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SUMMARY We derive a detailed earthquake catalogue and Vp, Vs and Vp/Vs models for the region around the 2019 Mw 6.4 and Mw7.1 Ridgecrest, California, earthquake sequence using data recorded by rapid-response, densely deployed sensors following the Ridgecrest main shock and the regional network. The new catalogue spans a 4-month period, starting on 1 June 2019, and it includes nearly 95 000 events detected and located with iterative updates to our velocity models. The final Vp and Vs models correlate well with surface geology in the top 4 km of the crust and spatial seismicity patterns at depth. Joint interpretation of the derived catalogue, velocity models, and surface geology suggests that (i) a compliant low-velocity zone near the Garlock Fault arrested the Mw 7.1 rupture at the southeast end; (ii) a stiff high-velocity zone beneath the Coso Mountains acted as a strong barrier that arrested the rupture at the northwest end and (iii) isolated seismicity on the Garlock Fault accommodated transtensional-stepover strain triggered by the main events. The derived catalogue and velocity models can be useful for multiple future studies, including further analysis of seismicity patterns, derivations of accurate source properties (e.g. focal mechanisms) and simulations of earthquake processes and radiated seismic wavefields.

Author(s): Sanderson, Richard W; Matoza, Robin S; Haymon, Rachel M; Steidl, Jamison H | Abstract: Abstract Erosion, hydrothermal activity, and magmatism at volcanoes can cause large and unexpected mass wasting events. Large fluidized debris flows have occurred within the past 6000nyr at Mount Adams, Washington, and present a hazard to communities downstream. In August 2017, we began a pilot experiment to investigate the potential of infrasound arrays for detecting and tracking debris flows at Mount Adams. We deployed a telemetered four-element infrasound array (BEAR, 85nm aperture), ~11nkm from a geologically unstable area where mass wasting has repeatedly originated. We present a preliminary analysis of BEAR data, representing a survey of the ambient infrasound and noise environment at this quiescent stratovolcano. Array processing reveals near continuous and persistent infrasound signals arriving from the direction of Mount Adams, which we hypothesize are fluvial sounds from the steep drainages on the southwest flank. We interpret observed fluctuations in the detectability of these signals as resulting from a combination of (1)nwind-noise variations at the array, (2)nchanges in local infrasound propagation conditions associated with atmospheric boundary layer variability, and (3)nchanging water flow speeds and volumes in the channels due to freezing, thawing, and precipitation events. Suspected mass movement events during the study period are small (volumes l105 m3 and durations l2nmin), with one of five visually confirmed events detected infrasonically at BEAR. We locate this small event, which satellite imagery suggests was a glacial avalanche, using three additional temporary arrays operating for five days in August 2018. Events large enough to threaten downstream communities would likely produce stronger infrasonic signals detectable at BEAR. In complement to recent literature demonstrating the potential for infrasonic detection of volcano mass movements (Allstadt etnal., 2018), this study highlights the practical and computational challenges involved in identifying signals of interest in the expected noisy background environment of volcanic topography and drainages.

The 2019 Ridgecrest, California, earthquake sequence included Mw 6.4 and 7.1 earthquakes that occurred on successive days beginning on 4 July 2019. These two largest earthquakes of the sequence occurred on orthogonal faults that ruptured the Earth’s surface. To better evaluate the 3D subsurface fault structure, (P- and S-wave) velocity, 3D and temporal variations in seismicity, and other important aspects of the earthquake sequence, we recorded aftershocks and ambient noise using up to 461 three-component nodal seismographs for about two months, beginning about one day after the Mw 7.1 mainshock. The ∼30,000Mw≥1 earthquakes that were recorded on the dense arrays provide an unusually large volume of data with which to evaluate the earthquake sequence. This report describes the recording arrays and is intended to provide metadata for researchers interested in evaluating various aspects of the 2019 Ridgecrest earthquake sequence using the nodal data set.

Rapid seismic deployments following large earthquakes capture ephemeral near-field recordings of aftershocks and ambient noise that can provide valuable data for seismological studies. The U.S. Geological Survey installed 19 temporary seismic stations following the 4 July 2019 Mw 6.4 and 6 July 2019 (UTC) Mw 7.1 earthquakes near the city of Ridgecrest, California. The stations record the aftershock sequence beginning two days after the mainshock and are expected to remain in the field through approximately January 2020. The deployment augments the permanent seismic network in the area to improve azimuthal coverage and provide additional near-field observations. This article summarizes the motivation and goals of the deployment; details of station installation, instrumentation, and configurations; and initial data quality and observations from the network. We expect these data to be useful for a range of studies including detailing near-field variability in strong ground motions, determining stress drops and rupture directivity of small events, imaging the fault zone, documenting the evolution of crustal properties within and outside of the fault zone, and others.

We present a data set of ground motion recordings and site information from vertical array sites in California. The recordings include two horizontal components of ground shaking at the ground surface level and from downhole sensors. The availability of both surface and downhole recordings at the same site facilitates direct observations of site response. The site data include measured shear-and compression-wave velocities, and, where available, geotechnical boring logs. We considered 39 vertical array sites in California and chose 21 for inclusion in the database on the basis of having at least four pairs of surface/downhole recordings. The recordings and site data are presented in a data repository, which is accessible at the DesignSafe platform (DOI: 10.17603/146DS2N680). The original digital accelerograms are processed in a manner consistent with NGA-West2 protocols. In this paper, this data set is compared to a similar but larger data set from Japanese vertical arrays compiled by others.

Characteristics of rotational earthquake ground motions and their effects on structural response are not yet well-defined. Recording rotational ground motions directly in free field is in its infancy, and simultaneous six-component earthquake measurements are being accumulated slowly. A six-degree-of-freedom (6DOF) ground motion observation system was installed in the Garner Valley Downhole Array (GVDA), a very well-characterized and well-instrumented geotechnical array in Southern California. Since 2008, six-component free-field earthquake ground motions have been recorded from hundreds of earthquakes with a relatively wide range of hypocentral distances and magnitudes. In this paper, analysis was conducted to develop the characteristics of these measured rotational ground motions. Linear relationships between peak rotation velocity and peak ground acceleration were found, similar to previous 6DOF measurements in Taiwan and Japan. Ratios of rotation to translation as a function of hypocentral distance show larger ground rotations at closer distance, and that rotational ground motions tend to attenuate more rapidly than corresponding translational ground motions. Measured rotational motions show differences from estimations using elastic plane wave theory when using simple local apparent wave velocities. Finally, preliminary empirical relationships for rotational response spectra are estimated for earthquake engineering applications.

We analyze the nonlinear and near‐surface geological effects of two Network for Earthquake Engineering Simulation at University of California, Santa Barbara (NEES@UCSB) instrumented sites: the Garner Valley Downhole Array (GVDA) and the Wildlife Liquefaction Array (WLA). The seismic interferometry by deconvolution method is applied to earthquake data recorded by the multisensor vertical array between January 2005 and September 2013. Along the cross section, local shear‐wave velocity is extracted by estimating travel time between sensors. The S ‐wave velocity profiles are constructed and compared with classical in situ geophysical surveys. We show that velocity values change according to the amplitude of the ground motion, and we find anisotropy between east–west and north–south directions at the GVDA site. The ratio between average peak particle velocity v * and local S ‐wave velocity between two boreholes is tested as a deformation proxy. Using average peak particle acceleration a * , the a * versus curve is used to represent the stress–strain curve for observing the site’s nonlinear responses under different levels of excitation. Nonlinearity is observed from quite low shear‐strain levels (∼1×10 −5 ) and a classic hyperbolic model is derived. proves to be a good deformation proxy. Finally, the shear modulus degradation curves are constructed for each depth and test site, and they are similar to previous laboratory measurements or in situ geophysical surveys. A simple comparison regarding nonlinear behavior between GVDA and WLA is performed.

The paper compares two liquefaction case histories in California: (1) the response of the Wildlife site in the Imperial Valley to the 2010 El-Mayor Cucapah earthquake (Mw=7.2, amax=0.15 g)...

Field observations suggest that preshaken natural sands in some seismic regions have high liquefaction resistance as a result of geologic aging and/or preshaking. This paper focuses on the young silty sand deposits located in the Imperial Valley of California. Recent deposition and intense seismic activity in the Valley suggest that preshaking is the main cause of their increased liquefaction resistance. The first part of the paper examines the liquefiable layer at the Wildlife site, which may have been deposited by flooding approximately between 1905–1907. The site was instrumented with accelerometers and piezometers in 2005, providing data over the last 10 years. The following conclusions are reached from this and from the catalog information on earthquakes before 2005: (1) Since 1907, the Wildlife layer has been subjected to approximately 60–70 earthquakes having amax≥0.1 g at the site, which caused pore pressure buildup in the layer; (2) most of these earthquakes generated excess pore pressur...

