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A
Al-Amri, AM, Mellors R, Vernon FL.  1999.  Broadband seismic noise characteristics of the Arabian Shield. Arabian Journal for Science and Engineering. 24:99-113. AbstractWebsite

A total of nine portable broadband stations were deployed across the Arabian Shield from November 1995 to March 1997. The stations consisted of STS-2 seismometers recorded continuously at 40 samples per second on RefTek dataloggers. Noise studies showed that most stations were exceptionally quiet with noise levels near the USGS low noise model for frequencies higher than 0.1 Hz. At lower frequencies, the horizontal components showed high noise levels, possibly due to instrumental characteristics. High frequency (>1 Hz) noise varied as much as 10 dB between day and night for some stations (RAYN and TAIF) while for more isolated stations (HALM) was constant. Seasonal noise levels also varied, with April to June being the quietest months. Slight changes in peak microseism frequency also occurred seasonally. The quietest stations were HALM, RAYN, AFIF, and UQSK, ail of which were located in central Saudi Arabia and show noise levels near the low noise model for frequencies between 0.1 and 4 Hz. The optimal site for a new quiet station would be near HALM which showed very little diurnal variations of cultural noise. These stations appear to be among the best sites in the world for the properties of detection thresholds and ground noise levels. Events with mb >3.5 could be detected at distances from 10 to 100 degrees.

Allam, AA, Ben-Zion Y, Kurzon I, Vernon F.  2014.  Seismic velocity structure in the Hot Springs and Trifurcation areas of the San Jacinto fault zone, California, from double-difference tomography. Geophysical Journal International. 198:978-999.   10.1093/gji/ggu176   AbstractWebsite

We present tomographic images of crustal velocity structures in the complex Hot Springs and Trifurcation areas of the San Jacinto Fault Zone (SJFZ) based on double-difference inversions of earthquake arrival times. We invert for V-P, V-S and hypocentre location within 50 x 50 x 20 km(3) volumes, using 266 969 P and 148 249 S arrival times. We obtain high-fidelity images of seismic velocities with resolution on the order of a few kilometres from 2 to 12 km depth and validate the results using checkerboard tests. Due to the relatively large proportion of S-wave arrival times, we also obtain stable maps of V-P/V-S ratios in both regions. The velocity of the Trifurcation Area as a whole is lower than adjacent unfaulted material. We interpret a 4-km-wide low velocity zone with high V-P/V-S ratio in the trifurcation itself as related to fault zone damage. We also observe clear velocity contrasts across the Buck Ridge, Clark and Coyote Creek segments of the SJFZ. The Anza segment of the SJFZ, to the NW of the trifurcation area, displays a strong (up to 27 per cent) contrast of V-S from 2 to 9 km depth. In the Hot Springs area, a low velocity zone between the Claremont and Casa Loma Strands narrows with depth, with clear velocity contrasts observed across both segments. A roughly 10-km-wide zone of low velocity and low V-P/V-S ratio at the NW tip of the Hot Springs fault is indicative of either unconsolidated sediments associated with the San Jacinto basin, or fluid-filled cracks within a broad deformation zone. High V-P/V-S ratios along the Anza segment could indicate a preferred nucleation location for future large earthquakes, while the across-fault velocity contrast suggests a preferred northwest rupture propagation direction for such events.

AlShukri, HJ, Pavlis GL, Vernon FL.  1995.  Site effect observations from broadband arrays. Bulletin of the Seismological Society of America. 85:1758-1769. AbstractWebsite

