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Sahakian, V, Baltay A, Hanks T, Buehler J, Vernon F, Kilb D, Abrahamson N.  2018.  Decomposing leftovers: Event, path, and site residuals for a small‐magnitude Anza region GMPE. Bulletin of the Seismological Society of America.   10.1785/0120170376   Abstract

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 ≤180km ) 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 VS30 , 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 VS30 and suggest applying the provided PGA site correction terms to future ground‐motion studies for increased accuracy.

Sahakian, VJ, Baltay A, Hanks TC, Buehler J, Vernon FL, Kilb D, Abrahamson NA.  2019.  Ground motion residuals, path effects, and crustal properties: A pilot study in Southern California. Journal of Geophysical Research: Solid Earth. 124:5738-5753.   10.1029/2018jb016796   AbstractWebsite

Abstract 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.

Sales, BC, Maple MB, Vernon FL.  1978.  Initial Oxidation-Kinetics near Curie-Temperature of Nickel. Physical Review B. 18:486-491.   10.1103/PhysRevB.18.486   Website
Sandvol, E, Seber D, Barazangi M, Vernon F, Mellors R, Al-Amri A.  1998.  Lithospheric seismic velocity discontinuities beneath the Arabian Shield. Geophysical Research Letters. 25:2873-2876.   10.1029/98gl02214   AbstractWebsite

We determined crustal and lithospheric mantle velocity structure beneath the Arabian Shield through the modeling of receiver function stacks obtained from teleseismic P waves recorded by the 9 station temporary broadband array in western Saudi Arabia. The receiver function deconvolution technique was used to isolate the receiver-side PS mode conversions. A grid search method, which should yield an unbiased global minimum, was used to solve for a shear wave velocity model that is optimal and has the minimum number of layers needed to fit the receiver function waveform. Results from this analysis show that the crustal thickness in the shield area varies from 35 to 40 km in the west, adjacent to the Red Sea, to 45 km in central Arabia. Stability tests of each solution indicate that the models are relatively well constrained. We have also observed evidence for a large positive velocity contrast at sub-Moho depths at four stations at depths of 80 to 100 km. This discontinuity may represent a change in rheology in the lower part of the lithosphere or remnant structure from the formation of the Arabian Shield.

Schofield, O, Kohut J, Glenn S, Morell J, Capella J, Corredor J, Orcutt J, Arrott M, Krueger I, Meisinger M, Peach C, Vernon F, Chave A, Chao Y, Chien S, Thompson D, Brown W, Oliver M, Boicourt W.  2010.  A Regional Slocum Glider Network in the Mid-Atlantic Bight Leverages Broad Community Engagement. Marine Technology Society Journal. 44:185-195. AbstractWebsite

Autonomous underwater gliders have proven to be a cost-effective technology for measuring the 3-D ocean and now represent a critical component during the design and implementation of the Mid-Atlantic Regional Ocean Observing System (MARCOOS), a Region of the U.S. Integrated Ocean Observing System. The gliders have been conducting regional surveys of the Mid-Atlantic (MA) Bight, and during the 3 years of MARCOOS, the glider fleet has conducted 22 missions spanning 10,867 km and collecting 62,824 vertical profiles of data. In addition to collecting regional data, the gliders have facilitated collaboration for partners outside of MARCOOS. The existence of the MA glider observatory provided a unique test bed for cyber-infrastructure tools being developed as part of the National Science Foundation's Ocean Observatory Initiative. This effort allowed the Ocean Observatory Initiative software to integrate the MARCOOS assets and provided a successful demonstration of an ocean sensor net. The hands-on experience of the MA glider technicians supported training and provided assistance of collaborators within the Caribbean Regional Association, also a region of the U.S. Integrated Ocean Observing System, to assess the efficacy of gliders to resolve internal waves. Finally, the glider fleet has enabled sensor development and testing in a cost-effective manner. Generally, new sensors were tested within the MARCOOS domain before they were deployed in more extreme locations throughout the world's oceans. On the basis of this experience, the goal of the MARCOOS glider team will be to expand the MA network in coming years. The potential of how an expanded network of gliders might serve national needs was illustrated during the 2010 Macondo Gulf of Mexico oil spill, where gliders from many institutions collected subsurface mesoscale data to support regional models and oil response planning. The experience gained over the last 5 years suggests that it is time to develop a national glider network.

