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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.

Jacques, AA, Horel JD, Crosman ET, Vernon FL.  2017.  Tracking Mesoscale Pressure Perturbations Using the USArray Transportable Array. Monthly Weather Review. 145:3119-3142.   10.1175/mwr-d-16-0450.1   AbstractWebsite

Mesoscale convective phenomena induce pressure perturbations that can alter the strength and magnitude of surface winds, precipitation, and other sensible weather, which, in some cases, can inflict injuries and damage to property. This work extends prior research to identify and characterize mesoscale pressure features using a unique resource of 1-Hz pressure observations available from the USArray Transportable Array (TA) seismic field campaign. A two-dimensional variational technique is used to obtain 5-km surface pressure analysis grids every 5 min from 1 March to 31 August 2011 from the TA observations and gridded surface pressure from the Real-Time Mesoscale Analysis over a swath of the central United States. Bandpass-filtering and feature-tracking algorithms are employed to isolate, identify, and assess prominent mesoscale pressure perturbations and their properties. Two case studies, the first involving mesoscale convective systems and the second using a solitary gravity wave, are analyzed using additional surface observation and gridded data resources. Summary statistics for tracked features during the period reviewed indicate a majority of perturbations last less than 3 h, produce maximum perturbation magnitudes between 2 and 5 hPa, and move at speeds ranging from 15 to 35ms(-1). The results of this study combined with improvements nationwide in real-time access to pressure observations at subhourly reporting intervals highlight the potential for improved detection and nowcasting of high-impact mesoscale weather features.

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.

Qiu, H, Ben-Zion Y, Ross ZE, Share PE, Vernon FL.  2017.  Internal structure of the San Jacinto fault zone at Jackass Flat from data recorded by a dense linear array. Geophysical Journal International. 209:1369-1388.   10.1093/gji/ggx096   AbstractWebsite
Ross, ZE, Ben-Zion Y, White MC, Vernon FL.  2016.  Analysis of earthquake body wave spectra for potency and magnitude values: implications for magnitude scaling relations. Geophysical Journal International. 207:1158-1164.   10.1093/gji/ggw327   AbstractWebsite

We develop a simple methodology for reliable automated estimation of the low-frequency asymptote in seismic body wave spectra of small to moderate local earthquakes. The procedure corrects individual P- and S-wave spectra for propagation and site effects and estimates the seismic potency from a stacked spectrum. The method is applied to > 11 000 earthquakes with local magnitudes 0 < M-L < 4 that occurred in the Southern California plate-boundary region around the San Jacinto fault zone during 2013. Moment magnitude M-w values, derived from the spectra and the scaling relation of Hanks & Kanamori, follow a Gutenberg-Richter distribution with a larger b-value (1.22) from that associated with the M-L values (0.93) for the same earthquakes. The completeness magnitude for the M-w values is 1.6 while for M-L it is 1.0. The quantity (M-w - M-L) linearly increases in the analysed magnitude range as M-L decreases. An average earthquake with M-L = 0 in the study area has an M-w of about 0.9. The developed methodology and results have important implications for earthquake source studies and statistical seismology.

Tanimoto, T, Lin C-J, Hadziioannou C, Igel H, Vernon F.  2016.  Estimate of Rayleigh-to-Love wave ratio in the secondary microseism by a small array at Piñon Flat observatory, California. Geophysical Research Letters. 43:11,173-11,181.   10.1002/2016GL071133   Abstract

Using closely located seismographs at Piñon Flat (PFO), California, for 1 year long record (2015), we estimated the Rayleigh-to-Love wave energy ratio in the secondary microseism (0.1–0.35 Hz) in four seasons. Rayleigh wave energy was estimated from a vertical component seismograph. Love wave energy was estimated from rotation seismograms that were derived from a small array at PFO. Derived ratios are 2–2.5, meaning that there is 2–2.5 times more Rayleigh wave energy than Love wave energy at PFO. In our previous study at Wettzell, Germany, this ratio was 0.9–1.0, indicating comparable energy between Rayleigh waves and Love waves. This difference suggests that the Rayleigh-to-Love wave ratios in the secondary microseism may differ greatly from region to region. It also implies that an assumption of the diffuse wavefield is not likely to be valid for this low frequency range as the equipartition of energy should make this ratio much closer.

