Valerie Sahakian | University of Oregon (original) (raw)

Papers by Valerie Sahakian

To improve models of ground motion estimation and probabilistic seismic hazard analyses, the engi... more To improve models of ground motion estimation and probabilistic seismic hazard analyses, the engineering seismology field is moving toward developing fully nonergodic ground motion models, models specific for individual source-to-site paths. Previous work on this topic has examined systematic variations in ground-motion along particular paths (from either recorded or simulated earthquake data) and has not included physical properties of the path. We present here a framework to include physical path properties, by seeking correlations between ground motion amplitudes along specific paths and crustal properties, specifically seismic velocity and anelastic attenuation, along that path. Using a large data set of small-magnitude earthquakes recorded in Southern California, we find a correlation between the gradient of seismic S wave velocity and the path term residual, after accounting for an average geometric spreading and anelastic attenuation, indicating that heterogeneity in crustal velocity primarily controls the path-specific attenuation. Even in aseismic regions, details of path-specific ground motion prediction equations can be developed from crustal structure and property data.

Ground-motion prediction equations (GMPEs) are critical elements of probabilistic seismic hazard ... more Ground-motion prediction equations (GMPEs) are critical elements of probabilistic seismic hazard analysis (PSHA), as well as for other applications of ground motions. To isolate the path component for the purpose of building nonergodic GMPEs, we compute a regional GMPE using a large dataset of peak ground accelerations (PGAs) from small-magnitude earthquakes (0:5 ≤ M ≤ 4:5 with >10; 000 events, yielding ∼120; 000 recordings) that occurred in 2013 centered around the ANZA seismic network (hypocentral distances ≤180 km) in southern California. We examine two separate methods of obtaining residuals from the observed and predicted ground motions: a pooled ordinary least-squares model and a mixed-effects maximum-likelihood model. Whereas the former is often used by the broader seis-mological community, the latter is widely used by the ground-motion and engineering seismology community. We confirm that mixed-effects models are the preferred and most statistically robust method to obtain event, path, and site residuals and discuss the reasoning behind this. Our results show that these methods yield different consequences for the uncertainty of the residuals, particularly for the event residuals. Finally , our results show no correlation (correlation coefficient [CC] <0:03) between site residuals and the classic site-characterization term V S30 , the time-averaged shear-wave velocity in the top 30 m at a site. We propose that this is due to the relative homogeneity of the site response in the region and perhaps due to shortcomings in the formulation of V S30 and suggest applying the provided PGA site correction terms to future ground-motion studies for increased accuracy. Electronic Supplement: Peak ground acceleration (PGA) dataset.

The 2017 M 8.2 Tehuantepec and M 7.1 Puebla-Morelos earthquakes were deep inslab normal-faulting ... more The 2017 M 8.2 Tehuantepec and M 7.1 Puebla-Morelos earthquakes were deep inslab normal-faulting events that caused significant damage to several central to southern regions of Mexico. Inslab earthquakes are an important component of seismicity and seismic hazard in Mexico. Ground-motion prediction equations (GMPEs) are an integral part of seismic hazard assessment as well as risk and rapid-response products. This work examines the observed ground motions from these two events in comparison to the predicted median ground motions from four GMPEs. The residuals between the observed and modeled ground motions allow us to study regional differences in shaking, the effects of each earthquake, and basin effects in Mexico City, Puebla, and Oaxaca. We find that the ground motions from these two earthquakes are generally well modeled by the GMPEs. However, the Tehuantepec event shows larger than expected ground motions at greater distances and longer periods , which suggests a waveguide effect from the subduction zone geometry. Finally, Mexico City and the cities of Puebla and Oaxaca exhibit very large ground motions, indicative of well-known site and basin effects that are much stronger than the basin terms included in some of the GMPEs. Simple and rapid ground-motion parameter estimates that include site effects are key for hazard and real-time risk assessments in regions such as Mexico, where the vast majority of the population lives in areas where the aforementioned effects are relevant. However, GMPEs based on site correction terms dependent on topographic slope proxies underestimate, at least in the three cities tackled in this work, the observed amplification. Therefore, there is a need to improve models of seismic amplification in basins that could be included in GMPEs. Electronic Supplement: Tables of ground-motion intensity measures for each station and earthquake, as well as the residual uncertainties for each model, over all distances, and figures showing comprehensive ground-motion prediction equation (GMPE) and residuals results, for every period considered in this study, and the uncertainties .

The southern San Andreas fault (SSAF) accommodates a significant amount of strain between the Pac... more The southern San Andreas fault (SSAF) accommodates a significant amount of strain between the Pacific and North American plates; thus, the fault represents a major geohazard to the populated areas of southern California, in particular the larger Los Angeles metropolitan area. Paleoseismic chronology of ruptures along the SSAF segment suggests this fault is near the end of its interseismic period (∼180 years), because it has not ruptured in historic times (∼320 years). A recent active-source seismic experiment performed in the Salton Sea west of the SSAF provides evidence for extensional deformation along the northeastern shore of the Salton Sea. This study pos-its that the extensional deformation is due to a previously unmapped fault, here named the Salton trough fault (STF). The seismic reflection data image a divergent sediment package that dips toward the northeast with thicknesses up to at least 2 km. Refraction inversion produces a southwestward-dipping velocity discontinuity that crops out east of the SSAF surface trace, consistent with the existence of a southwest to northeast gradient in lithology. If present, the existence of the STF has scientific and societal relevance. First, the STF appears to control the recent Salton trough architecture north of Bombay Beach. Second, from a seismological hazards perspective, the presence of this structure could alter the current understanding of stress transfer and rupture dynamics in the region , as well as community fault models and ground-motion simulations on the SSAF.

Faulting in the Inner California Borderlands is complex. In the past, this region has undergone v... more Faulting in the Inner California Borderlands is complex. In the past, this region has undergone various deformational events such as extensional and rotational deformation and variable strike-slip deformation; this has imparted the geomorphology and fault structures observed offshore Southern California. Several hypotheses have been proposed to explain the current fault structures and the hazards they pose to populated coastal regions. The geometry and architecture of these structures can have significant implications for ground motions in the event of a rupture, and therefore impact working models of hazard assessment. Here, focusing on the Newport-Inglewood/Rose Canyon (NI/RC) and Palos Verdes (PV) fault zones, we use new and existing multibeam, CHIRP and Multi-Channel Seismic (MCS) data to describe the geometry of the fault system. We interpret reprocessed (prestack time migration) MCS data collected in 1979, 1986, and 2006 as well as newly acquired high-res MCS datasets collecte...

Pure and Applied Geophysics, 2013

We investigate interseismic deformation across the San Jacinto fault at Anza, California where pr... more We investigate interseismic deformation across the San Jacinto fault at Anza, California where previous geodetic observations have indicated an anomalously high shear strain rate. We present an updated set of secular velocities from GPS and InSAR observations that reveal a 2-3 km wide shear zone deforming at a rate that exceeds the background strain rate by more than a factor of two. GPS occupations of an alignment array installed in 1990 across the fault trace at Anza allow us to rule out shallow creep as a possible contributor to the observed strain rate. Using a dislocation model in a heterogeneous elastic half space, we show that a reduction in shear modulus within the fault zone by a factor of 1.2-1.6 as imaged tomographically by ALLAM and BEN-ZION (Geophys J Int 190:1181-1196 can explain about 50 % of the observed anomalous strain rate. However, the best-fitting locking depth in this case (10.4 ± 1.3 km) is significantly less than the local depth extent of seismicity (14-18 km). We show that a deep fault zone with a shear modulus reduction of at least a factor of 2.4 would be required to explain fully the geodetic strain rate, assuming the locking depth is 15 km. Two alternative possibilities include fault creep at a substantial fraction of the long-term slip rate within the region of deep microseismicity, or a reduced yield strength within the upper fault zone leading to distributed plastic failure during the interseismic period.