Schreiber, KU, Hautmann JN, Velikoseltsev A, Wassermann J, Igel H, Otero J, Vernon F, Wells JPR.  2009.  Ring Laser Measurements of Ground Rotations for Seismology. Bulletin of the Seismological Society of America. 99:1190-1198.   10.1785/0120080171   AbstractWebsite

Since the discovery of the wave nature of light, optical interferometry has assumed an important place in high precision metrology. This is mostly due to the inherent high sensor resolution for operational wavelengths in the vicinity of several hundred nanometers. In this context, interferometers in the Michelson configuration are most prominently used in gravitational wave antennas, such as the large projects VIRGO, LIGO, TAMA, and GEO600. In the Sagnac configuration they are used for high resolution rotation monitoring such as the precise observation of Earth rotation. Modern large-scale ring lasers reach a sensitivity for the measurement of rotation of 1 prad/sec (with approximately 1 hr of averaging). Because of the comparatively short wavelengths employed, optical interferometers are extremely sensitive to small mechanical perturbations of the entire apparatus. These can be caused by deformations, thermal or mechanical stress, and instabilities in the alignment of the optical components at the level of about lambda/100. Ring lasers suitable for geophysical applications require a sensor resolution in the range of 10(-8) rad/sec and below. This demands a scale factor of the instrument that is only achievable with mechanical dimensions of the interferometer on the order of about 1 m(2) or larger. At the same time the necessary mechanical rigidity of the entire instrument has to be on the order of 5 nm. Currently, this has only been achieved with monolithic ring lasers made from blocks of Zerodur and installed in a temperature stabilized underground environment. However if long-term sensor stability is not required, compromises can be made and, in particular for studies of regional seismic events, it becomes feasible to build a heterolithic rotation sensor in a simpler and much cheaper way. Here, we report the design and first results from the GEOsensor, which has been specifically constructed for studies in rotational seismology. The sensor is operated at the Pi on Flat Seismological Observatory in Southern California.

Schulte-Pelkum, V, Vernon FL, Eakins J.  2003.  Large teleseismic P wavefront deflections observed with broadband arrays. Bulletin of the Seismological Society of America. 93:747-756.   10.1785/0120020126   AbstractWebsite

We measure the plane wavefront incidence azimuth for teleseismic P at large-aperture (similar to50 km) broadband arrays. The incidence azimuth is determined by crosscorrelation of the P arrivals on the vertical component seismograms filtered in successive frequency bands. The periods considered range from 10 to 35 see. At the Anza array in southern California, the plane wave direction is deflected from the great circle azimuth of the event by up to 20degrees. In addition, we find a surprisingly strong frequency dependence of the same magnitude and a striking antisymmetric pattern of the deflection as a function of backazimuth, whereas the curvature of the wavefront is small. Similar characteristics are found at the Grafenberg array in Germany and the NORSAR array in Norway, however, with much weaker amplitudes of similar to5degrees. We ascribe the behavior at Anza to structure in the lower crust and uppermost mantle beneath the array, given that the observations are only a function of source backazimuth and not of source depth and source mechanism, that the wavelengths under consideration range from 50 to 270 km, and that the sign of the deviation is opposite to that predicted from shallow crustal structure and Moho topography. We are able to reproduce the magnitude and frequency dependence of the wavefront deflection using finite difference numerical modeling of plane wave propagation through simple 2D structures.

Schulte-Pelkum, V, Earle PS, Vernon FL.  2004.  Strong directivity of ocean-generated seismic noise. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000520   AbstractWebsite

We measure direction and amplitude of ocean-generated continuous seismic noise in the western United States. Slowness direction of the noise is determined using array beamforming, and particle motion direction from individual three-component stations. We find two surprising results. First, the noise is highly monodirectional at all sites, regardless of coastal distance. A single narrow generation area dominates for most of the time period, interrupted by a second well defined direction during ocean swell events. Second, we find that a storm off the Labrador coast with not unusual wave heights generates coherent noise across the entire continent. We show the causal relationship between swells arriving at different North American coastal areas and the triggered microseisms in time-lapse movies (Animations 1 and 2) of ocean swells and concurrent microseisms. Our results have a number of implications for different fields of research. A useful by-product of our finding that microseisms are a strongly directional noise source is the possibility of using automated processing of the continuous noise as a near real-time check on station polarity and calibration problems, which would be a simply implemented indicator for the state of health of a seismic network. Consistent monodirectional noise may have an influence on seismic azimuthal measurements such as shear wave splitting. Most importantly, our findings should be taken into account in proposed studies which will use seismic noise as a proxy for ocean wave height in investigations of interdecadal climate change.