The effects of near-surface soil stratigraphy on the amplitude and frequency content of ground motion are accounted for in most modern U.S. seismic design codes for building structures as a function of the soil conditions prevailing in the area of interest. Nonetheless, currently employed site-classification criteria do not adequately describe the nonlinearity susceptibility of soil formations, which prohibits the development of standardized procedures for the computationally efficient integration of nonlinear ground response analyses in broadband ground-motion simulations. In turn, the lack of a unified methodology for nonlinear site-response analyses affects both the prediction accuracy of site-specific ground-motion intensity measures and the evaluation of site-amplification factors when broadband simulations are used for the development of hybrid attenuation relations. In this article, we introduce a set of criteria for quantification of the nonlinearity susceptibility of soil profiles based on the site conditions and incident ground-motion characteristics, and we implement them to identify the least complex ground response prediction methodology required for the simulation of nonlinear site effects at three sites in the Los Angeles basin. The criteria are developed on the basis of a comprehensive nonlinear site-response modeling uncertainty analysis, which includes both detailed soil profile descriptions and statistical adequacy of ground-motion time histories. Approximate and incremental nonlinear models are implemented, and the limited site-response observations are initially compared to the ensemble site-response estimates. A suite of synthetic ground motions for rupture scenarios of weak, medium, and large magnitude events (M 3.5–7.5) is next generated, parametric studies are conducted for each fixed magnitude scenario by varying the source-to-site distance, and the variability introduced in ground-motion predictions is quantified for each nonlinear site-response methodology. A frequency index is developed to describe the frequency content of incident ground motion relative to the resonant frequencies of the soil profile, and this index is used in conjunction with the rock-outcrop acceleration peak amplitude (PGA_(RO)) to identify the site conditions and ground-motion characteristics where incremental nonlinear analyses should be employed in lieu of approximate methodologies. We show that the proposed intensity-frequency representation of ground motion may be implemented to describe the nonlinearity susceptibility of soil formations in broadband simulations by accounting both for the magnitude-distance-orientation characteristics of seismic motion and the profile stiffness characteristics. The synthetic ground-motion predictions are next used for the development of site-amplification factors for the alternative site-response methodologies, and the results are compared to published site factors of attenuation relations. For the site conditions investigated, currently established amplification factors compare well with synthetic simulations for class C and D site conditions, while long-period amplification factors are overestimated by a factor of 1.5 at the class E site, where site-specific nonlinear analyses should be employed for levels of PGA_(RO)>0.2g.

Automatic trip hammers have advantages for standard penetration test (SPT) of consistent drop height and low friction loss during hammer fall. These advantages, however, generate high energy transfer ratios (ER), typically about 90%. This efficiency causes lower sensitivity and higher energy correction coefficients, CE . To reduce ER and CE and to increase the sensitivity of SPT conducted at the Wildlife Liquefaction Array (WLA) and the Garner Valley Downhole Array, instrumented Network for Earthquake Engineering Simulation sites, a 127 mm (5.00 in.) long sleeve was placed in the hammer mechanism to reduce the drop height from 762 mm (30 in.) to 635 mm (25in.) . To calibrate the energy for these drop heights, measurements were made for a series of SPT tests in Borehole X2 at WLA on November 21, 2003. For these SPT, sleeves were inserted with lengths of 50 mm (2 in.) , 127 mm (5 in.) 177 mm (7 in.) , and no sleeve. Resulting drop heights were 762 mm (30 in.) , 711 mm (28 in.) , 635 mm (25 in.) , and 584 mm...

Weak-motion geotechnical array recordings at 38 stations of the Japanese strong-motion network KiK-Net from the 2003 M_w 7:0 Miyagi-Oki aftershock sequence are used here to quantify the amplification and attenuation effects of near-surface formations to incident seismic motion. Initially, a seismic waveform optimization algorithm is implemented for the evaluation of high-resolution, low-strain velocity (V_s), attenuation (Q_s), and density (ρ) profiles at the sites of interest. Based on the inversion results, V_s versus Q_s correlations are developed, and scattering versus intrinsic attenuation effects are accounted for in their physical interpretation. Surface-to-downhole traditional spectral ratios (SSR), cross-spectral ratios (c-SSR), and horizontal-to-vertical (H/V) site-response estimates are next evaluated and compared, while their effectiveness is assessed as a function of the site conditions classified on the basis of the weighted average Vs of the upper 30 m (V_(s30)) of the formations. Single and reference-station site-response estimates are successively compared to surface-to-rock outcrop amplification spectra and are evaluated by deconvolution of the downhole records based on the inversion results; comparison of the observed SSR and estimated surface-to-rock outcrop amplification spectra illustrates the effects of destructive interference of downgoing waves at the downhole instrument level as a function of the site class. Site amplification factors are successively computed in reference to the National Earthquake Hazards Reduction Program (NEHRP) B–C boundary site conditions (V_(s30) = 760 m/sec), and results are compared to published values developed on the basis of strong-motion data and site-response analyses. Finally, weak-motion SSR estimates are compared to the mainshock spectra, and conclusions are drawn for the implications of soil nonlinearity in the near surface. Results presented in this article suggest that currently employed site classification criteria need to be reevaluated to ensure intraclass consistency in the assessment of amplification potentials and nonlinearity susceptibility of near-surficial soil formations.