We present evidence of significant variations in time-domain amplitude and spectral estimates of local earthquake recorded by small-aperture arrays. We examine data from three arrays: two arrays of different scales deployed at Pinon mat, California, in 1990 and 1991 and an array deployed in the summer of 1993 in the former Soviet republic of Turkmenistan. We find consistent evidence in all cases for significant variations in measured amplitudes over scale lengths comparable to feasible wavelengths of incident wave fields. This phenomenon, however, is strongly frequency dependent. At the Pinon Flat site, variations in power spectral estimates exceed a factor of 100 at frequencies over 4 Hz. Analysis of teleseismic signals, however, demonstrates that these variations diminish rapidly at lower frequencies and falls to negligible levels below 0.6 Hz. The Turkmenistan site shows similar overall characteristics; high-frequency variations are less dramatic. Variations comparable to the Pinon mat site do not occur below 20 Hz. Analysis of teleseismic signals yield results similar to Pinon Flat, although the transition to negligible variability seems to occur at a slightly lower frequency of 0.3 Hz. The 1990 Pinon Flat experiment utilized simultaneous recording in two boreholes directly beneath the array. Comparison of spectral estimates from these boreholes to the surface sensors strongly suggests that the deviations in high-frequency spectral estimates we observe across the array are due to interaction of the wave field with the near-surface, weathered layer. We suggest the differences in high-frequency variations of signals recorded at Pinon Flat compared with that of the Turkmenistan site can be explained by differences in near-surface conditions at the two sites. The low-frequency transition to negligible signal variation seen at both sites occurs when the array aperture becomes small compared with the wavelength of surface waves, suggesting that signal variations in intermediate frequencies may be influenced by body-wave to surface-wave conversions caused by crustal velocity variations and topography.

Anchieta, MC, Wolfe CJ, Pavlis GL, Vernon FL, Eakins JA, Solomon SC, Laske G, Collins JA.  2011.  Seismicity around the Hawaiian Islands Recorded by the PLUME Seismometer Networks: Insight into Faulting near Maui, Molokai, and Oahu. Bulletin of the Seismological Society of America. 101:1742-1758.   10.1785/0120100271   AbstractWebsite

Instrumental limitations have long prevented the detailed characterization of offshore earthquakes around the Hawaiian Islands, and little is known about the spatial distribution of earthquakes in regions outside the vicinity of the well-monitored island of Hawaii. Here, we analyze data from the deployment of two successive networks of ocean-bottom seismometers (OBSs) as part of the Plume-Lithosphere Undersea Melt Experiment (PLUME) to better determine seismicity patterns along the Hawaiian Islands and their offshore regions. We find that earthquake detection rates are improved when seismograms are high-pass filtered above similar to 5 Hz to reduce the background seismic noise. Hypocentral solutions have been determined for 1147 previously undetected microearthquakes, and an additional 2880 events correspond to earthquakes already in the catalog of the United States Geological Survey (USGS) Hawaiian Volcano Observatory (HVO). The spatial patterns of earthquakes identified solely on the PLUME network provide complementary information to patterns identified by the HVO network. A diffuse pattern of seismicity is found to the southeast of the island of Hawaii, and clusters of earthquakes are located west of the island. Many microearthquakes are observed in the vicinity of Maui and Molokai, including some located at mantle depths. A small number of microearthquakes are found to occur near Oahu. There is no evidence from our analyses that the Molokai fracture zone (MFZ) is seismically active at this time, and no evidence was found of a previously hypothesized Diamond Head fault (DHF) near Oahu. However, on the basis of both the PLUME and HVO locations, there is a northeast-southwest-trending swath of epicenters extending northeastward of Oahu that may indicate the locus of moderate-sized historic earthquakes attributed to the Oahu region.

B
Barker, B, Clark M, Davis P, Fisk M, Israelsson H, Khalturin V, Kim WY, McLaughin K, Meade C, Murphy J, North R, Orcutt J, Powell C, Richards PG, Stead R, Stevens R, Vernon F, Wallace T.  1998.  Seismology - Monitoring nuclear tests (vol 282, pg 1967, 1998). Science. 282:882-882.Website
Ben-Zion, Y, Vernon FL, Ozakin Y, Zigone D, Ross ZE, Meng HR, White M, Reyes J, Hollis D, Barklage M.  2015.  Basic data features and results from a spatially dense seismic array on the San Jacinto fault zone. Geophysical Journal International. 202:370-380.   10.1093/gji/ggv142   AbstractWebsite