Scott, JS, Masters TG, Vernon FL.  1994.  3-D Velocity Structure of the San-Jacinto Fault Zone near Anza, California .1. P-Waves. Geophysical Journal International. 119:611-626.   10.1111/j.1365-246X.1994.tb00145.x   AbstractWebsite

Seismic arrival times from microearthquakes (M(L) < 4) On the San Jacinto fault near Anza, California, are used to find spatial variations in the seismic velocity that are related to the crustal structure of the fault zone. Preliminary modelling of the 1-D P-wave velocity structure of the upper 25 km of crust reveals that most of the variation in velocity is lateral rather than depth dependent. The traveltime anomalies due to lateral structure can be partially compensated for by applying station corrections, however the variance of the traveltime residuals is still 2.25 times larger than the variance of the picking error. The spatially correlated residuals show that this variance is due to localized velocity anomalies and that the data require further modelling using a 3-D velocity structure. Because the 3-D inverse problem is non-unique, smoothness constraints are applied to find the model that has the minimum structure required to fit the data to the picking error, where a smooth model is defined such that the gradient of the velocity perturbation from the original 1-D model is small. With small non-zero station corrections, a 3-D velocity model can be found that fits the data well. The structure is well resolved from 3 to 9 km depth where lateral perturbations of up to 7 per cent are determined independently of the trade-off between station corrections and poorly resolved near surface structure. The model shows a horizontal gradient with overall faster velocities in the north-east side of the fault zone. At 3-6 km depth, the signature of the fault zone is evident in the lower velocities beneath the surface trace of the fault. However, at 9 km depth, higher seismic velocities are found extending into the fault zone from the north-east block. This higher velocity region occurs where there is a distinct lack of seismicity on the fault. There is also a localized feature in the south-west of the modelled region that is more than 10 km from the main trace of the fault with velocities 3 per cent slower than average.

Share, PE, Allam AA, Ben-Zion Y, Lin FC, Vernon FL.  2019.  Structural properties of the San Jacinto Fault Zone at Blackburn Saddle from seismic data of a dense linear array. Pure and Applied Geophysics. 176:1169-1191.   10.1007/s00024-018-1988-5   AbstractWebsite

We image the San Jacinto fault zone at Blackburn Saddle using earthquake waveforms recorded by a similar to 2-km across-fault linear array with 108 three-component sensors separated by similar to 10-30m. The length and spatiotemporal sampling of the array allow us to derive high-resolution information on the internal fault zone structure with spatial extent that can be merged with regional tomography models. Across-fault variations in polarization, amplitude, and arrival time of teleseismic P waves indicate abrupt changes in subsurface structure near the surface trace of the fault (sensor BS55) and similar to 270m to the northeast (sensor BS34). Analysis of fault zone head waves from local events reveals the existence of a deep bimaterial interface that extends from the array to at least 50km southeast and has a section with>10% velocity contrast. This analysis also corroborates the teleseismic results and indicates a broad damage zone primarily northeast of the fault bounded by a shallow bimaterial interface near BS34 that merges with the deep interface. Detection and waveform inversions of Love-type fault zone trapped waves generated by local events indicate a trapping structure within the broader damage zone with width of similar to 150m, velocity reduction of similar to 55% from the surrounding rock and depth extent of similar to 2km. The performed analyses provide consistent results on the subsurface location of the main seismogenic fault and properties of a major bimaterial interface and damage structure. The imaged fault zone properties are consistent with preferred propagation direction of earthquake ruptures in the area to the northwest.

Share, PE, Ben-Zion Y, Ross ZE, Qiu HR, Vernon FL.  2017.  Internal structure of the San Jacinto fault zone at Blackburn Saddle from seismic data of a linear array. Geophysical Journal International. 210:819-832.   10.1093/gji/ggx191   AbstractWebsite

Local and teleseismic earthquake waveforms recorded by a 180-m-long linear array (BB) with seven seismometers crossing the Clark fault of the San Jacinto fault zone northwest of Anza are used to image a deep bimaterial interface and core damage structure of the fault. Delay times of P waves across the array indicate an increase in slowness from the southwest most (BB01) to the northeast most (BB07) station. Automatic algorithms combined with visual inspection and additional analyses are used to identify local events generating fault zone head and trapped waves. The observed fault zone head waves imply that the Clark fault in the area is a sharp bimaterial interface, with lower seismic velocity on the southwest side. The moveout between the head and direct P arrivals for events within similar to 40 km epicentral distance indicates an average velocity contrast across the fault over that section and the top 20 km of 3.2 per cent. A constant moveout for events beyond similar to 40 km to the southeast is due to off-fault locations of these events or because the imaged deep bimaterial interface is discontinuous or ends at that distance. The lack of head waves from events beyond similar to 20 km to the northwest is associated with structural complexity near the Hemet stepover. Events located in a broad region generate fault zone trapped waves at stations BB04-BB07. Waveform inversions indicate that the most likely parameters of the trapping structure are width of similar to 200 m, S velocity reduction of 30-40 per cent with respect to the bounding blocks, Q value of 10-20 and depth of similar to 3.5 km. The trapping structure and zone with largest slowness are on the northeast side of the fault. The observed sense of velocity contrast and asymmetric damage across the fault suggest preferred rupture direction of earthquakes to the northwest. This inference is consistent with results of other geological and seismological studies.