The Constorium of Organizations for Strong-Motion Observation Systems (COSMOS) Virtual Data Center (VDC) is an unrestricted Web portal to strong-motion seismic data records of the United States and 14 contributing countries for use by the engineering and scientific communities (http://db.cosmos-eq.org). As of January 2006, the VDC contains the searchable metadata for 514 earthquakes, 3,052 stations, and 26,708 acceleration traces. A flexible, full range of search methods, including map-based, parameter-entry, and earthquake- and station-based searches, enables the user to quickly find records of interest. A range of display and download options allows users to view data in multiple contexts, extract and download metadata, and download time history and spectra files. Although the portal provides the user a consistent set of tools for discovery and retrieval, the data files continue to be acquired, processed, managed, and for most of the larger providers, archived by the data providers to ensure currency and integrity of the data. COSMOS oversees the development of the VDC through a working group of representatives from both data providers and users, including government agencies, engineering firms, and academic institutions. This article summarizes the VDC content, provides some statistics on its usage, and describes some of its basic search capabilities. The COSMOS Virtual Data Center began as the Strong-Motion Data Base (SMDB) in 1992 at the Institute for Crustal Studies at the University of California, Santa Barbara (UCSB), with support from the Southern California Earthquake Center (SCEC). The initial goal of the SMDB was to organize the distribution of strong-motion data in a single “one-stop” shop that provides open access to the data. The global strong-motion dataset was no longer small enough for an individual to know which records were relevant for his/her project. The SMDB was designed so that the strong-motion data could be accessed according to the needs …

The M w 7.0 Miyagi-Oki earthquake, which occurred on 26 May 2003, was well recorded by the KiK-net and K-net networks. A large number of stations recorded very high peak ground accelerations above 0.5g and large peak ground velocities above 0.5 m/sec. These high ground-motion values are thought to come from a combination of the effect of shallow sediment layers of the upper couple of meters and the enhanced high-frequency ground-motion content associated with this intraslab earthquake. The objective of this study is to examine the effect of sediment amplification at network stations with peak ground acceleration ! 0.3g. Linear site response is first estimated from observed weak motion (aftershock) records. In this case, we use a spectral inversion method, without reference stations, to separate the source, path, and site-response effects. The resulting weak motion analysis for the source, path, and site response agree with other previous studies. The mainshock site response is obtained separately using the same spectral inversion technique with the addition of a frequency-dependent radiation pattern. The comparison of the site am- plification from aftershocks with the mainshock indicates the possibility of nonlinear site response at many stations during the Mw 7.0 event. The results also suggest a correlation between lownear-surface materialvelocity and thedegreeofnonlinearity.

A seismic waveform inversion algorithm is proposed for the estimation of elastic soil properties using low amplitude, downhole array recordings. Based on a global optimization scheme in the wavelet domain, complemented by a local least-square's fit operator in the frequency domain, the hybrid scheme can efficiently identify the optimal solution vicinity in the stochastic search space, whereas the best-fit model detection is substantially accelerated through the local deterministic inversion. Results presented for selected aftershocks of the Mw 7.0 Sanriku-Minami earthquake in Japan, recorded by the Kik-Net Strong Motion Network, illustrate robustness of the impedance structure estimation. By contrast, the attenuation structure is shown to be sensitive to the frequency content of seismic input data, attributed to the deterministic description of the continuum in the forward model that cannot simulate late arrivals of multiple-scattered energy. Sensitivity analyses illustrate that for the same forward model, results can be substantially different based on the definition of the objective function. It is concluded that even for engineering purposes, inversion should aim to decouple intrinsic and scattering attenuation mechanisms.

