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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.

Prieto, GA, Parker RL, Thomson DJ, Vernon FL, Graham RL.  2007.  Reducing the bias of multitaper spectrum estimates. Geophysical Journal International. 171:1269-1281.   10.1111/j.1365-246X.2007.03592.x   AbstractWebsite

The power spectral density of geophysical signals provides information about the processes that generated them. We present a new approach to determine power spectra based on Thomson's multitaper analysis method. Our method reduces the bias due to the curvature of the spectrum close to the frequency of interest. Even while maintaining the same resolution bandwidth, bias is reduced in areas where the power spectrum is significantly quadratic. No additional sidelobe leakage is introduced. In addition, our methodology reliably estimates the derivatives (slope and curvature) of the spectrum. The extra information gleaned from the signal is useful for parameter estimation or to compare different signals.

Lewis, JL, Day SM, Magistrale H, Eakins J, Vernon F.  2000.  Regional crustal thickness variations of the Peninsular Ranges, southern California. Geology. 28:303-306.   10.1130/0091-7613(2000)28<303:rctvot>;2   AbstractWebsite

We used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern Peninsular Ranges, California. Depth to the Moho varies from similar to 37 inn beneath the western Peninsular Ranges batholith to similar to 27 km at the western edge of the Salton trough, an average apparent dip of similar to 10 degrees to the west over a lateral distance of 60 km, We previously obtained a similar result for a profile similar to 100 km to the south (a Moho dip of similar to 20 degrees over 30 km lateral distance). In both cases, the Moho depth variations do not correlate with topography of the eastern batholith, but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes, These observations suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges, and that compensation is through lateral variations in crustal or upper mantle density rather than through an Airy root.

Schofield, O, Kohut J, Glenn S, Morell J, Capella J, Corredor J, Orcutt J, Arrott M, Krueger I, Meisinger M, Peach C, Vernon F, Chave A, Chao Y, