction surveys, in-situ downhole velocity measurements and array monitoring of local seismicity. The UCSB campus sits on a raised marine terrace caught between the blind, north-dipping North Channel fault and the steeply south-dipping More Ranch fault. Uplift rates based on a dated marine coral are about 1 mm/yr, although these faults also likely include a significant strike-slip component. Over much of the UCSB campus, approximately 5 m of dry Quaternary terrace deposits (Vp ~350-500 m/s; Vs ~200 m/s) overlie low-density saturated Sisquoc Formation (Vp ~1500 m/s; Vs ~400-500 m/s). The campus is also situated above a sedimentary syncline. Amplification effects due to focusing from the syncline and the near-surface low velocities may represent a significant hazard to the campus. As part of Phase II, two 75-m boreholes were drilled this spring to provide additional information on subsurface material properties and to install uphole/downhole instruments to record strong and weak ground motion. Earthquakes of magnitude 4.9 and 3.2 at distances of 300 and 150 km, respectively, have been recorded by the new instrumentation. The data are provided real-time to the SCEC data center at Caltech. The uphole and downhole data will be used to provide empirical estimates of local site effects, to calibrate theoretical models of site response, and to predict future ground motion for use in modeling the 3-D response of various buildings on the UCSB campus.

 

Ralph Archuleta

10/1/97-9/30/98 $24,395 1/15/98-12/31/99 $89,440

Lawrence Livermore National Laboratory National Science Foundation

IGPP 98-GS012 EAR 97-25709

Dynamic Earthquake Rupture Simulation on Dipping Faults

David Oglesby in collaboration with Ralph Archuleta and Stefan Nielsen, has centered on investigating the effect that geometry has on the dynamics of dip-slip earthquake faults. Due to the asymmetric geometry of dipping faults, dynamic simulations of earthquakes on such faults produce asymmetric ground motion near the fault. The ground motion from a thrust/reverse fault is larger than that of a normal fault by a factor of two or more, given identical initial stress magnitudes. The motion of the hanging wall is larger than that of the footwall in both thrust/reverse and normal earthquakes. The asymmetry between normal and thrust/reverse faults results from time-dependent normal stress caused by the interaction of the earthquake-generated stress field with the earth’s free surface. The asymmetry between hanging wall and footwall results from the asymmetric mass and geometry on the two sides of the fault. Both the hanging wall vs. footwall effect and the thrust vs. normal fault effect are maximum for a fault that intercepts the free surface, and decay rapidly with depth of fault burial. Oglesby is currently performing preliminary simulations of thrust faults that change dip with depth.

 

Ralph Archuleta

4/1/91 - 1/31/98 $2,470,491

University of Southern California

Southern California Earthquake Center, USC 572726

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

 

Ralph Archuleta

4/1/91 - 1/31/98

University of Southern California

Southern California Earthquake Center, USC 572726

The Portable Broadband Instrument Center (PBIC)
Southern California Earthquake Center (SCEC)

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.

The vast majority of the PBIC equipment was used the entire year. The seismic hazard assessment phase of the Campus Laboratory Collaborative (CLC) project finished in early March freeing up the equipment for the Los Angeles Basin Passive Seismic Experiment (LABPSE) run out of UCLA by Dr. Monica Kohler. This experiment, consisting of eighteen seismic stations distributed from Seal Beach to the base of the San Gabriel Canyon, collected data to supplement the active source data collected by the LARSE project in late 1994.

Dr. Jamison Steidl of the Institute for Crustal Studies (UCSB) began his portable borehole study in January 1998. This study will investigate site responses using earthquake data recorded from sensors installed in 100m boreholes at several sites in the Los Angeles Basin.

The PBIC instruments were used by Javier Favella of Caltech to acquire data for a project that involved shaking the Millikan library, on the Caltech campus, with a large mechanical shaker mounted to the roof of the building.

Outreach programs continue to play an important role in the PBIC. Development of the PBIC World Wide Web (WWW) page has continued this past year including the addition of a java applet for calculating data collection rates and expanded vendor and equipment information. ICS researchers and the PBIC participated in seismological demonstrations at several local schools, including Isla Vista Elementary, Adams Elementary and La Colina Jr. High.

 

Ralph Archuleta

4/1/91 - 1/31/98

University of Southern California

Southern California Earthquake Center, USC 572726

Strong Motion DataBase

The UCSB Strong Motion DataBase (SMDB) is a relational database containing parametric information about 5,559 accelerograms, 121 earthquakes, and 654 strong motion stations, all of the data recorded within the state of California. Users can query the database directly from the World Wide Web (http://smdb.crustal.ucsb.edu/) and download the strong motion data from an on-site FTP server, or, whenever possible, from outside FTP sites, such as the U.S. Geological Survey and the California Strong Motion Instrumentation Program sites.