The Campus Earthquake Program (CEP) of the University of California (UC) started in March 1996, and involved a partnership among seven campuses of the UC—Berkeley, Davis, Los Angeles, Riverside, San Diego, Santa Barbara, Santa Cruz—and the Lawrence Livermore National Laboratory (LLNL). The aim of the CEP was to provide University campuses with site-specific assessments of their earthquake strong motion exposure, to complement estimates they obtain from consultants according to the state-of-the-practice (SOP), i.e. Building Codes (UBC 97, IBC 2000), and Probabilistic Seismic Hazard Analysis (PSHA). The Building Codes are highly simplified tools, while the more sophisticated PSHA is still somewhat generic in its approach because it usually draws from many earthquakes not necessarily related to the faults threatening the site under study. Between 1996 and 2001, the site-specific studies focused on three campuses: Riverside, San Diego, and Santa Barbara. Each campus selected 1–3 sites to demonstrate the methods and procedures used by the CEP: Rivera Library and Parking Lots (PL) 13 and 16 at UCR, Thornton Hospital, the Cancer Center, and PL 601 at UCSD, and Engineering I building at UCSB. The project provided an estimate of strong ground motions at each selected site, for selected earthquake scenarios. These estimates were obtained by using an integrated geological, seismological, geophysical, and geotechnical approach, that brings together the capabilities of campus and laboratory personnel. Most of the site-specific results are also applicable to risk evaluation of other sites on the respective campuses. The CEP studies have provided a critical assessment of whether existing campus seismic design bases are appropriate. Generally speaking, the current assumptions are not acknowledging the severity of the majority of expected motions. Eventually, both the results from the SOP and from the CEP should be analyzed, to arrive at decisions concerning the design-basis for buildings on UC campuses. Published by Elsevier Ltd.

To estimate the broadband strong ground motion one might expect at a given site we develop a method that includes heterogeneous slip on a finite-fault, full wave propagation with high frequencies, and site-specific material properties with nonlinear soil response. The faulting is simulated as a stochastic process with the spatial variation of the key parameters determined by probability distribution functions. The wave propagation from source to site is accounted for by using small earthquake recordings as empirical Green's functions (EGF). This accounts for the regional effects of scattering, attenuation and structure while providing the basis for a broadband (0.5-10 Hz) time history. Because we are interested in sites where the ground motion is expected to be severe, we have included nonlinear wave propagation through the soil. The material properties of the soil column have been determined from laboratory tests, borehole logs and confirmed through seismological modeling of weak motion. We have computed 240 three-component acceleration time histories to represent the range of ground motion one might expect from a M 6.8 earthquake for a site that is located 10 km above the hanging wall of blind thrust (7.1 km closest distance). Based on the suite of time histories we computed a S.D. of 0.45 (natural log units) for the acceleration response spectra in the passband 0.5-10 Hz. The total S.D. (modeling plus parameterization) is 0.6 in natural log units. The mean acceleration response spectrum is near the median 10% in 50-year probabilistic seismic hazard analysis (PSHA) spectrum for the site; the 84% spectrum of the simulations is closer to the 5% in 50 years median spectrum. © 2003 Elsevier Science B.V. All rights reserved.

The Garner Valley Downhole Array (GVDA) consists of a set of seven downhole strong-motion instruments ranging from 0- to 500-m depth. One of the objectives of this experiment is to estimate site response and study wave propagation as the energy travels from the bedrock underneath the site up through the soil column. The GVDA velocity structure is studied by computing synthetic accelerograms for a small event located at an epicentral distance of 10 km. These synthetics simulate well the data recorded at the borehole stations. In addition, theoretical transfer functions are calculated using the obtained velocity model and compare well with the empirical transfer functions from 54 recorded events. It is also observed that the downgoing wave effect is predominant in the first 87 m and is strongly reduced at depth. Using the velocity structure at GVDA and the transfer function results, it has also been possible to develop a simple method to compute the incident wave field, which is needed in nonlinear site response for instance. Recently there have been many comparative studies between horizontal-to-vertical (H/V) spectral ratios and traditional spectral ratios. Although many of these studies show that H/V spectral ratios can reproduce the shape of the site response curve, most show differences in the amplitude level. In the case of Garner Valley, where we have both surface and multiple borehole instruments, we find that this discrepancy in amplitude of the site response estimates is because the vertical component has significant site response associated with it due to S -to- P conversions that begin in the weathered granite boundary at 87-m depth. Manuscript received 27 August 2001.

