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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.

Orcutt, JA, Vernon FL, Arrott M, Chave A, Schofield O, Peach C, Krueger I, Meisinger M.  2007.  Cyberinfrastructure for the NSF Ocean Observatories Initiative. ( Orcutt JA, Ed.).: American Geophysical Union AbstractWebsite

The Ocean Observatories Initiative (OOI) is an environmental observatory covering a diversity of oceanic environments, ranging from the coastal to the deep ocean. The physical infrastructure comprises a combination of seafloor cables, buoys and autonomous vehicles. It is currently in the final design phase, with construction planned to begin in mid-2010 and deployment phased over five years. The Consortium for Ocean Leadership manages this Major Research Equipment and Facilities Construction program with subcontracts to Scripps Institution of Oceanography, University of Washington and Woods Hole Oceanographic Institution. High-level requirements for the CI include the delivery of near-real-time data with minimal latencies, open data, data analysis and data assimilation into models, and subsequent interactive modification of the network (including autonomous vehicles) by the cyberinfrastructure. Network connections include a heterogeneous combination of fiber optics, acoustic modems, and Iridium satellite telemetry. The cyberinfrastructure design loosely couples services that exist throughout the network and share common software and middleware as necessary. In this sense, the system appears to be identical at all scales, so it is self-similar or fractal by design. The system provides near-real-time access to data and developed knowledge by the OOI's Education and Public Engagement program, to the physical infrastructure by the marine operators and to the larger community including scientists, the public, schools and decision makers. Social networking is employed to facilitate the virtual organization that builds, operates and maintains the OOI as well as providing a variety of interfaces to the data and knowledge generated by the program. We are working closely with NOAA to exchange near-real-time data through interfaces to their Data Interchange Facility (DIF) program within the Integrated Ocean Observing System (IOOS). Efficiencies have been emphasized through the use of university and commercial computing clouds.

Orcutt, JA, Vernon FL, Peach CL, Arrott M, Chave AD, Schofield O, Meisinger MJ, Farcas C, Farcas E, Krueger I, Kleinert J.  2010.  The cyberinfrastructure model for the NSF Ocean Observatories Initiative: A 20-year prospective. ( Orcutt JA, Ed.).: American Geophysical Union, 2000 Florida Ave., N.W. Washington DC 20009 USA, [URL:http://www.agu.org] AbstractWebsite

The NSF Ocean Observatories Initiative (OOI) began a five-year construction period in October 2009. The Consortium on Ocean Leadership (COL) manages the overall program with Implementing Organizations for Coastal/Global Scale Nodes (CGSN) at Woods Hole, Oregon State and Scripps; the Regional Cabled Network (RCN) at U of Washington and Cyberinfrastructure (CI) at UCSD. The CI component is a substantial departure from previous approaches to data distribution and management, which we believe will have a significant impact on oceanography over the next twenty years. These innovations include the availability of data in near-real-time with latencies of seconds, open access to all data, analysis of the data streams for detection and modeling, use of the derived knowledge to modify the network with minimal or no human interaction and maintenance of data provenance through time as new versions of the data are created through QA/QC processes. The network architecture is designed to be scalable so that addition of new sensors is straightforward and inexpensive with costs increasing linearly at worst. Rather than building new computer infrastructure (disk farms and computer clusters), we are exploiting Amazons Extensible Computing Cloud (EC2) and Simple Storage System (S3) to reduce long-term commitments to hardware and maintenance in order to minimize operations and maintenance costs. The OOI CI is actively partnering with other organizations (e.g. NOAAs IOOS) to integrate existing data systems using many of the same technologies to improve broad access to existing and planned observing systems, including those that provide critical climate data. We welcome interest and participation in the OOIs CI construction, testing and transition to operations over the coming five years.