Presentations by Valerie Sahakian

Space geodetic investigation of interseismic deformation due to the San Jacinto fault near Anza, CA

ABSTRACT We performed campaign GPS measurements of near-field interseismic deformation in the Anz... more ABSTRACT We performed campaign GPS measurements of near-field interseismic deformation in the Anza section of the San Jacinto fault zone. Our survey occupied 27 benchmarks within 4 km of the fault trace, including a 400-m long alignment array established across the fault in 1990 by SDSU investigators. To constrain the fault slip rate and locking depth, we used SCEC CMM4 velocities in addition to our near field campaign GPS measurements. We inverted GPS velocities using a non-linear Monte-Carlo algorithm. The best-fitting slip rate and fault locking depths are 14 ± 2 mm/yr and 17 ± 3 km, respectively. We also investigated the occurrence of shallow creep on the fault by comparing GPS positions of the alignment array monuments to the total station measurements conducted in the early 1990's. This comparison yields a maximum rate of rotation of .002 degrees per year within the array (see Figure). We combined these observations with simple models of shallow creep to place an upper bound on the amount of surface creep. The data appear to rule out shallow creep at a rate greater than 0.5 mm/yr. This upper bound is well below predictions of models of interseismic deformation on rate-and-state faults assuming a two to three kilometer deep velocity-strengthening layer.

Neotectonics of the San Diego Trough and Coronado Bank fault systems, Inner California Borderlands

Geodetic data indicate that faults offshore of Southern California currently accommodate 6-8 mm/y... more Geodetic data indicate that faults offshore of Southern California currently accommodate 6-8 mm/yr of dextral Pacific-North American relative plate motion. In the Inner California Borderlands (ICB), modern strike-slip deformation overprints a prominent system of basins and ridges that formed during plate boundary reorganization 30-15 Ma. Despite its proximity to densely populated Southern California, the hazard posed by faults in the ICB remains poorly understood due to unknown fault geometry and loosely constrained slip rates. The San Diego Trough fault and Coronado Bank fault systems are northwest striking, sub-parallel, right-lateral faults within the ICB system. We use reprocessed legacy 2D multichannel seismic (MCS) reflection data, newly acquired high-resolution 2D MCS reflection data, and multibeam bathymetry to constrain the architecture and evolution of these faults. We interpret the MCS data using a sequence stratigraphic approach to establish a chronostratigraphy and dete...

The Inner California Borderlands (ICB) is situated off the coast of southern California and north... more The Inner California Borderlands (ICB) is situated off the coast of southern California and northern Baja. The structural and geomorphic characteristics of the area record a middle Oligocene transition from subduction to microplate capture along the California coast. Marine stratigraphic evidence shows large-scale extension and rotation overprinted by modern strike-slip deformation. Geodetic and geologic observations reported by others indicate that approximately 6-8 mm/yr of Pacific-North American relative plate motion is currently accommodated by offshore strike-slip faulting in the ICB. The farthest inshore fault system, the Newport-Inglewood/Rose Canyon (NI/RC) fault complex is a dextral strike-slip system that extends primarily offshore approximately 120 km from San Diego to the San Joaquin Hills near Newport Beach, California. The NI/RC fault system Holocene slip rate is 1.5-2.0 mm/yr to the south and 0.5 mm/yr along its northern extent based on well data from Freeman, et al (...

Reducing Uncertainty in Ground Motion Prediction Equations by Understanding Path Effects - poster presented at the SCEC 2016 Annual Meeting

The differences between recorded ground motions and median predictions made by Ground Motion Pre... more The differences between recorded ground motions and median predictions made by Ground Motion Prediction Equations (GMPEs) can be widely scattered, leading to large model standard deviations which can result in large ground motion amplitudes at low probabilities of exceedance. As such, reducing uncertainty in GMPEs is integral in decreasing the variability in predicted ground motions at low probabilities. Our approach for reducing uncertainty in the predictions is to understand the separate effects of source-, path-, and site-specific information. The knowable, repeatable parts of these effects can thus be identified and removed from population of residuals, which results location-specific GMPEs. These terms can then be associated with other geophysical information. This study focuses on correlating the path term to material properties along each recording’s raypath.
We employ a small database of approximately 3000 events recorded on the Anza seismic network, as shown in Baltay et al. (submitted), with magnitudes ranging from M 1 to M 5.4, with the majority of the events in the range 1 < M < 3. The Anza network has been in operation since ~1981, resulting in redundancy in source-to-station paths. We show various models for a regional GMPE, resulting from an inversion of all of the data. Next, we decompose the residuals for the preferred model into source, site, and path components, and focus on the path term. We examine two tomographic models of the Southern California region, the joint body wave and ambient noise model of Fang et al. (2016) and Allam et al. (2014). We raytrace through the Fang et al. (2016) model, and discuss various indices that can be computed from both models representing the effects of seismic attenuation, velocity structure on the raypath, and how they correlate to the computed path term. Finally, we describe a plan for moving forward with robust estimation of the effects of material properties on the path term, and how to incorporate this new path-specific knowledge into GMPE’s. We will discuss the implications on the resulting reduction in uncertainty and hence hazard level for low-probability earthquakes.

Papers by Valerie Sahakian

Images for CHIRP SEG-Y sub-bottom data from the All-American Canal near Calexico, CA, April 2019

Images in PNG format for each CHIRP line collected in the All-American Canal, April 2019, near Ca... more Images in PNG format for each CHIRP line collected in the All-American Canal, April 2019, near Calexico, CA. The purpose of these data was to image deformation features in shallow sediments in the Imperial Fault system and surrounding regions, to understand strain partitioning and identify locations for geologic slip rate studies. Data are plotted with a time-varying gain, and the right-hand axis presents depth converted using a nominal water velocity of 1500m/s. Funding was provided by the Southern California Earthquake Center grant #18119.

ASCII navigation for CHIRP SEG-Y sub-bottom data from the All-American Canal near Calexico, CA, April 2019

This data set presents ASCII files with navigation information for each shot of the CHIRP SEG-Y d... more This data set presents ASCII files with navigation information for each shot of the CHIRP SEG-Y data collected in the All American Canal. The purpose of these data was to image deformation features in shallow sediments in the Imperial Fault system and surrounding regions, to understand strain partitioning and identify locations for geologic slip rate studies. Navigation data were collected with a Septentrio Altus NR3, pole-mounted on the CHIRP floating frame. Elevations are corrected to the instrument, and are given as meters relative to sea level. These ASCII files contain six columns: Shot number, Longitude, Latitude, Elevation in meters, UTM X, UTM Y. UTM zone is 11N. Funding was provided by the Southern California Earthquake Center grant #18119.

Geology, 2022

In the tectonically complex Imperial Valley, California (USA), the Imperial fault (IF) is often c... more In the tectonically complex Imperial Valley, California (USA), the Imperial fault (IF) is often considered to be the primary fault at the U.S.-Mexico border; however, its strain partitioning and interactions with other faults are not well understood. Despite inferred evidence of other major faults (e.g., seismicity), it is difficult to obtain a holistic view of this system due to anthropogenic surface modifications. To better define the structural configuration of the plate-boundary strain in this region, we collected high-resolution shallow seismic imaging data in the All American Canal, crossing the Imperial, Dixieland, and Michoacán faults. These data image shallow (<25 m) structures on and near the mapped trace of the Imperial fault, as well as the Michoacán fault and adjacent stepover. Integration of our data with nearby terrestrial cores provides age constraints on Imperial fault deformation. These data suggest that the Michoacán fault, unmapped in the United States, is act...

Full margin Cascadia earthquake models

Rupture models for full margin Cascadia earthquakes. Companion data set tot he paper "Deep C... more Rupture models for full margin Cascadia earthquakes. Companion data set tot he paper "Deep Coseismic Slip in the Cascadia Megathrust can be Consistent with Coastal Subsidence "

Waveforms from Mentawai 2010 Event Strong-Motion Stations

Miniseed files (.mseed), and .chan file for the waveforms of the 2010 M7.8 Mentawai tsunami earth... more Miniseed files (.mseed), and .chan file for the waveforms of the 2010 M7.8 Mentawai tsunami earthquake, used in the study of Sahakian et al. (2019). This includes one miniseed (.mseed) file per channel of a station recording the event that was used here, from beginning to end of the event. The units of the data inside each file are cm/sec<sup>2</sup>,and the time in seconds. Inside the .zip file is the .chan file, describing the station's information (network, station, location, channel, longitude/latitude/elevation, sampling rate, gain, units). The gain is 1 (the units in the miniseed file are the same as described in the .chan file). If you use these data, please cite the associated publication: V.J. Sahakian, D. Melgar, M. Muzli (2019). Weak Near-Field Behavior of a Tsunami Earthquake: Towards Real-Time Identification for Local Warning, <em>Geophysical Research Letters</em>, doi:10/1029/2019GL083989.