As part of the Southern California Earthquake Center (SCEC) Phase III effort to include site effects in hazard models for southern California, a regional database of strong-motion observations was developed. The observations consist of the peak ground acceleration (PGA) and 5% damped response spectral acceleration (SA) at 0.3-, 1.0-, and 3.0-sec periods from 28 earthquakes and 281 stations. A total of 449 pairs of horizontal PGA and SA observations that were taken from the SCEC Strong-Motion Database (SMDB) are presented here. The phase III database includes earthquakes with moment magnitudes larger than 5.0 and stations in southern California with locations between 32° and 36° north latitude. Observations from buildings with more than two stories and dam abutments or crests are excluded from the database. Observations with distances of 150 km or greater were also excluded. The agencies that provided the data to SMDB are the U.S. Geological Survey, California Strong Motion Instrumentation Program, University of Southern California, and the Los Angeles Department of Water and Power. The database also contains site classification information for each station. A first general classification is based on the 1:750,000 California map of Quaternary, Tertiary, and Mesozoic geologic units by Jennings (1977) as modified by Park and Elrick (1998). A second more detailed classification is based on Quaternary mapping in the Los Angeles region by Tinsley and Fumal (1985) as modified by Park and Elrick (1998). A third classification is based on the correlation of surface geology with shear-wave velocity in the upper 30 m (Wills et al., 2000). An arbitrary “depth-to-basement” parameter is assigned to stations that are located within the boundary of the 3D velocity model used by Olsen (2000), which is based on the SCEC 3D velocity model (version 1). This parameter is defined as the depth to the 2.5km/sec velocity isosurface. Special parameters associated with particular attenuation relations, such as different distance measures, a hanging wall flag, and fault-type flag are also assigned to each observation. These data are all presented within the tables and figures of this article, and also have been made available via a downloadable file on the Internet ( http://smdb.crustal.ucsb.edu/~phase3 ).

This study determines site-response factors that can be applied as cor- rections to a rock-attenuation relationship for use in probabilistic seismic-hazard analysis. The site-response factors are amplitude and site-class dependent. These amplification factors are determined by averaging ratios between observed and pre- dicted ground motions for peak ground acceleration (PGA) and for 5% damped re- sponse spectral acceleration at 0.3, 1.0, and 3.0 sec oscillator periods. The observa- tions come from the strong-motion database of the Southern California Earthquake Center (SCEC), and the predictions are based on the Sadigh (1993) rock-attenuation relation. When separated and averaged according to surface geology, significantly different site-response factors are found for Quaternary and Mesozoic units, but a subclassification of Quaternary is generally not justified by the data. The low input- motion amplification factors are consistent with those obtained from independent aftershock studies at the PGA and 0.3-second period. An observed trend of decreasing Quaternary site amplification with higher input motion is consistent with nonlinear soil behavior; however, the trend exists for Mesozoic sites as well, implying that this may be an artifact of the Sadigh relationship. There is a correlation between larger site-response factors and lower average shear-wave velocity in the upper 30 m for low predicted PGA input motions, with an increase in the correlation with increasing period. The 0.3-sec site response factors for Quaternary data in southern California determined in this study are consistent with 0.3-sec NEHRP site-response correction factors; however, at 1.0-sec period some inconsistencies remain. A trend is also seen with respect to sediment basin depth, where deeper sites have higher average site- response factors. These results constitute a customized attenuation relationship for southern California. The implication of these customized attenuation models with respect to probabilistic hazard analysis is examined in Field and Petersen (2000).

During the months that followed the 17 January 1994 M 6.7 Northridge, California, earthquake, portable digital seismic stations were deployed in the San Fernando basin to record aftershock data and estimate site-amplification factors. This study analyzes data, recorded on 31 three-component stations, from 38 aftershocks ranging from M 3.0 to M 5.1, and depths from 0.2 to 19 km. Site responses from the 31 stations are estimated from coda waves, S waves, and ratios of horizontal to vertical (H/V) recordings. For the coda and the S waves, site response is estimated using both direct spectral ratios and a generalized inversion scheme. Results from the inversions indicate that the effect of Qs can be significant, especially at high frequencies. Site amplifications estimated from the coda of the vertical and horizontal components can be significantly different from each other, depending on the choice of the reference site. The difference is reduced when an average of six rock sites is used as a reference site. In addition, when using this multi-reference site, the coda amplification from rock sites is usually within a factor of 2 of the amplification determined from the direct spectral ratios and the inversion of the S waves. However, for nonrock sites, the coda amplification can be larger by a factor of 2 or more when compared with the amplification estimated from the direct spectral ratios and the inversion of the S waves. The H/V method for estimating site response is found to extract the same predominant peaks as the direct spectral ratio and the inversion methods. The amplifications determined from the H/V method are, however, different from the amplifications determined from the other methods. Finally, the stations were grouped into classes based on two different classifications, general geology and a more detailed classification using a quaternary geology map for the Los Angeles and San Fernando areas. Average site-response estimates using the site characterization based on the detailed geology show better correlation between amplification and surface geology than the general geology classification.