We discuss several outstanding aspects of seismograms recorded during >4 weeks by a spatially dense Nodal array, straddling the damage zone of the San Jacinto fault in southern California, and some example results. The waveforms contain numerous spikes and bursts of high-frequency waves (up to the recorded 200 Hz) produced in part by minute failure events in the shallow crust. The high spatial density of the array facilitates the detection of 120 small local earthquakes in a single day, most of which not detected by the surrounding ANZA and regional southern California networks. Beamforming results identify likely ongoing cultural noise sources dominant in the frequency range 1-10 Hz and likely ongoing earthquake sources dominant in the frequency range 20-40 Hz. Matched-field processing and back-projection of seismograms provide alternate event location. The median noise levels during the experiment at different stations, waves generated by Betsy gunshots, and wavefields from nearby earthquakes point consistently to several structural units across the fault. Seismic trapping structure and local sedimentary basin produce localized motion amplification and stronger attenuation than adjacent regions. Cross correlations of high-frequency noise recorded at closely spaced stations provide a structural image of the subsurface material across the fault zone. The high spatial density and broad frequency range of the data can be used for additional high resolution studies of structure and source properties in the shallow crust.

Berger, J, Eissler HK, Vernon FL, Nersesov IL, Gokhberg MB, Stolyrov OA, Tarasov NT.  1988.  Studies of High-Frequency Seismic Noise in Eastern Kazakhstan. Bulletin of the Seismological Society of America. 78:1744-1758.Website
Berger, J, Orcutt J, Vernon F.  2005.  HiSeasNet: Providing Internet to the UNOLS fleet. Sea Technology. 46:17-20.Website
Burdick, S, van der Hilst RD, Vernon FL, Martynov V, Cox T, Eakins J, Karasu GH, Tylell J, Astiz L, Pavlis GL.  2010.  Model Update January 2010: Upper Mantle Heterogeneity beneath North America from Traveltime Tomography with Global and USArray Transportable Array Data. Seismological Research Letters. 81:689-693.   10.1785/gssrl.81.5.689   Website
Burdick, S, van der Hilst RD, Vernon FL, Martynov V, Cox T, Eakins J, Karasu GH, Tylell J, Astiz L, Pavlis GL.  2012.  Model Update March 2011: Upper Mantle Heterogeneity beneath North America from Traveltime Tomography with Global and USArray Transportable Array Data. Seismological Research Letters. 83:23-28.   10.1785/gssrl.83.1.23   Website
Burdick, S, Li C, Martynov V, Cox T, Eakins J, Mulder T, Astiz L, Vernon FL, Pavlis GL, van der Hilst RD.  2008.  Upper mantle heterogeneity beneath north America from travel time tomography with global and USArray transportable array data. Seismological Research Letters. 79:384-392.   10.1785/gssrt.79.3.384   Website
Burdick, S, van der Hilst RD, Vernon FL, Martynov V, Cox T, Eakins J, Mulder T, Astiz L, Pavlis GL.  2009.  Model Update December 2008: Upper Mantle Heterogeneity beneath North America from P-wave Travel Time Tomography with Global and USArray Transportable Array Data. Seismological Research Letters. 80:638-645.   10.1785/gssrl.80.4.638   Website
C
Castro, RR, Shearer PM, Astiz L, Suter M, Jacques-Ayala C, Vernon F.  2010.  The Long-Lasting Aftershock Series of the 3 May 1887 M-w 7.5 Sonora Earthquake in the Mexican Basin and Range Province. Bulletin of the Seismological Society of America. 100:1153-1164.   10.1785/0120090180   AbstractWebsite

We study local and regional body-wave arrival times from several seismic networks to better define the active regional fault pattern in the epicentral region of the 3 May 1887 M-w 7.5 Sonora, Mexico (southern Basin and Range Province) earthquake. We determine hypocenter coordinates of earthquakes that originated between 2003 and 2007 from arrival times recorded by the local network RESNES (Red Sismica del Noreste de Sonora) and stations of the Network of Autonomously Recording Seismographs (NARS)-Baja array. For events between April and December 2007, we also incorporated arrival times from USArray stations located within 150 km of the United States-Mexico border. We first obtained preliminary earthquake locations with the Hypoinverse program (Klein, 2002) and then relocated these initial hypocenter coordinates with the source-specific station term (SSST) method (Lin and Shearer, 2005). Most relocated epicenters cluster in the upper crust near the faults that ruptured during the 1887 earthquake and can be interpreted to be part of its long-lasting series of aftershocks. The region of aftershock activity extends, along the same fault zone, 40-50 km south of the documented southern tip of the 1887 rupture and includes faults in the epicentral region of the 17 May 1913 (I-max VIII, M-I 5.0-0.4) and 18 December 1923 (I-max IX, M-I 5.7-0.4) Granados-Huasabas, Sonora, earthquakes, which themselves are likely to be aftershocks of the 1887 event. The long aftershock duration can be explained by the unusually large magnitude of the mainshock and by the low slip rates and long mainshock recurrence times of the faults that ruptured in 1887.