Thomson, DJ, Vernon FL.  2016.  Some comments on the analysis of "big" scientific time series. Proceedings of the Ieee. 104:2220-2249.   10.1109/jproc.2016.2598218   AbstractWebsite

Experience with long time series from space, climate, seismology, and engineering has demonstrated the need for even longer data series with better precision, timing, and larger instrument arrays. We find that almost all the data we have examined, including atmospheric, seismic data, and dropped calls in cellular phone networks contain evidence for solar mode oscillations that couple into Earth systems through magnetic fields, and that these are often the strongest signals present. We show two examples suggesting that robustness has been overused and that many of the extremes in geomagnetic and space physics data may be the result of a superposition of numerous modes. We also present initial evidence that the evolution of turbulence in interplanetary space may be controlled by modes. Returning to the theme of "big data," our experience has been that theoretical predictions that spectra would be asymptotically unbiased have turned out to be largely irrelevant with very long time series primarily showing that we simply did not understand the problems. Data that were considered to have excessively variable spectra appear to evolve into processes with dense sets of modes. In short data blocks, these modes are not resolved and as the relative phase of the modes within the estimator varies, so does the apparent power. Ideas that data series become uncorrelated at modest distances in either time or space do not seem to be true with the long duration continuous time series data we have examined.

Ross, ZE, White MC, Vernon FL, Ben-Zion Y.  2016.  An improved algorithm for real-time S-wave picking with application to the (augmented) ANZA network in Southern California. Bulletin of the Seismological Society of America. 106:2013-2022.   10.1785/0120150230   AbstractWebsite

We develop an automatic shear-wave picking algorithm suitable for real-time applications as well as with existing databases. The method can scan through packets of continuous waveforms and make picks without prior knowledge of whether earthquakes have occurred. This makes the algorithm suitable for detecting earthquakes at the same time. Expanding upon and improving the method of Ross and Ben-Zion (2014a), the algorithm first uses polarization filters to remove P-wave energy from the seismogram. Then, short-term average/long-term average and kurtosis detectors are applied to the data in tandem to lock in on the phase arrival. The method is tested by applying it to a full month of continuous waveform data recorded by a regional network at 123 stations and comparing the resulting automatic picks with 11,353 handmade picks. The automatic picks are found to be within 0.16 s of the analyst picks 75% of the time, and S picks are successful 92% of the time that a P-wave pick is made. The algorithm is then applied to an entire year of continuous data and detects 11,197 earthquakes. The hypocenters of these earthquakes are, on average, improved by more than 1 km when compared with the regional network's automated catalog.

Roux, P, Moreau L, Lecointre A, Hillers G, Campillo M, Ben-Zion Y, Zigone D, Vernon F.  2016.  A methodological approach towards high-resolution surface wave imaging of the San Jacinto Fault Zone using ambient-noise recordings at a spatially dense array. Geophysical Journal International. 206:980-992.   10.1093/gji/ggw193   AbstractWebsite

We present a new technique for deriving detailed information on seismic velocities of the subsurface material from continuous ambient noise recorded by spatially dense seismic arrays. This method uses iterative double beamforming between various subarrays to extract surface wave contributions from the ambient-noise data in complex environments with unfavourable noise-source distributions. The iterative double beamforming extraction makes it possible to retrieve large amounts of Rayleigh wave traveltime information in a wide frequency band. The method is applied to data recorded by a highly dense Nodal array with 1108 vertical geophones, centred on the damage zone of the Clark branch of the San Jacinto Fault Zone south of Anza, California. The array covers a region of similar to 650 x 700 m(2), with instrument spacing of 10-30 m, and continuous recording at 500 samples s(-1) over 30 d in 2014. Using this iterative double beamforming on subarrays of 25 sensors and cross-correlations between all of the station pairs, we separate surface waves from body waves that are abundant in the raw cross-correlation data. Focusing solely on surface waves, maps of traveltimes are obtained at different frequencies with unprecedented accuracy at each point of a 15-m-spacing grid. Group velocity inversions at 2-4 Hz reveal depth and lateral variations in the structural properties within and around the San Jacinto Fault Zone in the study area. This method can be used over wider frequency ranges and can be combined with other imaging techniques, such as eikonal tomography, to provide unprecedented detailed structural images of the subsurface material.