Many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a “reference” motion. The critical assumption in these methods is that the surface-rock-site record (reference) is equivalent to the input motion at the base of the soil layers. Data collected in this study show that surface-rock sites can have a site response of their own, which could lead to an underestimation of the seismic hazard when these sites are used as reference sites. Data were collected from local and regional earthquakes on digital recorders, both at the surface and in boreholes, at two rock sites and one basin site in the San Jacinto mountains, southern California. The two rock sites, Keenwild and Piñon Flat, are located on granitic bedrock of the southern California peninsular ranges batholith. The basin site, Garner Valley, is an ancestral lake bed with watersaturated sediments, on top of a section of decomposed granite, which overlies the competent bedrock. Ground motion is recorded simultaneously at the surface and in the bedrock at all three sites. When the surface-rock sites are used as the reference site, i.e., the surface-rock motion is used as the input to the basin, the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 Hz. This underestimation, by a factor of 2 to 4 depending on frequency and site, results from the rock sites having a site response of their own above the 2-to 5-Hz frequencies. The near-surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest, even at sites that appear to be located on competent crystalline rock. The bedrock borehole ground motion can be used as the reference motion, but the effect of the downgoing wave field 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 20 km from the soil site.

Ten Kinemetrics model SSA-2 Solid State Accelerographs were deployed in two dense arrays following the Landers-Big Bear earthquake sequence. The two arrays were separated by approximately three km, the first at a shallow alluvial soil site and the second at a rock site. We examine the soil and rock sites in terms of spectral ratio and cross-spectrum estimates of the site response. In order to construct an accurate representation of the motion in the horizontal plane, we treat the two horizontal components simultaneously as a complex signal. The Fourier transform of this complex signal represents the true motion in the horizontal plane as expressed in the frequency domain. The spectral ratio estimate is the ratio of this Fourier transform at the soil sites to the rock sites. The cross-spectrum estimate is the ratio of the cross-spectral density between the soil and rock sites to the power spectral density of the rock site. Spectral ratio estimates of site amplification are consistently higher than cross-spectrum estimates. On average the soil sites show amplification factors on the order of 2 to 4 relative to the rock sites between the frequencies of 4 to 15 Hz. There are, however, large variations in the ground motion recorded at sites with separations as small as 80 m. These variations demonstrate that site response studies can be biased by the choice of location of the sensor at distances of 80 m. We conclude that in the analysis of site-specific amplification both cross-spectrum and spectral ratio techniques should be used along with ensemble averages over many events.

Strong motion records of the 1989 Loma Prieta earthquake are inverted to determine a model of the rupture history. Uncorrected horizontal and vertical accelerograms are integrated to particle velocity time histories for 38 stations within an epicentral range of 75 km. The time histories are bandpassed filtered with corners at 0.05 and 1.0 Hz. These bandpassed time histories are inverted using a nonlinear method to solve for the distribution of slip amplitudes and rupture times at specified locations on the fault plane. The fault plane is specified a priori: 38 km long and 17 km wide, extending from 3 to 19 km depth at a constant dip of 70°. Starting models have rupture times based on constant rupture velocities of 2.5, 2.8, and 3.0 km/sec and uniform slip with rise times of 0.5, 1.0, 1.5, and 3.0 sec. The waveform inversion results show the strike-slip displacement is concentrated at the southern end of the rupture (rake = 156°) and the dip-slip displacement is concentrated at the northern end of the rupture (rake = 115°). The average total slip is partitioned almost equally between strike slip and dip slip (rake = 137°). The hypocentral area has an unusually small amount of slip with almost no slip in a region just to the north and up dip from the hypocenter. The rupture front is complex, propagating up dip to the south faster than it propagates to the north. The region of maximum strike slip to the southeast radiates simultaneously with the region of maximum dip slip to the northwest. The average rupture velocity is 3.0 km/sec, approximately 0.83 times the local shear wave speed. The calculated seismic moment is 3.5 ± 0.5 × 1026 dyne-cm.

The COSMOS VDC is a comprehensive, unrestricted, on-line, interactive strong ground-motion search engine for engineers, seismologists, and other earthquake professionals which implements a variety of search interfaces and allows users to preview data and configure design spectra overlays on response spectra. Users may download data files from data providers’ servers transparently through this web portal: http://db.cosmos-eq.org.

One part of the NEES portfolio is two permanently instrumented field site for monitoring soil-foundation-structure interaction (SFSI), ground motion, ground deformation, and pore-water pressure response to both active and passive seismic sources. The two sites, the Garner Valley Downhole Array and the Wildlife Liquefaction Array, are located in the southern California region. Both sites have many years of previous monitoring history and have been well characterized. These field sites will be used to monitor response generated by local and regional earthquakes and for active experiments using shakers. A reconfigurable steel-framed SFSI test structure is being constructed at the GVDA site and instrumented with sensors installed in the structure, foundation, and underlying soil. The focus of this wimple structure is the study of physics of SFSI without the complexities of "real" structures. This pair of NEES field sites will also provide an excellent bed for new in-situ site characterization techniques and new sensor technologies. Data from both sites will be contributed to the NEES Data repository and the advanced National Seismic System.

This paper presents the findings and recommendations of the User Scenario Work Group (USWG) in identifying a baseline of current practices within the geo-professional community and prioritizing desired functional requirements in the development of a comprehensive geotechnical information management system. This work was conducted as an initial phase of a larger project to demonstrate the effectiveness of a web based virtual data center for the dissemination of geotechnical data from multiple linked databases of various government and private sector organizations. An online survey was administered over the course of several months to practitioners across the nation. The results from the survey were compiled and examined to provide direction to the other project teams in the development if user-driven prototype data system.

The approach of the Campus Earthquake Program (CEP) is to combine the substantial expertise that exists within the UC system in geology, seismology, and geotechnical engineering, to estimate the earthquake strong motion exposure of UC facilities. These estimates draw upon recent advances in hazard assessment, seismic wave propagation modeling in rocks and soils, and dynamic soil testing. The UC campuses currently chosen for application of our integrated methodology are Riverside, San Diego, and Santa Barbara. The procedure starts with the identification of possible earthquake sources in the region and the determination of the most critical fault(s) related to earthquake exposure of the campus. Combined geological, geophysical, and geotechnical studies are then conducted to characterize each campus with specific focus on the location of particular target buildings of special interest to the campus administrators. We drill and geophysically log deep boreholes next to the target structure, to provide direct in-situ measurements of subsurface material properties, and to install uphole and downhole 3-component seismic sensors capable of recording both weak and strong motions. The boreholes provide access below the soil layers, to deeper materials that have relatively high seismic shear-wave velocities. Analyses of conjugate downhole and uphole records provide a basis for optimizing the representation of the low-strain response of the sites. Earthquake rupture scenarios of identified causative faults are combined with the earthquake records and with nonlinear soil models to provide site-specific estimates of strong motions at the selected target locations. The predicted ground motions are shared with the UC consultants, so that they can be used as input to the dynamic analysis of the buildings. Thus, for each campus targeted by the CEP project, the strong motion studies consist of two phases, Phase 1--initial source and site characterization, drilling, geophysical logging, installation of the seismic station, and initial seismic monitoring, and Phase 2--extended seismic monitoring, dynamic soil testing, calculation of estimated site-specific earthquake strong motions at depth and at the surface, and, where applicable, estimation of the response of selected buildings to the CEP-estimated strong motions.

The University of California Campus-Laboratory Collaboration (CLC) project is an integrated 3 year effort involving Lawrence Livermore National Laboratory (LLNL) and four UC campuses - Los Angeles (UCLA), Riverside (UCR), Santa Barbara (UCSB), and San Diego (UCSD) - plus additional collaborators at San Diego State University (SDSU), at Los Alamos National Laboratory and in industry. The primary purpose of the project is to estimate potential ground motions from large earthquakes and to predict site-specific ground motions for one critical structure on each campus. This project thus combines the disciplines of geology, seismology, geodesy, soil dynamics, and earthquake engineering into a fully integrated approach. Once completed, the CLC project will provide a template to evaluate other buildings at each of the four UC campuses, 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 professional in the application of this integrated, multidisciplinary, state-of-the-art approach to the assessment of earthquake hazard. For each campus targeted by the CLC project, the 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. This report cover Phase I for the UCSB campus and incudes results up through March 1997.

We estimate nonlinear site response by comparing site response estimates from the 16 August 2005 Mj=7.2 and 26 May 2003 Mj=7.0 Miyagi-Oki earthquakes with site response estimates from aftershocks of the 2003 event. Site response is solved by a spectral inversion technique to separate source, path, and site components. The constraint motion in the inversion is a regional attenuation model derived from fitting the spectra of data recorded at borehole KiK-net stations in the region and a theoretical source spectrum for each event determined using the same borehole stations. Site response is calculated at the surface of the KiK-net and K-NET stations. In general, the average aftershock site response is larger than for the two mainshocks, especially at a higher frequency. When comparing site response with input ground motion level, the predominant frequency and the site response values tend to decrease as the level of input ground motion increases.