Some of the parametric information in the database include peak ground acceleration, epicentral distance, hypocentral distance, closest distance to the fault, response spectral amplitudes at 0.3, 1.0, and 3.0 seconds, instrument trigger times, earthquake locations, magnitudes, and focal mechanisms, station locations and site geology, and selected references.

Six access methods have been developed and can be found on the home page. Station and event summaries allow the user to view summaries of all of the data which is available for a particular station or earthquake. Users can also query the database through two HTML forms pages, a basic search page and a custom search page. These pages allow the user to search on all of the parametric information in the database. An interactive Java map applet has also been written that allows the user to easily search for earthquakes and stations in particular locations. Finally, for those users familiar with the database query language, SQL, a search page is available that allows the user to input a SQL query directly. The data returned by any of the above access methods can be downloaded directly from the search results.

The database is currently being served on a Sun Ultra 10 computer with a 300 MHz UltraSPARC processor, 17 GB of disk space, and 640 MB of RAM. The database software is the latest version of Oracle8 and includes the Oracle Web Application Server. A dedicated ethernet line is connected to a single port on a 10Base-T ethernet switch that is connected directly to the UCSB campus FDDI backbone.

http://smdb.crustal.ucsb.edu/

 

Ralph Archuleta

Kim Olsen

3/1/97 - 2/28/99 $55,000

US Geological Survey, 1434HQ97GR03100

Three-Dimensional Ground Motion Modeling in the San Francisco Bay Area

During January and February 1998, Dr. Paul Spudich spent a month at ICS working with Ralph Archuleta, graduate student Eleanore Jewel and Kim Olsen on 3D ground motion calculations for the San Francisco Bay area. They received and visualized the recently developed 3D crustal model of the bay area from USGS. The 1979 Coyote Lake earthquake was simulated in 1D and 3D models and the ground motions were compared to those from other studies. They developed 2.5D models of the Calaveras fault zone in order to study the radiation pattern and waves trapped in the fault zone. Finally, a study has been initiated to examine the effects of 3D crustal structure on kinematic slip inversion.

 

 

Ralph Archuleta

Alexei Tumarkin

9/1/97 - 6/30/98 $62,000

University of Southern California,

Southern California Earthquake Center, PO 030905

Integrated Approach to Ground Motion Prediction

The work on this SCEC project related to ground motion prediction led to formulation of this problem in terms of two global characteristics of the scenario earthquake -- seismic moment and radiated energy. 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. With the recent advances in compiling energy estimates for major earthquakes it is now possible to examine whether state-of-the-art models of earthquake sources produce realistic values of both the seismic moment and the radiated energy. For this purpose we use an estimate of the radiated energy for general models of extended earthquake sources based on the knowledge of the slip-rate history on the fault (Rudnicki and Freund, 1981). Kim Olsen and Alexei Tumarkin calculated the radiated energy for three-dimensional dynamic fault models that include a length scale. The resulting total moment rate functions can be described by a simple shape. Moreover, the average stress drops were almost four times larger than the apparent stresses, suggesting that the average radiational friction is of the same magnitude as the apparent stress. Tumarkin also studied the energy and apparent stress parameters as a function of the rise time and rupture velocity for kinematic models. The addition of the energy parameter allowed them to constrain the range of predictions thus reducing the uncertainty of the results. A talk was given at the SSA Annual Meeting (April 1998, Boulder, CO) and an abstract submitted for the 1998 Fall AGU meeting.

 

Ed Keller

2/1/96-1/31/98

University of Southern California

Southern California Earthquake Center, USC 572726

Earthquake Hazard of the Santa Barbara Fold Belt, California

The major accomplishment in this project (1997-98) has been to develop a new method of correlating uplifted marine platforms based upon analyses of oxygen isotopes from marine fossils collected at the interface between the bedrock, wave-cut platform and overlying sediments. The method we are developing will allow for correlation of terraces that have been deformed by uplift, faulting, or folding, as well as those dismembered by erosional processes. The method will then allow for rates of uplift to be calculated for marine platforms that are correlated to other platforms of known numerical ages.