Sohn, RA, Vernon F, Hildebrand JA, Webb SC.  2000.  Field measurements of sonic boom penetration into the ocean. Journal of the Acoustical Society of America. 107:3073-3083.   10.1121/1.429336   AbstractWebsite

Six sonic booms, generated by F-4 aircraft under steady flight at a range of altitudes (610-6100 nl) and Mach numbers (1.07-1.26), were measured just above the air/sea interface, and at five depths in the water column. The measurements were made with a vertical hydrophone array suspended from a small spar buoy at the sea surface, and telemetered to a nearby research vessel. The sonic boom pressure amplitude decays exponentially with depth, and the signal fades into the ambient noise field by 30-50 m, depending on the strength of the boom at the sea surface. Low-frequency components of the boom waveform penetrate significantly deeper than high frequencies. Frequencies greater than 20 Hz are difficult to observe at depths greater than about 10 m. Underwater sonic boom pressure measurements exhibit excellent agreement with predictions from analytical theory, despite the assumption of a flat air/sea interface. Significant scattering of the sonic boom signal by the rough ocean surface is not detected. Real ocean conditions appear to exert a negligible effect on the penetration of sonic booms into the ocean unless steady vehicle speeds exceed Mach 3, when the boom incidence angle is sufficient to cause scattering on realistic open ocean surfaces. (C) 2000 Acoustical Society of America. [S0001-4966(00)03106-4].

Stephen, RA, Spiess FN, Collins JA, Hildebrand JA, Orcutt JA, Peal KR, Vernon FL, Wooding FB.  2003.  Ocean Seismic Network Pilot Experiment. Geochemistry Geophysics Geosystems. 4   10.1029/2002gc000485   AbstractWebsite

[1] The primary goal of the Ocean Seismic Network Pilot Experiment ( OSNPE) was to learn how to make high quality broadband seismic measurements on the ocean bottom in preparation for a permanent ocean seismic network. The experiment also had implications for the development of a capability for temporary (e.g., 1 year duration) seismic experiments on the ocean floor. Equipment for installing, operating and monitoring borehole observatories in the deep sea was also tested including a lead-in package, a logging probe, a wire line packer and a control vehicle. The control vehicle was used in three modes during the experiment: for observation of seafloor features and equipment, for equipment launch and recovery, and for power supply and telemetry between ocean bottom units and the ship. The OSNPE which was completed in June 1998 acquired almost four months of continuous data and it demonstrated clearly that a combination of shallow buried and borehole broadband sensors could provide comparable quality data to broadband seismic installations on islands and continents. Burial in soft mud appears to be adequate at frequencies below the microseism peak. Although the borehole sensor was subject to installation noise at low frequencies (0.6 to 50 mHz), analysis of the OSNPE data provides new insights into our understanding of ocean bottom ambient noise. The OSNPE results clearly demonstrate the importance of sediment borne shear modes in ocean bottom ambient noise behavior. Ambient noise drops significantly at high frequencies for a sensor placed just at the sediment basalt interface. At frequencies above the microseism peak, there are two reasons that ocean bottom stations have been generally regarded as noisier than island or land stations: ocean bottom stations are closer to the noise source ( the surface gravity waves) and most ocean bottom stations to date have been installed on low rigidity sediments where they are subject to the effects of shear wave resonances. When sensors are placed in boreholes in basement the performance of ocean bottom seismic stations approaches that of continental and island stations. A broadband borehole seismic station should be included in any real-time ocean bottom observatory.

Sutherland, FH, Vernon FL, Orcutt JA, Collins JA, Stephen RA.  2004.  Results from OSNPE: Improved teleseismic earthquake detection at the seafloor. Bulletin of the Seismological Society of America. 94:1868-1878.   10.1785/012003088   AbstractWebsite

Earthquake data from three ocean seismic network (OSN) sensors, located (1) on the seafloor, (2) buried in seafloor sediments and (3) in a borehole, together with those from Hawaiian Island stations, were compared by calculating threshold-detection magnitudes for P-, S-, Rayleigh-, and Love-wave arrivals. Our results show that the borehole seismometer had noise levels similar to those of the Island stations and produced high-quality high- and low-frequency body- and surface-wave data. Shallow burial of the seismometer in the sediments had no effect on higher frequencies but significantly reduced low-frequency noise levels so that data for S and Rayleigh waves were of high quality. In fact, the buried seismometer was characterized by the lowest noise levels at very low frequencies (<20 mHz; Collins et al., 2001). The ocean-floor seismometer was consistently noisy, and the data produced were of lower quality. Both observed magnitudes and calculated threshold magnitudes were lower by more than an order of magnitude than those observed in previous studies. Results for short-period body waves at the borehole instrument in particular were much better than those that were previously found for any ocean-bottom recording.