Castro, RR, Valdes-Gonzalez C, Shearer P, Wong V, Astiz L, Vernon F, Perez-Vertti A, Mendoza A.  2011.  The 3 August 2009 M-w 6.9 Canal de Ballenas Region, Gulf of California, Earthquake and Its Aftershocks. Bulletin of the Seismological Society of America. 101:929-939.   10.1785/0120100154   AbstractWebsite

On 3 August 2009 an earthquake of magnitude M-w 6.9 occurred near Canal de Ballenas, in the north-central region of the Gulf of California, Mexico. The focal mechanism of the main event, reported in the Global Centroid Moment Tensor (CMT) catalog, is right lateral strike-slip with a strike of 216 degrees and a dip of 78 degrees. The initial location reported by the National Seismological Service of Mexico [Servicio Sismologico Nacional (SSN)] and the Array Network Facility (ANF) suggested that the epicenter was on the North American plate near the Tiburon fault, which is considered inactive. This earthquake was preceded by a magnitude m(b) 5.5 event that occurred about 5 min before. In the next 40 min after the main event two aftershocks with magnitudes m(b) 4.9 and M-w 6.2 occurred, and on 5 August a third aftershock of M-w 5.7 was located in the Canal de Ballenas region. The events of August 2009 were recorded by the regional stations of the broadband network Red Sismologica de Banda Ancha (RESBAN) that Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE) operates and by stations of the SSN also located in the region of the Gulf of California. We used body-wave arrivals to determine precise epicentral locations and to estimate the rupture area of this important sequence of earthquakes. The resulting hypocentral coordinates indicate that the main event of this sequence occurred along the Canal de Ballenas transform fault, with a rupture length of 50 km. Based on the aftershock distribution, we estimate that the main event had a rupture area of approximately 600 km(2), an average slip of 1.3 m, and a stress drop of 2.2 MPa.

Collins, JA, Vernon FL, Orcutt JA, Stephen RA, Peal KR, Wooding FB, Spiess FN, Hildebrand JA.  2001.  Broadband seismology in the oceans: Lessons from the Ocean Seismic Network Pilot Experiment. Geophysical Research Letters. 28:49-52.   10.1029/2000gl011638   AbstractWebsite

The fundamental objective of the Ocean Seismic Network Pilot Experiment (OSNPE) - which was carried out over a period of about 4 months at a site 225 km southwest of Oahu, Hawaii - was to learn how to make high-quality, broadband seismic measurements in the deep oceans. The OSNPE results demonstrate that broadband data of quality similar to that of quiet land stations can be acquired with seafloor seismographs, but that the location of the seismometer - whether it be on the seafloor, surficially buried within the seabed, or in a deep borehole - has a profound effect on data quality. At long-periods (< 0.1 Hz), data quality was highest for a seismometer buried just beneath the seafloor, while at short-periods (> 0.1 Hz), data quality was best for a seismometer deployed 242 m below the seafloor in a borehole.

Collins, JA, Vernon FL, Orcutt JA, Stephen RA.  2002.  Upper mantle structure beneath the Hawaiian swell: Constraints from the ocean seismic network pilot experiment. Geophysical Research Letters. 29   10.1029/2001gl013302   AbstractWebsite

[1] Data from two broadband, ocean-bottom seismographic stations deployed similar to 225 km southwest of Oahu, Hawaii during the Ocean Seismic Network Pilot Experiment provide constraints on upper mantle structure beneath the Hawaiian swell. Receiver functions show that the mantle transition zone is thinned by >50 km relative to reference model PA5, which, in the absence of compositional changes, implies excess temperatures of >350 K in the transition zone. The combination of the measurements reported here and the thickness variations reported by Li et al. [2000] imply that the transition zone is thinned by 30 +/- 15 km over an along-swell dimension of at least 700 km. At similar to80 km depth, P-to-S converted phases are identified from the Gutenberg discontinuity marking the lid of the oceanic low-velocity zone and the base of the lithosphere. Shear-wave splitting measurements imply that fast-polarization azimuths are intermediate between the absolute plate-motion vector and the fossil spreading direction; multi-event stacked values of o and deltat are -80degrees and 1.5 s, respectively.

