Bulletin of the Seismological Society of America, 2020
We explore the detectability of M2 tidal tilt in the western part of the United States, using sei... more We explore the detectability of M2 tidal tilt in the western part of the United States, using seismic velocity data from 40 stations in the EarthScope Transportable Array (TA) network. We augment these data with data from two additional stations both collocated at the Piñon Flats Observatory (PFO) in southern California (networks TA and Incorporated Research Institutions for Seismology [IRIS] International Deployment of Accelerometers [IDA]). We find a good agreement between the acceleration-tilt derived from the TA seismic data with the theoretical model (body Earth and ocean loading for M2). These results are also consistent with prior studies using borehole tiltmeters operated at PFO (Wyatt and Berger, 1980; Wyatt et al., 1982). We find statistically significant M2 tilt anomaly responses that correlate with large lateral variations in rock properties in Yellowstone National Park, which stem from volcanic sources in the region. We also examined deviations in the M2 tidal tilt mode...
Bulletin of the Seismological Society of America, 2016
Technological advances in combination with the onslaught of data availability allow for large sei... more Technological advances in combination with the onslaught of data availability allow for large seismic data streams to automatically and systematically be recorded, processed, and stored. Here, we develop an automated approach to identify small, local earthquakes within these large continuous seismic data records. Our aim is to automate the process of detecting small seismic events triggered by a distant large earthquake, recorded at a single station. Specifically, we apply time-domain shortterm average (STA) to long-term average (LTA) ratio algorithms to three-component data to create a catalog of detections. We remove some of the false detections by requiring the detection be recorded on a minimum of two channels. To calibrate the algorithm, we compare our automatic detection catalog to a set of analyst-derived P-wave arrival times for a subset of small earthquakes occurring in the December 2008 Yellowstone swarm. Of the four STA/LTA algorithms we test (1 s/10 s; 4 s/40 s; 8 s/80 s; 16 s/160 s), the 1 s/10 s and 4 s/40 s detectors proved most effective at identifying the majority of events in the swarm. We apply these detectors to 45 hrs and 5 hrs of USArray data from the 2011 Japan M 9.0 and the 2010 Chile M 8.8 earthquakes, respectively. Using time-of-day versus number of detection relationships, we identify 38 of the 728 available stations that exhibit strong anthropogenic noise following the 2011 Japan earthquake. Our detection algorithm identified three regional earthquakes concurrent with the passage of the Sand surface waves of the Chile mainshock at USArray station R11A that locate in the Coso region of California, as well as events in Texas following the Japan earthquake.
Journal of Geophysical Research: Solid Earth, 2014
To better understand earthquake source processes involved in dynamically triggering remote afters... more To better understand earthquake source processes involved in dynamically triggering remote aftershocks, we use data from the EarthScope Transportable Array (TA) that provide uniform station sampling, similar recording capabilities, large spatial coverage, and, in many cases, repeat sampling at each site. To avoid spurious detections, which are an inevitable part of automated time domain amplitude threshold detection methods, we develop a frequency domain earthquake detection algorithm that identifies coherent signal patterns through array visualization. This method is tractable for large data sets, ensures robust catalogs, and delivers higher resolution observations than what are available in current catalogs. We explore seismicity rate changes local to the TA stations following 18 global main shocks (M ≥ 7) that generate median peak dynamic stress amplitudes of 0.001-0.028 MPa across the array. From these main shocks, we find no evidence of prolific or widespread remote dynamic triggering in the continental U.S. within the main shock's wave train or following main shock stress transients within 2 days. However, limited evidence for rate increases exist in localized source regions. These results suggest that for these data, prolific, remote earthquake triggering is a rare phenomenon throughout a wide range of observable magnitudes. We further conclude that within the lower range of previously reported triggering thresholds, surface wave amplitude does not correlate well with observed cases of dynamic triggering. We propose that other characteristics of the triggering wavefield, in addition to specific conditions at the site, will drive the occurrence of triggering at these amplitudes. LINVILLE ET AL.