Journal of Geophysical Research: Solid Earth, 2019

The July 4, 2019 Mw6.4 and subsequent July 6, 2019 Mw7.1 Ridgecrest Sequence earthquakes ruptured... more The July 4, 2019 Mw6.4 and subsequent July 6, 2019 Mw7.1 Ridgecrest Sequence earthquakes ruptured orthogonal fault planes in the Little Lake Fault Zone, a low slip rate (1 mm/yr) dextral fault zone in the area linking the Eastern California Shear Zone and Walker Lane. This region accommodates nearly one fourth of plate boundary motion and has been proposed to be an incipient transform fault system that could eventually become the main tectonic boundary, replacing the San Andreas. We investigate the rupture process of these events using a novel simultaneous kinematic slip method with joint inversion of high-rate GNSS, strong motion, GNSS static offset, and InSAR data. We model the Coulomb stress change to evaluate how the first mainshock may have affected the second. Our findings suggest complex interactions between several fault structures, including dynamic and static triggering, and provide important context for regional seismic source characterization and hazard models.

Bulletin of the Seismological Society of America, 2018

One important element of understanding basin response to strong shaking is the analysis of spectr... more One important element of understanding basin response to strong shaking is the analysis of spectral ratios, which may provide information about the dominant frequency of ground motion at specific locations. Spectral ratios computed from accelerations recorded by strong-motion stations in Mexico City during the mainshock of the 19 September 2017 M 7.1 Puebla-Morelos earthquake reveal predominate periods consistent with those mapped in the 2004 Mexican seismic design code. Furthermore, the predominant periods thus computed validate those studies using mainshock and aftershock recordings of the handful strong-motion stations that recorded the 19 September 1985 M 8.1 Michoacán earthquake. Even though the number of stations in each of the zones (zones I, II, IIIa, b, c, and d) is not the same, they still allow confirmation of site frequencies (periods) attributable to the specific zones (particularly those in zones IIIa, b, c, and d). Spectral ratios are computed with two different methods: (1) horizontal to horizontal (H/H) ratio of smoothed amplitude spectrum of a horizontal channel in direction X of a station with respect to the smoothed amplitude spectrum of the horizontal channel in the same X direction of a reference stiff soil (or rock) station and (2) horizontal to vertical (H/V) ratio (or also known as the Nakamura method) of both horizontal (H) and vertical (V) channels of the same station. We show a comparison of the identified frequencies (periods) derived by both methods and find they are very similar and in good agreement with those indicated in the zoning maps of Mexico City in the 2004 seismic design code.

Pure and Applied Geophysics, 2013

Space geodetic investigation of interseismic deformation due to the San Jacinto fault near Anza, CA

Neotectonics of the San Diego Trough and Coronado Bank fault systems, Inner California Borderlands

Reducing Uncertainty in Ground Motion Prediction Equations by Understanding Path Effects - poster presented at the SCEC 2016 Annual Meeting

Images for CHIRP SEG-Y sub-bottom data from the All-American Canal near Calexico, CA, April 2019

ASCII navigation for CHIRP SEG-Y sub-bottom data from the All-American Canal near Calexico, CA, April 2019

