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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.

Zigone, D, Ben-Zion Y, Lehujeur M, Campillo M, Hillers G, Vernon FL.  2019.  Imaging subsurface structures in the San Jacinto fault zone with high-frequency noise recorded by dense linear arrays. Geophysical Journal International. 217:879-893.   10.1093/gji/ggz069   AbstractWebsite

Cross-correlations of 2-35 Hz ambient seismic noise recorded by three linear arrays across the San Jacinto Fault Zone (SJFZ) in Southern California are used to derive high-resolution shear wave velocity models for the top 50-90m of the crust at the array locations. Coherent Rayleigh surface waves are inverted to construct 2-D maps of group velocities in the range 0.2-0.6 km s(-1). These maps are inverted to shear wave velocities around the fault using a Markov Chain Monte Carlo approach. The results show marked low-velocity zones in the top 20-30 m with velocity reduction up to 35 per cent and shallow flower structures at depth shallower than 50 m. The derived velocities, location of low-velocity zone with respect to main surface traces and shape with depth are generally consistent with borehole measurements and previous imaging of deeper sections of the SJFZ at the same sites or nearby. The imaging technique requires only similar to 30 d of data (90 per cent of the signal-to-noise ratio is obtained in 15 d) and it bridges an observational gap between surface geology and typical tomography studies with no resolution in the top 100 m.

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

Abstract To improve models of ground motion estimation and probabilistic seismic hazard analyses, the engineering seismology field is moving toward developing fully nonergodic ground motion models, models specific for individual source-to-site paths. Previous work on this topic has examined systematic variations in ground-motion along particular paths (from either recorded or simulated earthquake data) and has not included physical properties of the path. We present here a framework to include physical path properties, by seeking correlations between ground motion amplitudes along specific paths and crustal properties, specifically seismic velocity and anelastic attenuation, along that path. Using a large data set of small-magnitude earthquakes recorded in Southern California, we find a correlation between the gradient of seismic S wave velocity and the path term residual, after accounting for an average geometric spreading and anelastic attenuation, indicating that heterogeneity in crustal velocity primarily controls the path-specific attenuation. Even in aseismic regions, details of path-specific ground motion prediction equations can be developed from crustal structure and property data.

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

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

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

Ground‐motion prediction equations (GMPEs) are critical elements of probabilistic seismic hazard analysis (PSHA), as well as for other applications of ground motions. To isolate the path component for the purpose of building nonergodic GMPEs, we compute a regional GMPE using a large dataset of peak ground accelerations (PGAs) from small‐magnitude earthquakes ( 0.5≤M≤4.5 with >10,000 events, yielding ∼120,000 recordings) that occurred in 2013 centered around the ANZA seismic network (hypocentral distances ≤180km ) in southern California. We examine two separate methods of obtaining residuals from the observed and predicted ground motions: a pooled ordinary least‐squares model and a mixed‐effects maximum‐likelihood model. Whereas the former is often used by the broader seismological community, the latter is widely used by the ground‐motion and engineering seismology community. We confirm that mixed‐effects models are the preferred and most statistically robust method to obtain event, path, and site residuals and discuss the reasoning behind this. Our results show that these methods yield different consequences for the uncertainty of the residuals, particularly for the event residuals. Finally, our results show no correlation (correlation coefficient [CC] <0.03 ) between site residuals and the classic site‐characterization term VS30 , the time‐averaged shear‐wave velocity in the top 30 m at a site. We propose that this is due to the relative homogeneity of the site response in the region and perhaps due to shortcomings in the formulation of VS30 and suggest applying the provided PGA site correction terms to future ground‐motion studies for increased accuracy.

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.

Qin, L, Ben-Zion Y, Qiu H, Share PE, Ross ZE, Vernon FL.  2018.  Internal structure of the San Jacinto fault zone in the trifurcation area southeast of Anza, California, from data of dense seismic arrays. Geophysical Journal International. 213:98-114.   10.1093/gji/ggx540   AbstractWebsite

We image the internal structure of the San Jacinto fault zone (SJFZ) in the trifurcation area southeast of Anza, California, with seismic records from dense linear and rectangular arrays. The examined data include recordings from more than 20 000 local earthquakes and nine teleseismic events. Automatic detection algorithms and visual inspection are used to identify P and S body waves, along with P- and S-types fault zone trapped waves (FZTW). The location at depth of the main branch of the SJFZ, the Clark fault, is identified from systematic waveform changes across lines of sensors within the dense rectangular array. Delay times of P arrivals from teleseismic and local events indicate damage asymmetry across the fault, with higher damage to the NE, producing a local reversal of the velocity contrast in the shallow crust with respect to the large-scale structure. A portion of the damage zone between the main fault and a second mapped surface trace to the NE generates P- and S-types FZTW. Inversions of high-quality S-type FZTW indicate that the most likely parameters of the trapping structure are width of similar to 70 m, S-wave velocity reduction of 60 per cent, Q value of 60 and depth of similar to 2 km. The local reversal of the shallow velocity contrast across the fault with respect to large-scale structure is consistent with preferred propagation of earthquake ruptures in the area to the NW.

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.