CHI '05 Extended Abstracts on Human Factors in Computing Systems, 2005
Authentic practice in science requires access to phenomena. In this paper, we introduce RoomQuake... more Authentic practice in science requires access to phenomena. In this paper, we introduce RoomQuake, an application designed to foster the growth of a community of learning around scientific practice in seismology. Rather than treating seismic activity as remote events, RoomQuake seeks to enhance salience by situating those phenomena directly in the classroom. Using fixed-position PDAs as simulated seismographs, students determine the magnitude and distance of a series of "randomly" timed events by reading characteristic waveforms and using calibrated tape measures to sweep out arcs from multiple stations until they literally collide, physically enacting mathematical trilateration. We describe our experience in a six-week unit in a fifth-grade classroom.
2009 WRI World Congress on Computer Science and Information Engineering, 2009
Previously, the methods used by scientists to locate an earthquake epicenter were so cumbersome, ... more Previously, the methods used by scientists to locate an earthquake epicenter were so cumbersome, even for scientists, that the epicenter location methodology needed to be simplified before it was presented to K-12 students. Today, because of the advances in computer technology, there is a smaller divide between what is performed in research laboratories and what can be introduced in classrooms. The Flash-based Earthquake Location Tool (FELT) provides an authentic earthquake locating experience for students. FELT retrieves seismic data from recent earthquakes and displays seismograms that students can interact with in order to determine an earthquake's epicenter. XML files provide reference points for the selected earthquakes and their seismograms. FELT allows its client to perform computations and interact with Google Maps API as it loads data, images and audios asynchronously, and redraws sections of the screen, all independent of the server with which it is connected.
The causes of the catastrophic eruption of the Lusi mud volcano in Indonesia are hotly debated. D... more The causes of the catastrophic eruption of the Lusi mud volcano in Indonesia are hotly debated. Data from a nearby exploration well and a new look at the stress regime suggest that drilling operations, and not an earthquake set the eruption off.
Journal of Geophysical Research: Solid Earth, 2002
We use seismic waveform cross correlation to determine the relative positions of 2747 microearthq... more We use seismic waveform cross correlation to determine the relative positions of 2747 microearthquakes near Mount Lewis, California, that have waveforms recorded from 1984 to 1999. These earthquakes include the aftershock sequence of the 1986 M L 5.7 Mount Lewis earthquake. Approximately 90% of these aftershocks are located beyond the tips of the approximately north striking main shock, defining an hourglass with the long axis aligned approximately with the main shock. Surprisingly, our relocation demonstrates that many of these aftershocks illuminate a series of near-vertical east-west faults that are $0.5-1 km long and separated by as little as $200 m. We propose that these structures result from the growth of a relatively young fault in which displacement across a right-lateral approximately north striking fault zone is accommodated by slip on secondary left-lateral approximately east striking faults. We derive the main shock-induced static Coulomb failure function (Ás f) on the dominant fault orientation in our study area using a three-dimensional (3-D) boundary element program. To bound viable friction coefficients, we measure the correlation between the rank ordering of relative amplitudes of Ás f and seismicity rate change. We find that likely friction coefficients are 0.2-0.6 and that the assumed main shock geometry introduces the largest uncertainties in the favored friction values. We obtain similar results from a visual correlation of calculated Ás f contours with the distribution of aftershocks. Viable rate-and-state constitutive parameters bound the observed relationship between magnitude of Ás f and seismicity rate change, and for our favored main shock model a maximum correlation is achieved when Ás f is computed with friction coefficients of 0.3-0.6. These values are below those previously cited for young faults.
... Authors: Nayak, AM; Vernon, F.; Kent, G.; Orcutt, J.; Kilb, D.; Newman, R.; Smarr, L.; Defant... more ... Authors: Nayak, AM; Vernon, F.; Kent, G.; Orcutt, J.; Kilb, D.; Newman, R.; Smarr, L.; Defanti, T.; Leigh, J.; Renambot, L.; Johnson, A. Affiliation: AA(Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics,Scripps Institution of Oceanography, University of California ...
We have undertaken a systematic study of spectral attenuation parameter kappa at permanent statio... more We have undertaken a systematic study of spectral attenuation parameter kappa at permanent stations of the Southern Great Basin Digital Seismic Network (SGBDSN). Objectives included estimating kappa itself, reconciling disparate small and moderate earthquake estimates, and exploring magnitude and distance dependence of kappa. In seismic engineering, kappa can be the controlling parameter for site-specific seismic design amplitudes. We estimated kappa
Remote earthquake triggering is a well-established phenomenon. Triggering is commonly identified ... more Remote earthquake triggering is a well-established phenomenon. Triggering is commonly identified from statistically significant increases in earthquake rate coincident with the passage of seismic energy. In establishing rate changes, short duration earthquake catalogs are commonly used, and triggered sequences are not typically analyzed within the context of background seismic activity. Using 500 mainshocks and four western USA 33-yearlong earthquake catalogs, we compare the ability of three different statistical methods to identify remote earthquake triggering. Counter to many prior studies, we find remote dynamic triggering is rare (conservatively, <2% of the time). For the mainshocks associated with remote rate increases, the spatial and temporal signatures of triggering differ. We find that a rate increase coincident in time with mainshock energy alone is insufficient to conclude that dynamic triggering occurred. To classify dynamically triggered sequences, we suggest moving ...
We study the scaling relationships of source parameters and the self-similarity of earthquake spe... more We study the scaling relationships of source parameters and the self-similarity of earthquake spectra by analyzing a cluster of over 400 small earthquakes (M L = 0.5 to 3.4) recorded by the Anza seismic network in southern California. We compute P, S, and preevent noise spectra from each seismogram using a multitaper technique and approximate source and receiver terms by iteratively stacking the spectra. To estimate scaling relationships, we average the spectra in size bins based on their relative moment. We correct for attenuation by using the smallest moment bin as an empirical Green's function (EGF) for the stacked spectra in the larger moment bins. The shapes of the log spectra agree within their estimated uncertainties after shifting along the w À3 line expected for self-similarity of the source spectra. We also estimate corner frequencies and radiated energy from the relative source spectra using a simple source model. The ratio between radiated seismic energy and seismic moment (proportional to apparent stress) is nearly constant with increasing moment over the magnitude range of our EGF-corrected data (M L = 1.8 to 3.4). Corner frequencies vary inversely as the cube root of moment, as expected from the observed self-similarity in the spectra. The ratio between P and S corner frequencies is observed to be 1.6 ± 0.2. We obtain values for absolute moment and energy by calibrating our results to local magnitudes for these earthquakes. This yields a S to P energy ratio of 9 ± 1.5 and a value of apparent stress of about 1 MPa.
Journal of Geophysical Research: Solid Earth, 2019
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.
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.
ABSTRACTSeasonal lake Gornersee forms at the confluence of Gornergletscher and Grenzgletscher, Sw... more ABSTRACTSeasonal lake Gornersee forms at the confluence of Gornergletscher and Grenzgletscher, Switzerland, and experiences outburst floods annually in midsummer. To study the interplay between lake drainage, glacier movement and crevasse activity, high-frequency seismometers and GPS receivers were deployed in networks near Gornersee during the summer ablation seasons of 2004, 2006 and 2007. We use a Rayleigh wave coherence method to locate 3289, 7939 and 4087 icequakes, respectively, primarily along well-defined surface crevasses. We calculate two-dimensional strains from triads of GPS stations and find mean differential strain rates of ~300 × 10−6 d−1 with diurnal variations up to 800 × 10−6 d−1. Crevasse icequake activity and glacial velocity are highest during early season, then decrease as meltwater channels erode and subglacial water pressure decreases. Glacial response to Gornersee drainage varied year-to-year, with icequake activity promoted at some crevasses and inhibited a...
Bulletin of the Seismological Society of America, 2020
We explore the detectability of M2 tidal tilt in the western part of the United States, using sei... more We explore the detectability of M2 tidal tilt in the western part of the United States, using seismic velocity data from 40 stations in the EarthScope Transportable Array (TA) network. We augment these data with data from two additional stations both collocated at the Piñon Flats Observatory (PFO) in southern California (networks TA and Incorporated Research Institutions for Seismology [IRIS] International Deployment of Accelerometers [IDA]). We find a good agreement between the acceleration-tilt derived from the TA seismic data with the theoretical model (body Earth and ocean loading for M2). These results are also consistent with prior studies using borehole tiltmeters operated at PFO (Wyatt and Berger, 1980; Wyatt et al., 1982). We find statistically significant M2 tilt anomaly responses that correlate with large lateral variations in rock properties in Yellowstone National Park, which stem from volcanic sources in the region. We also examined deviations in the M2 tidal tilt mode...
Bulletin of the Seismological Society of America, 2016
Technological advances in combination with the onslaught of data availability allow for large sei... more Technological advances in combination with the onslaught of data availability allow for large seismic data streams to automatically and systematically be recorded, processed, and stored. Here, we develop an automated approach to identify small, local earthquakes within these large continuous seismic data records. Our aim is to automate the process of detecting small seismic events triggered by a distant large earthquake, recorded at a single station. Specifically, we apply time-domain shortterm average (STA) to long-term average (LTA) ratio algorithms to three-component data to create a catalog of detections. We remove some of the false detections by requiring the detection be recorded on a minimum of two channels. To calibrate the algorithm, we compare our automatic detection catalog to a set of analyst-derived P-wave arrival times for a subset of small earthquakes occurring in the December 2008 Yellowstone swarm. Of the four STA/LTA algorithms we test (1 s/10 s; 4 s/40 s; 8 s/80 s; 16 s/160 s), the 1 s/10 s and 4 s/40 s detectors proved most effective at identifying the majority of events in the swarm. We apply these detectors to 45 hrs and 5 hrs of USArray data from the 2011 Japan M 9.0 and the 2010 Chile M 8.8 earthquakes, respectively. Using time-of-day versus number of detection relationships, we identify 38 of the 728 available stations that exhibit strong anthropogenic noise following the 2011 Japan earthquake. Our detection algorithm identified three regional earthquakes concurrent with the passage of the Sand surface waves of the Chile mainshock at USArray station R11A that locate in the Coso region of California, as well as events in Texas following the Japan earthquake.
Journal of Geophysical Research: Solid Earth, 2014
To better understand earthquake source processes involved in dynamically triggering remote afters... more To better understand earthquake source processes involved in dynamically triggering remote aftershocks, we use data from the EarthScope Transportable Array (TA) that provide uniform station sampling, similar recording capabilities, large spatial coverage, and, in many cases, repeat sampling at each site. To avoid spurious detections, which are an inevitable part of automated time domain amplitude threshold detection methods, we develop a frequency domain earthquake detection algorithm that identifies coherent signal patterns through array visualization. This method is tractable for large data sets, ensures robust catalogs, and delivers higher resolution observations than what are available in current catalogs. We explore seismicity rate changes local to the TA stations following 18 global main shocks (M ≥ 7) that generate median peak dynamic stress amplitudes of 0.001-0.028 MPa across the array. From these main shocks, we find no evidence of prolific or widespread remote dynamic triggering in the continental U.S. within the main shock's wave train or following main shock stress transients within 2 days. However, limited evidence for rate increases exist in localized source regions. These results suggest that for these data, prolific, remote earthquake triggering is a rare phenomenon throughout a wide range of observable magnitudes. We further conclude that within the lower range of previously reported triggering thresholds, surface wave amplitude does not correlate well with observed cases of dynamic triggering. We propose that other characteristics of the triggering wavefield, in addition to specific conditions at the site, will drive the occurrence of triggering at these amplitudes. LINVILLE ET AL.