Jacques, AA, Horel JD, Crosman ET, Vernon F, Tytell J.  2016.  The Earthscope US transportable array 1 Hz surface pressure dataset. Geoscience Data Journal. 3:29-36.   10.1002/gdj3.37   AbstractWebsite

A unique set of high temporal frequency surface atmospheric pressure observations have been collected and archived from a large-scale field campaign in the geosciences. The Earthscope U.S. Transportable Array (USArray TA) consists of approximately 400 deployable surface platforms. Stations were deployed in a Cartesian-like gridded fashion across a section of the contiguous United States for 1-2 year then retrieved and redeployed as new platforms further east. While primarily deployed for seismic measurements, platforms also recorded surface atmospheric pressure. These pressure data, collected and stored at a temporal frequency of 1 Hz, have been made available via the Research Data Archive at the National Center for Atmospheric Research (NCAR) for the time period 1 Jan 2010-31 Dec 2015. The 6 years of observations contain data from over 1000 locations ranging from the central to eastern United States, as well as some platforms in Alaska and the northwest United States. Data were organized as annual station files with supplemental metadata and quality control summary files. Several web-based interfaces are also available to rapidly explore the pressure archive. We describe the available dataset with several prominent atmospheric events shown as usage examples.

Tytell, J, Vernon F, Hedlin M, Hedlin CD, Reyes J, Busby B, Hafner K, Eakins J.  2016.  The USARRAY transportable array as a platform for weather observation and research. Bulletin of the American Meteorological Society. 97:603-619.   10.1175/bams-d-14-00204.1   AbstractWebsite
Li, ZF, Peng ZG, Ben-Zion Y, Vernon FL.  2015.  Spatial variations of shear wave anisotropy near the San Jacinto Fault Zone in Southern California. Journal of Geophysical Research-Solid Earth. 120:8334-8347.   10.1002/2015jb012483   AbstractWebsite

We examine crustal anisotropy at several scales along and across the San Jacinto Fault Zone (SJFZ) by systematically measuring shear wave splitting (SWS) parameters. The analyzed data are recorded by 86 stations during 2012-2014, including five linear dense arrays crossing the SJFZ at different locations and other autonomous stations within 15 km from the main fault trace. Shear phase arrivals and SWS parameters (fast directions and delay times) are obtained with automated methods. The measurement quality is then assessed using multiple criteria, resulting in 23,000 high-quality measurements. We find clear contrast of fast directions between the SW and NE sides of the SJFZ. Stations on the SW side have fast directions consistent overall with the maximum horizontal compression direction (SHmax), while stations on the NE side show mixed patterns likely reflecting lithological/topographic variations combined with fault zone damage. The fast directions in the Anza gap section with relatively simple fault geometry agree with the inferred SHmax, and the delay times at an array within that section are smaller than those observed at other across-fault arrays. These indications of less pronounced damage zone in the Anza section compared to other segments of the SJFZ are correlated generally with geometrical properties of the surface traces. Significant variations of fast directions on several across-fault arrays, with station spacing on the orders of a few tens of meters, suggest that shallow fault structures and near-surface layers play an important role in controlling the SWS parameters.

Thomson, DJ, Vernon FL.  2015.  Unexpected, high-Q, low-frequency peaks in seismic spectra. Geophysical Journal International. 202:1690-1710.   10.1093/gji/ggv175   AbstractWebsite

It was established over a decade ago that the normal modes of the Earth are continuously excited at times without large earthquakes, but the sources of the 'seismic hum' have remained unresolved. In addition to the normal modes of the Earth, we show spectral lines in seismic data with frequencies which correspond closely to normal modes of the Sun. Moreover, the widths of the low-frequency lines in the seismic spectra are similar to those of solar modes and much narrower than those of the Earth's normal mode peaks. These seismic lines are highly coherent with magnetic fields measured on both the Geostationary Operations Environmental Satellite (GOES)-10 satellite and the Advanced Composition Explorer (ACE) spacecraft located at L1, 1.5 million km sunward of Earth suggesting that the solar modes are transmitted to the Earth by the interplanetary magnetic field and solar wind. The solar modes are split by multiples of a cycle/day and, surprisingly, by the 'quasi two-day' mode and other frequencies. Both the phase of the coherences and slight frequency offsets between seismic and geomagnetic data at observatories exclude the possibility that these effects are simply spurious responses of the seismometers to the geomagnetic field. We emphasize data from low-noise seismic observatories: Black Forest (BFO), Pion Flat (PFO), Eskdalemuir (ESK) and Obninsk (OBN). Horizontal components of seismic velocity show higher coherences with the external (ACE) magnetic field than do the vertical components. This effect appears to be larger near the seismic torsional, or T-mode, frequencies.

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.

Jacques, AA, Horel JD, Crosman ET, Vernon FL.  2015.  Central and Eastern US surface pressure variations derived from the USArray Network. Monthly Weather Review. 143:1472-1493.   10.1175/mwr-d-14-00274.1   AbstractWebsite

Large-magnitude pressure signatures associated with a wide range of atmospheric phenomena (e.g., mesoscale gravity waves, convective complexes, tropical disturbances, and synoptic storm systems) are examined using a unique set of surface pressure sensors deployed as part of the National Science Foundation Earth-Scope USArray Transportable Array. As part of the USArray project, approximately 400 seismic stations were deployed in a pseudogrid fashion across a portion of the United States for 1-2 yr, then retrieved and redeployed farther east. Surface pressure observations at a sampling frequency of 1 Hz were examined during the period 1 January 2010-28 February 2014 when the seismic array was transitioning from the central to eastern continental United States. Surface pressure time series at over 900 locations were bandpass filtered to examine pressure perturbations on three temporal scales: meso-(10 min-4 h), subsynoptic (4-30 h), and synoptic (30 h-5 days) scales. Case studies of strong pressure perturbations are analyzed using web tools developed to visualize and track tens of thousands of such events with respect to archived radar imagery and surface wind observations. Seasonal assessments of the bandpass-filtered variance and frequency of large-magnitude events are conducted to identify prominent areas of activity. Large-magnitude mesoscale pressure perturbations occurred most frequently during spring in the southern Great Plains and shifted northward during summer. Synoptic-scale pressure perturbations are strongest during winter in the northern states with maxima located near the East Coast associated with frequent synoptic development along the coastal storm track.

Mikhalevsky, PN, Sagen H, Worcester PF, Baggeroer AB, Orcutt J, Moore SE, Lee CM, Vigness-Raposa KJ, Freitag L, Arrott M, Atakan K, Beszczynska-Moeller A, Duda TF, Dushaw BD, Gascard JC, Gavrilov AN, Keers H, Morozov AK, Munk WH, Rixen M, Sandven S, Skarsoulis E, Stafford KM, Vernon F, Yuen MY.  2015.  Multipurpose Acoustic Networks in the Integrated Arctic Ocean Observing System. Arctic. 68:11-27. AbstractWebsite

The dramatic reduction of sea ice in the Arctic Ocean will increase human activities in the coming years. This activity will be driven by increased demand for energy and the marine resources of an Arctic Ocean accessible to ships. Oil and gas exploration, fisheries, mineral extraction, marine transportation, research and development, tourism, and search and rescue will increase the pressure on the vulnerable Arctic environment. Technologies that allow synoptic in situ observations year-round are needed to monitor and forecast changes in the Arctic atmosphere-ice-ocean system at daily, seasonal, annual, and decadal scales. These data can inform and enable both sustainable development and enforcement of international Arctic agreements and treaties, while protecting this critical environment. In this paper, we discuss multipurpose acoustic networks, including subsea cable components, in the Arctic. These networks provide communication, power, underwater, and under-ice navigation, passive monitoring of ambient sound (ice, seismic, biologic, and anthropogenic), and acoustic remote sensing (tomography and thermometry), supporting and complementing data collection from platforms, moorings, and vehicles. We support the development and implementation of regional to basin-wide acoustic networks as an integral component of a multidisciplinary in situ Arctic Ocean observatory.

Yang, HF, Li ZF, Peng ZG, Ben-Zion Y, Vernon F.  2014.  Low-velocity zones along the San Jacinto Fault, Southern California, from body waves recorded in dense linear arrays. Journal of Geophysical Research-Solid Earth. 119:8976-8990.   10.1002/2014jb011548   AbstractWebsite

We derive high-resolution information on low-velocity fault zone (FZ) structures along the San Jacinto Fault Zone (SJFZ), Southern California, using waveforms of local earthquakes that are recorded at multiple linear cross-fault arrays. We observe clear across-fault delays of direct P and S waves, indicating damage zones at different segments of the SJFZ. We then compute synthetic traveltimes and waveforms using generalized ray theory and perform forward modeling to constrain the FZ parameters. At the southern section near the trifurcation area, the low-velocity zone (LVZ) of the Clark branch has a width of 200m, 30-45% reduction in Vp, and 50% reduction in Vs. From array data across the Anza seismic gap, we find a LVZ with 200m width and 50% reduction in both Vp and Vs, nearly as prominent as that on the southern section. We only find prominent LVZs beneath three out of the five arrays, indicating along-strike variations of the fault damage. FZ-reflected phases are considerably less clear than those observed above the rupture zone of the 1992 Landers earthquake shortly after the event. This may reflect partially healed LVZs with less sharp boundaries at the SJFZ, given the relatively long lapse time from the last large surface-rupturing event. Alternatively, the lack of observed FZ-reflected phases could be partially due to the relatively small aperture of the arrays. Nevertheless, the clear signatures of damage zones at Anza and other locations indicate very slow healing process, at least in the top few kilometers of the crust.

Kurzon, I, Vernon FL, Rosenberger A, Ben-Zion Y.  2014.  Real-time automatic detectors of P and S waves using singular value decomposition. Bulletin of the Seismological Society of America. 104:1696-1708.   10.1785/0120130295   AbstractWebsite

We implement a new method for automatic detection of P and S phases using singular value decomposition (SVD) analysis. The method is based on the real-time iteration algorithm of Rosenberger (2010) for the SVD of three-component seismograms. The algorithm identifies the apparent incidence angle by applying SVD and separates the waveforms into their P and S components. We apply the algorithm to filtered waveforms and then either set detectors on the incidence angle and singular values or apply signal-to-noise ratio (SNR) detectors for P and S picking on the filtered and SVD-separated channels. The Anza Seismic Network and the recent portable deployment in the San Jacinto fault zone area provide a very dense seismic network for testing the detection algorithm in a diverse setting, including events with different source mechanisms, stations with different site characteristics, and ray paths that diverge from the approximation used in the SVD algorithm. A 2-30 Hz Butterworth band-pass filter gives the best performance for a large variety of events and stations. We use the SVD detectors on many events and present results from the complex and intense aftershock sequence of the M-w 5.2 June 2005 event. This sequence was thoroughly reviewed by several analysts, identifying 294 events in the first hour, all located in a dense cluster around the mainshock. We used this dataset to fine-tune the automatic SVD detection, association, and location, achieving a 37% automatic identification and location of events. All detected events fall within the dense cluster, and there are no false events. An ordinary SNR detector does not exceed 11% success and has a wider spread of locations (not within the reviewed cluster). The preknowledge of the phases picked ( P or S) by the SVD detectors significantly reduces the noise created by phase-blind SNR detectors.

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.

Jacobeit, E, Thomas C, Vernon F.  2013.  Influence of station topography and Moho depth on the mislocation vectors for the Kyrgyz Broadband Seismic Network (KNET). Geophysical Journal International. 193:949-959.   10.1093/gji/ggt014   AbstractWebsite

Deviations of slowness and backazimuth from theoretically calculated values, the so-called mislocation vectors, are measured for the Kyrgyz Broadband Seismic Network (KNET) in the Tien Shan region. 870 events have been analysed for arrivals of P and PKP waves from all azimuths. The deviations of slowness and backazimuth show a strong trend with values up to 1 s deg(-1) for slowness values for waves arriving from the North and South and backazimuth deviations of, in some cases, more than 10 degrees for waves arriving from the East and West. Calculating the traveltime deviations of the stations for topography of the Tien Shan region and Moho depth values appropriate for this area shows that most slowness and backazimuth deviations can be reduced to very small values. The remaining mislocation vectors show no strong trends and are on average smaller than 0.2 s deg(-1) for slowness and 2 degrees for backazimuth values, which is within the error bars of these measurements. Results from array methods that rely on the knowledge of the backazimuth values show much improved resolution after the correction of the mislocation vectors which shows the importance of knowing and correcting for structures directly beneath arrays.

Kane, DL, Shearer PM, Goertz-Allmann BP, Vernon FL.  2013.  Rupture directivity of small earthquakes at Parkfield. Journal of Geophysical Research-Solid Earth. 118:212-221.   10.1029/2012jb009675   AbstractWebsite