 

Ed Keller

12/1/96 - 11/30/98 $76,091

US Geological Survey, 1434HQ97GR02978

Earthquake Hazard of the Santa Barbara Fold Belt, California

During the 1997-98 academic year, work on the project has identified the major potential seismic sources in the Santa Barbara Fold Belt. We have also completed our work to evaluate the Arroyo Parida Segment of the Mission Ridge Fault System. Our work is demonstrating the process of lateral propagation of prominent folds in the Santa Barbara Fold Belt including Mission Ridge and the newly identified Rincon Creek Anticline. Finally, rates of uplift have been calculated based upon mapping of shoreline angles of late Pleistocene marine platforms.

 

 

Grant Lindley

8/5/94 - 8/4/97 $213,370

Nuclear Regulatory Commission, NRC 04-94-079

Analysis of Source Spectra, Attenuation, and Site Effects

Using Broad Band Digital Recordings from the U.S. Seismograph

from Central and Eastern United States Earthquakes

A three-year project to conduct investigations of central and eastern United States earthquakes has recently been completed. The purpose of this project has been to improve our ability to predict ground motions in the central and eastern United States from future earthquakes. This project collected and analyzed data from the United States National Seismograph Network (USNSN); this data set included data from 207 earthquakes that occurred over a five-year period in the central and eastern United States and southeastern Canada. A total of 347 recordings were included in the analysis from 25 stations.

Several separate studies were conducted for the project. The most recently completed study compared the results of 200 source parameter measurements from 27 previous studies of eastern North American earthquakes. These studies were combined to test how the ground motions measured at the earth’s surface scale with increasing earthquake magnitude. One of the important parameters that measures the strength of an earthquake is the stress drop, which measures the drop in stress along an earthquake fault that occurs during an earthquake. This stress drop is often assumed to be roughly a constant, independent of the earthquake size. By combining the results from the 27 previous source parameter studies, it was found that these results are inconsistent with a constant earthquake stress drop for eastern North American earthquakes.

A second study completed for this project in the last year examined the attenuation of seismic waves and site responses of USNSN stations by analyzing the recordings of the regional earthquake phase, Lg. Generally, the ground motion recorded from an earthquake can be broken down into three components: the source effect, the path effect (including attenuation), and the site effect. The prediction of ground motion from an earthquake is often made easier and more reliable by estimating each of these three effects separately. The second study involved a combined analysis of data from various sources, paths, and station locations, in order to separate the three effects.

For the purposes of the study, the data were divided into five regions: the northeastern United States, the central United States, the southeastern United States, California and Nevada, and the Basin and Range province. Among the results of this study, large differences were found in the attenuation of the Lg phase between the western United States and the central and eastern United States. These differences are likely related to the rate of tectonic activity that occurs in the different regions.

 

 

Kim Olsen

2/3/97 - 9/30/99 $67,000

Los Alamos National Laboratory, F42200017-3Z

Modeling Non-Linear Ground Motion in the LA Basin

Dr. Eric Jones, Los Alamos National Laboratories, and Dr. Kim Olsen have developed a hybrid method to model non-linear soil amplification using finite-fault ground motion. They computed the source-time functions at a datum plane below the area of interest using a 3-D elastic finite-difference method. The source-time functions were then propagated up to the surface through a 1D soil column using a full non-linear method. The method has been tested on selected sites from the San Fernando Valley (JFP and SSA). The results are similar to those obtained from ratios of weak and strong motion recordings from the Northridge earthquake.

 

Kim Olsen

2/1/97 - 1/31/99

University of Southern California

Southern California Earthquake Center, USC 572726

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

In collaboration with Professor Raul Madariaga and Ralph Archuleta, Kim Olsen has developed a computer method to model an earthquake including a complex variation of the friction on the fault plane. They have used the method to simulate the 1992 M 7.3 Landers earthquake as the propagation of a spontaneous rupture. The simulation used an initial stress distribution on the fault calculated from fault movements derived in a prior study. The simulation shows the rupture propagating on the fault along a complex path with highly variable speed and pulse width. The results have implications for the state of stress on the fault following an earthquake as well as the seismic waves radiated during the earthquake, and the method may provide the framework to estimate earthquake rupture parameters from recorded seismograms in the future. In collaboration with post-doc Stefan Nielsen, Kim Olsen has improved the numerical accuracy of the 3-D dynamic modeling.