CHI '05 Extended Abstracts on Human Factors in Computing Systems, 2005
Authentic practice in science requires access to phenomena. In this paper, we introduce RoomQuake... more Authentic practice in science requires access to phenomena. In this paper, we introduce RoomQuake, an application designed to foster the growth of a community of learning around scientific practice in seismology. Rather than treating seismic activity as remote events, RoomQuake seeks to enhance salience by situating those phenomena directly in the classroom. Using fixed-position PDAs as simulated seismographs, students determine the magnitude and distance of a series of "randomly" timed events by reading characteristic waveforms and using calibrated tape measures to sweep out arcs from multiple stations until they literally collide, physically enacting mathematical trilateration. We describe our experience in a six-week unit in a fifth-grade classroom.
2009 WRI World Congress on Computer Science and Information Engineering, 2009
Previously, the methods used by scientists to locate an earthquake epicenter were so cumbersome, ... more Previously, the methods used by scientists to locate an earthquake epicenter were so cumbersome, even for scientists, that the epicenter location methodology needed to be simplified before it was presented to K-12 students. Today, because of the advances in computer technology, there is a smaller divide between what is performed in research laboratories and what can be introduced in classrooms. The Flash-based Earthquake Location Tool (FELT) provides an authentic earthquake locating experience for students. FELT retrieves seismic data from recent earthquakes and displays seismograms that students can interact with in order to determine an earthquake's epicenter. XML files provide reference points for the selected earthquakes and their seismograms. FELT allows its client to perform computations and interact with Google Maps API as it loads data, images and audios asynchronously, and redraws sections of the screen, all independent of the server with which it is connected.
The causes of the catastrophic eruption of the Lusi mud volcano in Indonesia are hotly debated. D... more The causes of the catastrophic eruption of the Lusi mud volcano in Indonesia are hotly debated. Data from a nearby exploration well and a new look at the stress regime suggest that drilling operations, and not an earthquake set the eruption off.
Journal of Geophysical Research: Solid Earth, 2002
We use seismic waveform cross correlation to determine the relative positions of 2747 microearthq... more We use seismic waveform cross correlation to determine the relative positions of 2747 microearthquakes near Mount Lewis, California, that have waveforms recorded from 1984 to 1999. These earthquakes include the aftershock sequence of the 1986 M L 5.7 Mount Lewis earthquake. Approximately 90% of these aftershocks are located beyond the tips of the approximately north striking main shock, defining an hourglass with the long axis aligned approximately with the main shock. Surprisingly, our relocation demonstrates that many of these aftershocks illuminate a series of near-vertical east-west faults that are $0.5-1 km long and separated by as little as $200 m. We propose that these structures result from the growth of a relatively young fault in which displacement across a right-lateral approximately north striking fault zone is accommodated by slip on secondary left-lateral approximately east striking faults. We derive the main shock-induced static Coulomb failure function (Ás f) on the dominant fault orientation in our study area using a three-dimensional (3-D) boundary element program. To bound viable friction coefficients, we measure the correlation between the rank ordering of relative amplitudes of Ás f and seismicity rate change. We find that likely friction coefficients are 0.2-0.6 and that the assumed main shock geometry introduces the largest uncertainties in the favored friction values. We obtain similar results from a visual correlation of calculated Ás f contours with the distribution of aftershocks. Viable rate-and-state constitutive parameters bound the observed relationship between magnitude of Ás f and seismicity rate change, and for our favored main shock model a maximum correlation is achieved when Ás f is computed with friction coefficients of 0.3-0.6. These values are below those previously cited for young faults.
... Authors: Nayak, AM; Vernon, F.; Kent, G.; Orcutt, J.; Kilb, D.; Newman, R.; Smarr, L.; Defant... more ... Authors: Nayak, AM; Vernon, F.; Kent, G.; Orcutt, J.; Kilb, D.; Newman, R.; Smarr, L.; Defanti, T.; Leigh, J.; Renambot, L.; Johnson, A. Affiliation: AA(Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics,Scripps Institution of Oceanography, University of California ...