Theoretical modeling of strike-slip ruptures along a bimaterial interface suggests that earthquakes initiating on the interface will have a preferred rupture direction. We test this model with 450 small earthquakes (2 < M < 5) from Parkfield, California, to look for evidence of consistent rupture directivity along the San Andreas Fault. We analyze azimuthal variations in earthquake source spectra after applying an iterative correction for wave propagation effects. Our approach avoids directly modeling source spectra because these models generally assume symmetric rupture; instead, we look for azimuthal variations in the amplitudes of the source spectra over specified frequency bands. Our overall results show similar proportions of events exhibiting characteristics of rupture directivity toward either the southeast or northwest. However, the proportion of events with southeast rupture directivity increases as we limit the data set to larger magnitudes, with 70% of the 46 events M > 3 exhibiting southeast rupture characteristics. Some spatial and temporal variability in rupture directivity is also apparent. We observe a higher proportion of northwest directivity ruptures following the 2004 M 6 Parkfield earthquake, which ruptured toward the northwest. Our results are generally consistent with the preferred southeast rupture directivity model but suggest that directivity is likely due to several contributing factors. Citation: Kane, D. L., P. M. Shearer, B. P. Goertz-Allmann, and F. L. Vernon (2013), Rupture directivity of small earthquakes at Parkfield, J. Geophys. Res. Solid Earth, 118, 212-221, doi: 10.1029/2012JB009675.

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
Kilb, D, Biasi G, Anderson J, Brune J, Peng ZG, Vernon FL.  2012.  A Comparison of Spectral Parameter Kappa from Small and Moderate Earthquakes Using Southern California ANZA Seismic Network Data. Bulletin of the Seismological Society of America. 102:284-300.   10.1785/0120100309   AbstractWebsite

Kappa is a one-parameter estimator of the spectral amplitude decay with frequency of a seismogram. Low values (similar to 5 ms) indicate limited attenuation of high-frequency energy whereas higher values (similar to 40 ms) indicate high-frequency energy has been removed. Kappa is often assumed to be a site term and used in seismic designs. We address two key questions about kappa: (1) how to identify source, path, and site contributions to kappa; and (2) can kappa estimates from smaller earthquakes, and more readily accessible weak- motion recordings, be reasonably extrapolated to estimate kappa of larger earthquakes? The use of small earthquakes (M-L < 1) presents many challenges and requires new approaches. We develop estimates of kappa for seismograms from 1137 small earthquakes recorded by the ANZA seismic network in southern California, and compare these to results from the stronger recorded shaking generated by 43 M-L > 3.5 earthquakes inside the network. We find kappa from small earthquakes predicts the relative values of kappa for larger earthquakes (e.g., measurements at stations PFO and KNW are small compared with those at stations TRO and SND). For the SND and TRO data, however, kappa values from small earthquakes overpredict those from moderate and large earthquakes. Site effects are the most important contributor to kappa estimates, but the scatter within kappa measurements at a given station is likely caused by a significant contribution from near the source, perhaps related to near-source scattering. Because of this source-side variability, care is recommended in using individual small events as Green's functions to study source-time effects of moderate and large events.

Pavlis, GL, Sigloch K, Burdick S, Fouch MJ, Vernon FL.  2012.  Unraveling the geometry of the Farallon plate: Synthesis of three-dimensional imaging results from USArray. Tectonophysics. 532:82-102.   10.1016/j.tecto.2012.02.008   AbstractWebsite

We compare 12 recent three-dimensional (3D) seismic imaging results that made extensive use of data from the Earthscope Transportable Array (TA). Our goal is to sort out what can be said about the geometry of the Farallon plate. Our main approach is 3D visualization using a kinematic plate motion model as a framework. Comparison of results from all 12 image volumes indicates that the results are most consistent with a single, coherent Farallon slab overridden by North American. The Farallon can be tracked from the trench in the Pacific Northwest to its remnants in the lower mantle under eastern North America. From the trench the lithosphere has a low dip to the volcanic arc. Immediately east of the arc the slab steepens sharply before undergoing a decrease in dip above the 410 km discontinuity. The gently dipping section varies along strike. Under Washington the deflection is minor but to the south the slab flattens to become nearly horizontal beneath southern Idaho. There is a strong agreement that the high velocity anomaly associated with the slab vanishes under eastern Oregon. Scattered wave imaging results, however, suggest the top of the anomaly is continuous. These can be reconciled if one assumes the wavespeed anomaly has been neutralized by processes linked to the Yellowstone system. We find that all results are consistent with a 4D kinematic model of the Mendocino slab window under Nevada and Utah. In the eastern US the larger scale models all show a lower mantle anomaly related to the older history of Farallon subduction. The link between the lower mantle and new results in the U.S. Cordillera lies under the High Plains where the required USArray coverage is not yet complete. Image volumes in a unified format are supplied in an electronic supplement. (C) 2012 Elsevier B.V. All rights reserved.

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.