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De Groot-Hedlin, CD, Vernon FL.  1998.  An evolutionary programming method for estimating layered velocity structure. Bulletin of the Seismological Society of America. 88:1023-1035. AbstractWebsite

For most geophysical inverse problems, one solves for an Earth model that minimizes the misfit between the observed data and observational predications determined from forward modeling. However, the solutions to such problems are almost always nonunique, as some of the model parameters are poorly resolved by the data. We have developed a novel form of evolutionary programming (EP) that solves directly for a broad cluster of models that adequately fit the data. We obtain improved performance of the EP algorithm by rotating the model parameter axes at each generation so that the axes are aligned perpendicular and parallel to long valleys in the misfit surface. The rotated axes are defined by the eigenvectors of the covariance matrix of the set of models; the mutation rate in each new direction varies with the square root of the corresponding eigenvalue, This modified EP method finds a wide range of models satisfying the data, allowing one to determine the uncertainty in the model parameters. We illustrate this EP method by applying it to the problem of determining velocity structure from regional travel-time data. Application of this method to a travel-time data set generated from a model that includes a low-velocity zone (LVZ) illustrates that it successfully finds a wide range of models within the broad minimum that includes the initial solution. We apply the method to regional travel-time data recorded at the Kyrgyz broadband network (KNET) and find that velocities are well resolved, except at depths of 25 to 50 km.

Donner, S, Lin CJ, Hadziioannou C, Gebauer A, Vernon F, Agnew DC, Igel H, Schreiber U, Wassermann J.  2017.  Comparing direct observation of strain, rotation, and displacement with array estimates at Pinon Flat Observatory, California. Seismological Research Letters. 88:1107-1116.   10.1785/0220160216   AbstractWebsite

The unique instrument setting at the Pinon Flat Observatory in California is used to simultaneously measure 10 out of the 12 components, completely describing the seismic-wave field. We compare the direct measurements of rotation and strain for the 13 September 2015 M-w 6.7 Gulf of California earthquake with array-derived observations using this configuration for the first time. In general, we find a very good fit between the observations of the two measurements with cross-correlation coefficients up to 0.99. These promising results indicate that the direct and array-derived measurements of rotation and strain are consistent. For the array-based measurement, we derived a relation to estimate the frequency range within which the array-derived observations provide reliable results. This relation depends on the phase velocity of the study area and the calibration error, as well as on the size of the array.

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Foley, S, Berger J, Orcutt JA, Vernon FL.  2010.  Advanced communications for remote ocean platforms in the coming 15 years. ( Foley S, Ed.).: American Geophysical Union, 2000 Florida Ave., N.W. Washington DC 20009 USA, [URL:http://www.agu.org] AbstractWebsite

Long-term measurements in the oceans are becoming a scientific and civil imperative that is having a profound impact on oceanography and particularly seagoing oceanography. Ocean observatories such as NSFs Ocean Observatories Initiative (OOI) and NOAAs Integrated Ocean Observing System (IOOS) are providing means for making measurements of change over decadal time scales, a practice of great importance for understanding climate variability and change as well as potential for natural disasters such as tsunamis. At the same time the costs for operating ships at sea are increasing quickly (fuel, personnel, capability) and pressure is mounting for targeted community measurements in which data collected are available openly. Both of these trends drive efforts to enhance communications at sea in coming decades. Ships are now platforms for deployment and testing of new sensors that might be later deployed at fixed observatories and observatories are increasingly common; communications to these remote sites become increasingly important. Streamed real-time data from a ship or observatory allow for rapid response to new data and greater flexibility on how the science facility can be used by the community. Cost effective transfers of large blocks of data with high reliability including surety of data return, coupled with real-time streams, allow data to be analyzed quickly by shore experts and even machine-to-machine interactions, and improve the quality of information derived from science programs. For those scientists working at sea, robust communication with shore will allow for increased contributions to ongoing programs ashore. Satellite bandwidth today is still largely too expensive for personal work by individual investigators, but bandwidth will gradually decrease in price as new spacecraft are launched and more commercial operators offer service at sea. Whether paid by the minute, byte, or month, satellite communications will make increase the quality of research by making data available to a wider audience. We shall review the current use of HiSeasNet for these purposes and present anticipated enhancements of bandwidth by government and industry for the foreseeable future.

