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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.

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.

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Kappus, ME, Vernon FL.  1991.  Acoustic Signature of Thunder from Seismic Records. Journal of Geophysical Research-Atmospheres. 96:10989-11006.   10.1029/91jd00789   AbstractWebsite

Thunder, the sound wave through the air associated with lightning, transfers sufficient energy to the ground to trigger seismometers set to record regional earthquakes. The acoustic signature recorded on seismometers, in the form of ground velocity as a function of time, contains the same type features as pressure variations recorded with microphones in air. At a seismic station in Kislovodsk, USSR, a nearly direct lightning strike caused electronic failure of borehole instruments while leaving a brief impulsive acoustic signature on the surface instruments. The peak frequency of 25-55 Hz is consistent with previously published values for cloud-to-ground lightning strikes, but spectra from this station are contaminated by very strong wind noise in this band. A thunderstorm near a similar station in Karasu triggered more than a dozen records of individual lightning strikes during a 2-hour period. The spectra for these events are fairly broadband, with peaks at low frequencies, varying from 6 to 13 Hz. The spectra were all computed by multitaper analysis, which deals appropriately with the nonstationary thunder signal. These independent measurements of low-frequency peaks corroborate the occasional occurrences in traditional microphone records, but a theory concerning the physical mechanism to account for them is still in question. Examined separately, the individual claps in each record have similar frequency distributions, discounting a need for multiple mechanisms to explain different phases of the thunder sequence. Particle motion, determined from polarization analysis of the three-component records, is predominantly vertical downward, with smaller horizontal components indicative of the direction to the lightning bolt. In three of the records the azimuth to the lightning bolt changes with time, confirming a significant horizontal component to the lightning channel itself.

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.

Kilb, D, Martynov VG, Vernon FL.  2007.  Aftershock detection thresholds as a function of time: Results from the ANZA seismic network following the 31 October 2001 M-L 5.1 ANZA, California, earthquake. Bulletin of the Seismological Society of America. 97:780-792.   10.1785/0120060116   AbstractWebsite

We examine aftershock detectability thresholds for events in the initial part of the 31 October 2001, M-L 5.1 sequence in southern California. This sequence occurred directly below the broadband ANZA seismic network, which recorded continuous waveform data at 13 azimuthally well-distributed stations within the study region (seven had epicentral distances < 20 km). Of the 608 aftershocks (0 < ML < similar to 2.8) in the initial 2 hr of this sequence, the first five aftershocks recorded were only identifiable at stations within 30 km after applying a high-pass filter. Using a cluster (radius <= 1.1 km) of 200 representative aftershocks, we track the maximum seismogram amplitude versus earthquake magnitude. This relationship helps us quantify the visibility of aftershocks within the mainshock coda and assess our detection capabilities. We estimate that detectable aftershocks within the mainshock coda include (1) those over magnitude similar to 3 that are within 15 km of the network centroid that occur 12 sec or more into the sequence, and (2) those over magnitude similar to 2 that are within 30 km of the centroid of the network that occur 60 see or more into the sequence. We find a lack of large aftershocks in this sequence. The largest aftershock (M-L similar to 2.8) is substantially smaller than the mainshock (M-L 5.1). We suggest this relatively large-magnitude differential is dictated by a combination of factors that includes complexity of the San Jacinto fault system and the lack of large earthquakes in the region in the past similar to 20 years.

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.

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
Lindquist, KG, Newman RL, Vernon FL.  2007.  The antelope interface to PHP and applications: Web-based real-time monitoring. Seismological Research Letters. 78:663-670.   10.1785/gssrl.78.6.663   Website
Pavlis, GL, Vernon FL.  2010.  Array processing of teleseismic body waves with the USArray. Computers & Geosciences. 36:910-920.   10.1016/j.cageo.2009.10.008   AbstractWebsite

We introduce a novel method of array processing for measuring arrival times and relative amplitudes of teleseismic body waves recorded on large aperture seismic arrays. The algorithm uses a robust stacking algorithm with three features: (1) an initial 'reference' signal is required for initial alignment by crosscorrelation; (2) a robust stacking method is used that penalizes signals that are not well matched to the stack; and (3) an iterative procedure alternates between cross-correlation with the current stack and the robust stacking algorithm. This procedure always converges in a few iterations making it well suited for interactive processing. We describe concepts behind a graphical interface developed to utilize this algorithm for processing body waves. We found it was important to compute several data quality metrics and allow the analyst to sort on these metrics. This is combined with a 'pick cutoff' function that simplifies data editing. Application of the algorithm to data from the USArray show four features of this method. (1) The program can produce superior results to that produced by a skilled analyst in approximately 1/5 of the time required for conventional interactive picking. (2) We show an illustrative example comparing residuals from S and SS for an event from northern Chile. The SS data show a remarkable +/- 10 s residual pattern across the USArray that we argue is caused by propagation approximately parallel to the subduction zones in Central and South America. (3) Quality metrics were found to be useful in identifying data problems. (4) We analyzed 50 events from the Tonga-Fiji region to compare residuals produced by this new algorithm with those measured by interactive picking. Both sets of residuals are approximately normally distributed, but corrupted by about 5% outliers. The scatter of the data estimated by waveform correlation was found to be approximately 1/2 that of the hand picked data. The outlier populations of both data sets are likely produced by cycle skips, but the distribution of hand picked data show a more diffuse departure from a Gaussian error model. (C) 2010 Elsevier Ltd. All rights reserved.

Johnson, CW, Vernon F, Nakata N, Ben-Zion Y.  2019.  Atmospheric processes modulating noise in Fairfield nodal 5 Hz geophones. Seismological Research Letters. 90:1612-1618.   10.1785/0220180383   AbstractWebsite

Atmospheric processes are documented to modulate seismic noise in Fairfield Nodal three-component geophones. Spectral analysis has shown high-amplitude signals between 40 and 50 Hz in all waveforms inspected. The changes in spectral amplitudes and frequency are found to be modified by daily variations in wind velocity and temperature, which are temporally correlated for much of the study. The wind velocity is shown to affect a wide spectral band with peak amplitudes that depend on the distance from in situ structures coupling wind energy into the shallow crust. The wind velocity increases the spectral amplitudes, most noticeably in the 40-50 Hz band; it produces a 15 Hz frequency modulation in the conditions of highest wind, with resonance frequencies up to 150 Hz. These signals likely reflect a superposition of multiple local and regional sources producing wind-generated ground motions and nonlinear wave propagation in the shallow subsurface. During periods of temperatures below 0 degrees C, a similar frequency modulation is observed, but the amplitudes are not as pronounced without the elevated wind velocity. A possible source of the continuous noise signal and the temperature-dependent frequency modulation is the spike mount that is attached to the nodal housing. The noise signals modulated by the wind and temperature variations require installation procedures in order to mitigate the effects of the contaminating noise on the geophysical processes of interest.

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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.

Tolstoy, M, Vernon FL, Orcutt JA, Wyatt FK.  2002.  Breathing of the seafloor: Tidal correlations of seismicity at Axial volcano. Geology. 30:503-506.   10.1130/0091-7613(2002)030<0503:botstc>2.0.co;2   AbstractWebsite

Tidal effects on seafloor microearthquakes have been postulated, but the search has been hindered by a lack of continuous long-term data sets. Making this observation is further complicated by the need to distinguish between Earth and ocean tidal influences on the seafloor. In the summer of 1994, a small ocean-bottom seismograph array located 402 microseismic events, over a period of two months, on the summit caldera of Axial volcano on the Juan de Fuca Ridge. Harmonic tremor was also observed on all instruments, and Earth and ocean tides were recorded on tiltmeters installed within the seismometer packages. Microearthquakes show a strong correlation with tidal lows, suggesting that faulting is occurring preferentially when ocean loading is at a minimum. The harmonic tremor, interpreted as the movement of superheated fluid in cracks, also has a tidal periodicity.

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.

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.

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Pavlis, GL, Vernon FL.  1994.  Calibration of Seismometers Using Ground Noise. Bulletin of the Seismological Society of America. 84:1243-1255. AbstractWebsite

We have developed and tested a new technique for calibration of seismometers using continuous recordings of ground noise. The method is founded on analytic techniques recently developed for estimation of transfer functions in magnetotellurics. We find that the technique can produce precise, absolute calibration measurements on sensors that do not have calibration coils. The data used are obtained by placing two sets of sensors close enough together that we can assume they record the same ground motion. It is further assumed that one of the sensors has a known, absolute calibration. One then records ground noise of sufficiently high amplitude to guarantee that one is recording above the amplifier noise floor across the entire frequency band of interest. Data are recorded for a time period that depends upon the lowest frequency that is to be resolved. The data is then divided into a series of N partially overlapping time windows, transfer-function estimates are calculated from each of these N time windows, and finally a robust mean estimation procedure is used to produce transfer-function estimates at a set of discrete frequencies. We applied this technique to produce calibration estimates for four different types of sensors (GS-13, triaxial 4.5-Hz L-28, STS-2, and triaxial L4) in various recording arrangements. We found that the technique worked extremely well in every case at frequencies above the point where the sensor output dropped into the instrument noise floor. Problems were consistently encountered above some high-frequency limit that depended upon the site and sensor being tested, and, as a result, we conclude that obtaining reliable results at higher frequencies requires more care in the experimental procedure. We show results from 4.5- and 1-Hz passive sensors plastered onto the same pier, which show nearly perfect coherence out to 100 Hz, and excellent agreement with theoretical predictions between 0.03 and 20 Hz. However, above 20 Hz, a systematic phase error plagues our results. Other cases were comparable when care was taken in the experimental procedure, but differed in detail. We argue that there are fundamental problems recording ground noise at these higher frequencies as a result of the following three experimental problems that can be difficult to control: (1) coupling of sensors to a common, stable platform, (2) contamination by acoustic and pier resonances in typical recording vaults, and (3) resonances of the sensor-pier-ground system.

Orcutt, JA, Vernon FL, Arrott M, Chave AD.  2007.  A Candidate Cyberinfrastructure for the NSF Ocean Observatories Initiative. ( Orcutt JA, Ed.).: American Geophysical Union, 2000 Florida Ave., N.W. Washington DC 20009 USA, [URL:http://www.agu.org] AbstractWebsite

We will describe a candidate cyberinfrastructure for the NSF Major Research Equipment and Facilities Construction project termed the Ocean Observatories Initiative. The system architecture departs substantially from earlier models in dealing with real-time data streams (not files), real-time workflow quality assurance and modeling/analysis, and the use of the knowledge developed in controlling the attached, real-time sensor network. The middleware, which facilitates these interactions, also provides the capability to support many separate virtual observatories developed to meet individual scientists needs. This transformative approach to scientific interaction with the ocean environment marks the beginning of a new epoch of the instrumented or digital Earth with a globally accessible continuous signal representing the now state of the Earth system. The data and inferred knowledge informs our understanding of the past, present, and predicted future of Earth systems as the observed signal grows exponentially for the foreseeable future.

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.

Li, YG, Vernon FL.  2001.  Characterization of the San Jacinto fault zone near Anza, California, by fault zone trapped waves. Journal of Geophysical Research-Solid Earth. 106:30671-30688.   10.1029/2000jb000107   AbstractWebsite

We installed three 350-m-long seismic arrays, each array consisting of 12 three-component stations, across the Coyote Creek fault (CCF), Clark Valley fault (CVF), and Buck Ridge fault (BRF) of the San Jacinto fault zone (SJFZ) near Anza, California, to record fault zone trapped waves from microearthquakes. We observed trapped waves with relatively large amplitudes and long duration at stations close to the fault traces for earthquakes occurring within the fault zone. The coda-normalized amplitude spectra of trapped waves showed peaks at 4-7 Hz, which decreased sharply with the distance from the fault trace. Observations and three-dimensional finite difference simulations of trapped waves revealed low-velocity and low-Q waveguides on these active faults with the width of 75- 100 m in which shear velocities are reduced by 25-30% from wall rock velocities and Q values are 40-90 at depths between the surface and 18 km. The locations of earthquakes for which we observed trapped waves delineate the most seismically active fault strands of the SJFZ in a region with complicated slip planes near Anza. The low-velocity waveguides inferred from trapped waves extend 15 to 20 km in the length on these active faults and are segmented by the fault discontinuities. The waveguide on the BRF dips southwestward to connect the waveguide on the CVF, which dips northeastward. This waveguide extends at the seismogenic depth through Anza slip gap to another low-velocity waveguide on the Casa Loma fault (CLF), which has been delineated in our previous study of the SJFZ using trapped waves [Li et al., 1997]. The waveguide on the CCF in Coyote Mountain is nearly vertical and disconnected from the CLF at the south edge of Anza gap. We interpret the low-velocity waveguides on these active strands to partly result from recent prehistoric significant earthquakes on them and evaluate the future earthquake in the Anza region.

Vernon, FL, Fletcher J, Carroll L, Chave A, Sembera E.  1991.  Coherence of Seismic Body Waves from Local Events as Measured by a Small-Aperture Array. Journal of Geophysical Research-Solid Earth and Planets. 96:11981-11996.   10.1029/91jb00193   AbstractWebsite

Eight local earthquakes were recorded during the operation of a small-aperture seismic array at Pinyon Flat, California. The site was chosen for its homogeneous granitic geology and its planar topography. Amplitude spectral ratios for the same signal measured at different stations had average values of less than 2 and maximum values of 7. Magnitude-squared coherences were estimated for all station pairs. These estimates were highest for the P wave arrivals on the vertical component and lowest for the P wave recorded on the transverse component. At 500 m station separation the P and S waves were incoherent above 15 Hz and 10 Hz, respectively. Coherence for both the P and S waves decrease as frequency increases and as distance increases. The coherence of signals from borehole sensors located at 300 and 150 m depth displays higher average coherence than equally spaced sites located on the surface. The results here suggest that even for sites that appear to be very similar, that is, those which are located on a planar surface within a few meters of granite bedrock, the measured seismic wavefield can be distorted substantially over scale lengths of 500 m. Coherence properties were calculated from synthetic seismograms which were computed for velocity models with exponential and self similar distribution perturbations. Standard deviations of 10% are not sufficient for the random velocity distributions to approximate the results from the small-aperture array.

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.

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.

Prieto, GA, Thomson DJ, Vernon FL, Shearer PM, Parker RL.  2007.  Confidence intervals for earthquake source parameters. Geophysical Journal International. 168:1227-1234.   10.1111/j.1365-246X.2006.03257.x   AbstractWebsite

We develop a method to obtain confidence intervals of earthquake source parameters, such as stress drop, seismic moment and corner frequency, from single station measurements. We use the idea of jackknife variance combined with a multitaper spectrum estimation to obtain the confidence regions. The approximately independent spectral estimates provide an ideal case to perform jackknife analysis. Given the particular properties of the problem to solve for source parameters, including high dynamic range, non-negativity, non-linearity, etc., a log transformation is necessary before performing the jackknife analysis. We use a Student's t distribution after transformation to obtain accurate confidence intervals. Even without the distribution assumption, we can generate typical standard deviation confidence regions. We apply this approach to four earthquakes recorded at 1.5 and 2.9 km depth at Cajon Pass, California. It is necessary to propagate the errors from all unknowns to obtain reliable confidence regions. From the example, it is shown that a 50 per cent error in stress drop is not unrealistic, and even higher errors are expected if velocity structure and location errors are present. An extension to multiple station measurement is discussed.

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.

Lewis, JL, Day SM, Magistrale H, Castro RR, Astiz L, Rebollar C, Eakins J, Vernon FL, Brune JN.  2001.  Crustal thickness of the peninsular ranges and gulf extensional province in the Californias. Journal of Geophysical Research-Solid Earth. 106:13599-13611.   10.1029/2001jb000178   AbstractWebsite

We estimate crustal thickness along an east-west transect of the Baja California peninsula and Gulf of California, Mexico, and investigate its relationship to surface elevation and crustal extension. We derive Moho depth estimates from P-to-S converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at similar to 31 degreesN latitude. Depth to the Moho is similar to 33 (+/-3) km near the Pacific coast of Baja California and increases gradually toward the east, reaching a maximum depth of similar to 40 (+/-4) km beneath the western part of the Peninsular Ranges batholith, The crust then thins rapidly under the topographically high eastern Peninsular Ranges and across the Main Gulf Escarpment, Crustal thickness is similar to 15-18 (+/-2) km within and on the margins of the Gulf of California. The Moho shallowing beneath the eastern Peninsular Ranges represents an average apparent westward dip of similar to 25 degrees. This range of Moho depths within the Peninsula Ranges, as well as the sharp similar to east-west gradient in depth in the eastern part of the range, is in agreement with earlier observations from north of the international border. The Moho depth variations do not correlate with topography of the eastern batholith, These findings suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges and that a local Airy crustal root does not support the highest elevations. We suggest that Moho shallowing under the eastern Peninsular Ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the Gulf Extensional Province that commenced in the late Cenozoic, Support of the eastern Peninsular Ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and/or upper mantle.

Ichinose, G, Day S, Magistrale H, Prush T, Vernon F, Edelman A.  1996.  Crustal thickness variations beneath the peninsular ranges, southern California. Geophysical Research Letters. 23:3095-3098.   10.1029/96gl03020   AbstractWebsite

We investigate the crustal thickness under the Peninsular Ranges using P-to-S converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method. Ps minus P times at sites west of a compositional boundary that separates the Peninsular Ranges batholith into east and west zones indicate a relatively hat, deep Moho. Ps minus P times at sites east of the compositional boundary decrease eastward. Moho depth estimates (made from the Ps delays and crustal velocities from seismic tomography) indicate a relatively constant 36 to 41 km thick crust in the western zone. In the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the Salton trough a lateral distance of 30 km. The lack of correlation between topography and Moho depths suggests compensation via lateral density variations in the lower crust or upper mantle. We propose that the compositional boundary decouples the eastern and western portions of the batholith and that the eastern portion has thinned in response to regional Miocene extension, or Salton trough sifting, or both.