Geology, 2022

Full margin Cascadia earthquake models

Waveforms from Mentawai 2010 Event Strong-Motion Stations

Journal of Geophysical Research: Solid Earth, 2019

Bulletin of the Seismological Society of America, 2018

Ground-motion prediction equations (GMPEs) are critical elements of probabilistic seismic hazard ... more Ground-motion prediction equations (GMPEs) are critical elements of probabilistic seismic hazard analysis (PSHA), as well as for other applications of ground motions. To isolate the path component for the purpose of building nonergodic GMPEs, we compute a regional GMPE using a large dataset of peak ground accelerations (PGAs) from small-magnitude earthquakes (0:5 ≤ M ≤ 4:5 with >10; 000 events, yielding ∼120; 000 recordings) that occurred in 2013 centered around the ANZA seismic network (hypocentral distances ≤180 km) in southern California. We examine two separate methods of obtaining residuals from the observed and predicted ground motions: a pooled ordinary least-squares model and a mixed-effects maximum-likelihood model. Whereas the former is often used by the broader seismological community, the latter is widely used by the ground-motion and engineering seismology community. We confirm that mixed-effects models are the preferred and most statistically robust method to obtain event, path, and site residuals and discuss the reasoning behind this. Our results show that these methods yield different consequences for the uncertainty of the residuals, particularly for the event residuals. Finally, our results show no correlation (correlation coefficient [CC] <0:03) between site residuals and the classic site-characterization term V S30 , the time-averaged shearwave velocity in the top 30 m at a site. We propose that this is due to the relative homogeneity of the site response in the region and perhaps due to shortcomings in the formulation of V S30 and suggest applying the provided PGA site correction terms to future ground-motion studies for increased accuracy.

Bulletin of the Seismological Society of America, 2018

The 2017 M 8.2 Tehuantepec and M 7.1 Puebla-Morelos earthquakes were deep inslab normal-faulting ... more The 2017 M 8.2 Tehuantepec and M 7.1 Puebla-Morelos earthquakes were deep inslab normal-faulting events that caused significant damage to several central-to-southern regions of Mexico. Inslab earthquakes are an important component of seismicity and seismic hazard in Mexico. Ground-motion prediction equations (GMPEs) are an integral part of seismic hazard assessment as well as risk and rapid-response products. This work examines the observed ground motions from these two events in comparison to the predicted median ground motions from four GMPEs. The residuals between the observed and modeled ground motions allow us to study regional differences in shaking, the effects of each earthquake, and basin effects in Mexico City, Puebla, and Oaxaca. We find that the ground motions from these two earthquakes are generally well modeled by the GMPEs. However, the Tehuantepec event shows larger than expected ground motions at greater distances and longer periods, which suggests a waveguide effect from the subduction zone geometry. Finally, Mexico City and the cities of Puebla and Oaxaca exhibit very large ground motions, indicative of well-known site and basin effects that are much stronger than the basin terms included in some of the GMPEs. Simple and rapid ground-motion parameter estimates that include site effects are key for hazard and real-time risk assessments in regions such as Mexico, where the vast majority of the population lives in areas where the aforementioned effects are relevant. However, GMPEs based on site correction terms dependent on topographic slope proxies underestimate, at least in the three cities tackled in this work, the observed amplification. Therefore, there is a need to improve models of seismic amplification in basins that could be included in GMPEs. Electronic Supplement: Tables of ground-motion intensity measures for each station and earthquake, as well as the residual uncertainties for each model, over all distances, and figures showing comprehensive ground-motion prediction equation (GMPE) and residuals results, for every period considered in this study, and the uncertainties.

Journal of Geophysical Research: Solid Earth, 2017

The Newport-Inglewood/Rose Canyon (NIRC) fault zone is an active strike-slip fault system within ... more The Newport-Inglewood/Rose Canyon (NIRC) fault zone is an active strike-slip fault system within the Pacific-North American plate boundary in Southern California, located in close proximity to populated regions of San Diego, Orange, and Los Angeles counties. Prior to this study, the NIRC fault zone's continuity and geometry were not well constrained. Nested marine seismic reflection data with different vertical resolutions are employed to characterize the offshore fault architecture. Four main fault strands are identified offshore, separated by three main stepovers along strike, all of which are 2 km or less in width. Empirical studies of historical ruptures worldwide show that earthquakes have ruptured through stepovers with this offset. Models of Coulomb stress change along the fault zone are presented to examine the potential extent of future earthquake ruptures on the fault zone, which appear to be dependent on the location of rupture initiation and fault geometry at the stepovers. These modeling results show that the southernmost stepover between the La Jolla and Torrey Pines fault strands may act as an inhibitor to throughgoing rupture due to the stepover width and change in fault geometry across the stepover; however, these results still suggest that rupture along the entire fault zone is possible. 2. Background: Prior Fault and Earthquake Studies Onshore the NIRC fault zone strikes northward from San Diego Bay to La Jolla, where it uplifts Mount Soledad as the result of a left jog, before stepping offshore, south of La Jolla Canyon (Figure 1). The fault extends to the SAHAKIAN ET AL.

Bulletin of the Seismological Society of America, 2016

The southern San Andreas fault (SSAF) accommodates a significant

Active faulting in the Inner California Borderlands: new constraints from high-resolution multichannel seismic and multibeam bathymetric data

κ 0 and Broadband Site Spectra in Southern California from Source Model‐Constrained Inversion

Bulletin of the Seismological Society of America, 2019

Ground‐motion modeling requires accurate representation of the earthquake source, path, and site.... more Ground‐motion modeling requires accurate representation of the earthquake source, path, and site. Site amplification is often modeled by VS30, the time‐averaged shear‐wave velocity of the top 30 m of the Earth’s surface, though recent studies find that its ability to accurately predict site effects varies. Another measure of the site is κ0, the attenuation of high‐frequency energy near the site (Anderson and Hough, 1984). We develop a novel application of the Andrews (1986) method to simultaneously invert the spectra of 3357 earthquakes in Southern California into source and site components. These earthquakes have magnitudes 2.5–5.72 and were recorded on 16 stations for a total of 52,297 records. We constrain the inversion with an individual earthquake, demonstrating the most Brune‐like shape to preserve the site spectra. We then solve for κ0 site amplification at each station in three frequency bands: 1–6 Hz, 6–14 Hz, and 14–35 Hz. The resulting values of κ0 range from 0.017 s at A...

κ 0 and Broadband Site Spectra in Southern California from Source Model‐Constrained Inversion

Bulletin of the Seismological Society of America, 2019

Supplemental Material: Shallow distributed faulting in the Imperial Valley, California, USA

Description of methods (seismic data processing, 14C dating, interpretations, and Coulomb stress ... more Description of methods (seismic data processing, 14C dating, interpretations, and Coulomb stress models), supplemental figures, and interpreted fault locations.

The occurrence and hazards of great subduction zone earthquakes

Nature Reviews Earth & Environment

Deep Coseismic Slip in the Cascadia Megathrust can be Consistent with Coastal Subsidence

Geophysical Research Letters

At subduction zones, the down-dip limit of slip represents how deep an earthquake can rupture. Fo... more At subduction zones, the down-dip limit of slip represents how deep an earthquake can rupture. For hazards it is important - it controls the intensity of shaking and the pattern of coseismic uplift and subsidence. In the Cascadia Subduction Zone, because no large magnitude events have been observed in instrumental times, the limit is inferred from geological estimates of coastal subsidence during previous earthquakes; it is typically assumed to coincide approximately with the coastline. This is at odds with geodetic coupling models, it leaves residual slip deficits unaccommodated on a large swath of the megathrust. Here we will show that ruptures can penetrate deeper into the megathrust and still produce coastal subsidence provided slip decreases with depth. We will discuss the impacts of this on expected shaking intensities

The Igdeas Initiative: A Department-Wide Collaboration to Promote Gender Equity in the Earth Sciences

The U.S. Pacific Northwest (PNW) is one of the largest Internet infrastructure hubs for several c... more The U.S. Pacific Northwest (PNW) is one of the largest Internet infrastructure hubs for several cloud and content providers, research networks, colocation facilities, and submarine cable deployments. Yet, this region is within the Cascadia Subduction Zone and currently lacks a quantitative understanding of the resilience of the Internet infrastructure due to seismic forces. The main goal of this work is to assess the resilience of critical Internet infrastructure in the PNW to shaking from earthquakes. To this end, we have developed a framework called ShakeNet to understand the levels of risk that earthquake-induced shaking poses to wired and wireless infrastructures in the PNW. We take a probabilistic approach to categorize the infrastructures into risk groups based on historical and predictive peak ground acceleration (PGA) data and estimate the extent of shaking-induced damages to Internet infrastructures. Our assessments show the following in the next 50 years: ∼65% of the fiber...