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Given, HK, Tarasov NT, Zhuravlev V, Vernon FL, Berger J, Nersesov IL.  1990.  High-Frequency Seismic Observations in Eastern Kazakhstan, USSR, with Emphasis on Chemical Explosion Experiments. Journal of Geophysical Research-Solid Earth and Planets. 95:295-307.   10.1029/JB095iB01p00295   Website
Golos, EM, Fang H, Yao H, Zhang H, Burdick S, Vernon F, Schaeffer A, Lebedev S, van der Hilst RD.  2018.  Shear wave tomography beneath the United States using a joint inversion of surface and body waves. Journal of Geophysical Research-Solid Earth. 123:5169-5189.   10.1029/2017jb014894   AbstractWebsite

Resolving both crustal and shallow-mantle heterogeneity, which is needed to study processes in and fluxes between crust and mantle, is still a challenge for seismic tomography. Body wave data can constrain deep features but often produce vertical smearing in the crust and upper mantle; in contrast, surface wave data can provide good vertical resolution of lithospheric structure but may lack lateral resolution and are less sensitive to the deeper Earth. These two data types are usually treated and inverted separately, and tomographic models therefore do not, in general, benefit from the complementary nature of sampling by body and surface waves. As a pragmatic alternative to full waveform inversions, we formulate linear equations for teleseismic S wave traveltimes and surface wave phase velocities and solve them simultaneously for variations in shear wave speed anomalies in the crust and upper mantle. We apply this technique to data from USArray and permanent seismic networks and present a model of seismic shear wave speed anomalies beneath the continental United States. Our joint model fits the individual data sets almost as well as separate inversions but provides a better explanation of the combined data set. It is generally consistent with previous models but shows improvements over both body wave-only and surface wave-only tomography and can lead to refinements in interpretation of features on the scale of the lithosphere and mantle transition zone. Plain Language Summary Variations in the speed at which seismic waves travel through the Earth reveal information about the structure and history of the planet. In this study, we investigate seismic velocity variations using two common types of data from seismograms: body waves, which travel through the deep Earth, and surface waves, which provide information about the shallower layers. Commonly, these two waves are studied separately, but we adopt the method of Fang et al. (2016, ) to produce a model of the crust and mantle of the whole Earth by using both types of data. The goal of this paper is to validate the application of this technique on a large scale, using the continental United States as a test region. We perform qualitative and quantitative tests to show that this method improves upon models made with only body or surface waves while maintaining the best fits of the individual models. We conclude that this technique is a valuable and efficient tool to study the Earth's interior at multiple scales.

Growdon, MA, Pavlis GL, Niu F, Vernon FL, Rendon H.  2009.  Constraints on mantle flow at the Caribbean-South American plate boundary inferred from shear wave splitting. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb005887   AbstractWebsite

We measured shear wave splitting from SKS and SKKS data recorded by temporary stations deployed as part of the Broadband Onshore-Offshore Lithospheric Investigation of Venezuela and the Antilles Arc Region project and the national seismic network of Venezuela. Approximately 3000 station-event pairs yielded similar to 300 with visible SKS and/or SKKS phases. We obtained 63 measurements at 39 of the 82 stations in the network using the method of Silver and Chan (1991) and conventional quality criteria. We combined our results with previous measurements made by Russo et al. (1996). The most prominent feature in the data is an area of large (> 2.0 s) lag times with roughly east-west fast axes in northeastern Venezuela. Mineral physics models show split times this large are difficult to explain with horizontal foliation, but are more feasible with anisotropy characterized by a coherent vertical foliation and an east-west fast axis extending over most of the upper 250 km of the mantle. We interpret the large split times in northeastern Venezuela as a consequence of eastward translation of the Atlantic slab, which has left a strong vertical foliation in its wake parallel to the plate boundary. The peak split times correspond closely with the point the slab intersects the base of the anisotropic asthenosphere at 250 km. Away from this area of large split times the measured times fall to more standard values, but an east-west fast axis still predominates. We suggest this is linked to the rapidly varying strain field at the southern edge of the Atlantic which quickly disrupts the coherent strain field that causes the very large split times in northeastern Venezuela.