 

Kim Olsen

8/15/96 - 7/31/99 $120,000

National Science Foundation, EAR 96-28682

Ground Motion Modeling in Los Angeles

Kim Olsen has carried out a study on ground motion amplification for the Santa Monica area with SCEC summer intern Carmen Alex. They examined whether amplification due to a proposed buried lens-shaped basin boundary could account for the anomalously large amplification observed in the area during the Northridge earthquake. The results suggested that the "lens-effect" could only account for less than 50% of the observed amplification. Currently, Olsen is working on extending the amplification study for Santa Monica to a 3D model.

Researchers from SCEC are currently trying to finish the "phase 3" project. Olsen is responsible for mapping the long-period amplification from the 3D Los Angeles basin. He has found a strong variation in amplification pattern and magnitude for nine different scenario earthquakes, with the largest amplification occurring above the deepest part of the basin. Sensitivity tests show that anelastic attenuation can significantly reduce the ground motion for frequencies below 0.4 hz, and that a randomized slip distribution on the fault generates ground motions similar to those for a smooth slip distribution.

 

 

Jamison Steidl

2/1/97 — 1/31/99

University of Southern California

Southern California Earthquake Center, USC 572726

Site Response: Completion of Phase III and Beyond

The past year’s efforts under this project have been focused on the completion of the SCEC Phase III report. Response spectral amplification factors from weak-motion data, strong-motion data, and analytical models, are examined to find a correlation between these factors and geotechnical site parameters. In other words, can we better predict ground motion (reduce the residuals to the attenuation relations) given more detailed geotechnical and geological information regarding the local site conditions? The underlying motivation being that if we can better predict the ground motion then we can do a better job in the seismic hazard calculation by including the site response. This multi-disciplinary approach has been useful in determining where improvements to our models can be made and what new measurements are needed.

Site response factors are determined for strong motion sites in Southern California by comparison of observed response spectral acceleration (RSA) to predicted rock RSA. The site response is calculated using a rock attenuation relation as the "reference" motion to produce factors that can be used to include site response in probabilistic seismic hazard analysis (PSHA). Strong motion records from 1933 through 1994, provided by many agencies to the Southern California Earthquake Center's (SCEC) strong motion database (SMDB), are used as the data. Site response factors are calculated at four periods; 0.1, 0.3, 1.0, and 3.0 seconds. These factors are averaged according to surface geology and predicted input peak ground acceleration (PGA). Correlation with weak motion site response, surface geology, and near-surface shear wave velocity is examined. There is a trend towards larger site response factors with younger geology. At short periods (0.1 & 0.3) on Quaternary geology, there is a decrease in the site response factors at high levels of input motion which suggests nonlinear soil behavior. Independent weak-motion site effect studies are found to produce similar results, and the weak-motion site response is found to be consistent with the low-input strong motion site response.

In addition to the above work, we began a new study to look at the correlation between theoretical site response and empirical estimates of site response. The theoretical estimates are based on numerical simulation of wave propagation using the measured wave velocity in the near surface. The empirical site response estimates are determined using borehole-surface sensor pairs at the same sites where the wave velocity is measured. The study focuses on the Van Norman Dam Complex where very large and variable ground motion was recorded during the 1994 Northridge earthquake. With the help of a SCEC undergraduate summer intern we are currently in the empirical data collection phase, and judging from the earthquakes recorded to date, the project should produce some very useful results.

 

Jamison Steidl

Ralph Archuleta

2/1/97 — 1/31/99

University of Southern California

Southern California Earthquake Center, USC 572726

SCEC Borehole Instrumentation Initiative

The variability of observed ground motion and damage patterns over short distances produces a large degree of uncertainty in our ability to predict shaking from future earthquakes. Part of the variability is caused by the local near-surface site conditions. Installing borehole instrumentation below the surface soil layers allows us to remove the near-surface site effect at a few select stations. These borehole stations produce data that has not been distorted by the effect of the surface materials. This will allow for direct estimation of site effects and provide a test for the calibration and improvement of physical models of soil response. It will also give us a much clearer picture of the incident ground motion, which can be used to study the earthquake source process and the regional crustal structure in more detail. In addition the borehole data can be used as empirical Green's functions (the input motion) for predicting ground shaking at surface sites in the region surrounding the borehole station.

This marks the second year of the SCEC borehole instrumentation initiative. The long-term objective of this project is to instrument three borehole sites per year in the Los Angeles region. Currently five sites have been selected and are now in the final stages of completion. All five of the sites have been drilled, logged for wave velocity, and cased for deployment of the downhole instrumentation package. Data from one of the stations is already being provided in real-time to the Caltech/USGS Southern California Seismic Network (SCSN) and is being stored online at the SCEC data center. The other four stations should be online by the end of 1998.