We have undertaken a systematic study of spectral attenuation parameter kappa at permanent statio... more We have undertaken a systematic study of spectral attenuation parameter kappa at permanent stations of the Southern Great Basin Digital Seismic Network (SGBDSN). Objectives included estimating kappa itself, reconciling disparate small and moderate earthquake estimates, and exploring magnitude and distance dependence of kappa. In seismic engineering, kappa can be the controlling parameter for site-specific seismic design amplitudes. We estimated kappa
Remote earthquake triggering is a well-established phenomenon. Triggering is commonly identified ... more Remote earthquake triggering is a well-established phenomenon. Triggering is commonly identified from statistically significant increases in earthquake rate coincident with the passage of seismic energy. In establishing rate changes, short duration earthquake catalogs are commonly used, and triggered sequences are not typically analyzed within the context of background seismic activity. Using 500 mainshocks and four western USA 33-yearlong earthquake catalogs, we compare the ability of three different statistical methods to identify remote earthquake triggering. Counter to many prior studies, we find remote dynamic triggering is rare (conservatively, <2% of the time). For the mainshocks associated with remote rate increases, the spatial and temporal signatures of triggering differ. We find that a rate increase coincident in time with mainshock energy alone is insufficient to conclude that dynamic triggering occurred. To classify dynamically triggered sequences, we suggest moving ...
We study the scaling relationships of source parameters and the self-similarity of earthquake spe... more We study the scaling relationships of source parameters and the self-similarity of earthquake spectra by analyzing a cluster of over 400 small earthquakes (M L = 0.5 to 3.4) recorded by the Anza seismic network in southern California. We compute P, S, and preevent noise spectra from each seismogram using a multitaper technique and approximate source and receiver terms by iteratively stacking the spectra. To estimate scaling relationships, we average the spectra in size bins based on their relative moment. We correct for attenuation by using the smallest moment bin as an empirical Green's function (EGF) for the stacked spectra in the larger moment bins. The shapes of the log spectra agree within their estimated uncertainties after shifting along the w À3 line expected for self-similarity of the source spectra. We also estimate corner frequencies and radiated energy from the relative source spectra using a simple source model. The ratio between radiated seismic energy and seismic moment (proportional to apparent stress) is nearly constant with increasing moment over the magnitude range of our EGF-corrected data (M L = 1.8 to 3.4). Corner frequencies vary inversely as the cube root of moment, as expected from the observed self-similarity in the spectra. The ratio between P and S corner frequencies is observed to be 1.6 ± 0.2. We obtain values for absolute moment and energy by calibrating our results to local magnitudes for these earthquakes. This yields a S to P energy ratio of 9 ± 1.5 and a value of apparent stress of about 1 MPa.
Journal of Geophysical Research: Solid Earth, 2019
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.
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.
ABSTRACTSeasonal lake Gornersee forms at the confluence of Gornergletscher and Grenzgletscher, Sw... more ABSTRACTSeasonal lake Gornersee forms at the confluence of Gornergletscher and Grenzgletscher, Switzerland, and experiences outburst floods annually in midsummer. To study the interplay between lake drainage, glacier movement and crevasse activity, high-frequency seismometers and GPS receivers were deployed in networks near Gornersee during the summer ablation seasons of 2004, 2006 and 2007. We use a Rayleigh wave coherence method to locate 3289, 7939 and 4087 icequakes, respectively, primarily along well-defined surface crevasses. We calculate two-dimensional strains from triads of GPS stations and find mean differential strain rates of ~300 × 10−6 d−1 with diurnal variations up to 800 × 10−6 d−1. Crevasse icequake activity and glacial velocity are highest during early season, then decrease as meltwater channels erode and subglacial water pressure decreases. Glacial response to Gornersee drainage varied year-to-year, with icequake activity promoted at some crevasses and inhibited a...
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