Gurrola, H, Minster JB, Given H, Vernon F, Berger J, Aster R.  1990.  Analysis of High-Frequency Seismic Noise in the Western United-States and Eastern Kazakhstan. Bulletin of the Seismological Society of America. 80:951-970.Website
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Haase, JS, Hauksson E, Vernon F, Edelman A.  1996.  Modeling of ground motion from a 1994 Northridge aftershock using a tomographic velocity model of the Los Angeles Basin. Bulletin of the Seismological Society of America. 86:S156-S167. AbstractWebsite

The 1994 Northridge mainshock and its aftershocks show a complex pattern of peak accelerations at stations located in the Los Angeles Basin. The waveforms contain multiples of body-wave phases and extensive surface waves at frequencies mostly below 1 Hz. In particular, for stations at distances greater than 18 km, secondary arrivals show larger accelerations than the direct S-wave arrivals. The mainshock waveforms are further complicated by irregularities of the source rupture. We use 2D finite difference to evaluate the effect of lateral variations in seismic velocity on the amplitude of shear-wave energy and to distinguish the effects of source and propagation path. We model waveforms from one aftershock recorded at nine stations deployed along a 60-km-long profile extending into the Los Angeles Basin. We use a two-dimensional slice through the 3D tomography model of the Los Angeles Basin in the 2D finite-difference calculations. These synthetic waveforms fit the aftershock waveforms significantly better than corresponding waveforms determined from simple 1D velocity models. With the addition of a thin low-velocity surface layer above the tomography model, the finite-difference synthetics reproduce most of the important features of the recorded data, in particular, the large-amplitude arrivals 7 to 10 sec following the direct S arrival. These arrivals correspond to the SS arrival, which is sharply refracted at the basin edge, and the S-wave with multiple legs trapped by the dipping near surface gradient. For large earthquakes located either inside or outside the basin, these phases can be the cause of the largest and hence potentially most hazardous shaking in the Los Angeles Basin.

Hartse, HE, Fehler MC, Aster RC, Scott JS, Vernon FL.  1994.  Small-Scale Stress Heterogeneity in the Anza Seismic Gap, Southern California. Journal of Geophysical Research-Solid Earth. 99:6801-6818.   10.1029/93jb02874   AbstractWebsite

Focal mechanism inversions reveal significant lateral variations in stress orientations along the Anza segment of the San Jacinto fault zone. The most notable stress anomaly is within the 20-km aseismic (seismic gap) portion of the fault zone, where sigma1, the maximum compressive stress, is nearly horizontal and is oriented at 74-degrees +/- 13-degrees relative to the fault strike. This contrasts with orientations ranging from 62-degrees +/- 11-degrees to 49-degrees +/- 7-degrees along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap. Regional stress results, found by inverting all focal mechanisms simultaneously, indicate that sigma1 is horizontal and trends north-south, while sigma3 is horizontal and trends east-west. Approximately, 15 km west of the seismic gap, in the off-fault Cahuilla swarm area, sigma1 and sigma3 solutions are rotated clockwise by about 25-degrees relative to the regional model. Roughly, 10 km southeast of the seismic gap near the Buck Ridge fault, sigma1 and sigma3 are rotated counterclockwise by about 10-degrees relative to the regional solution. Northwest of the seismic gap along the fault zone, sigma3 plunges about 30-degrees from the horizontal, correlating with a local increase in reverse faulting between the Hot Springs and San Jacinto faults. Southeast of the seismic gap, sigma1 plunges about 45-degrees from the horizontal, correlating with a local increase in normal faulting in the trifurcation region of the Buck Ridge, Clark, and Coyote Creek faults. We propose a simple mechanical model in which a block rotation superimposed on the dominant right-lateral strike-slip motion of the fault zone satisfies the first-order observations of stress orientation, faulting, and horizontal surface strain. Under this model the Anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault, and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap.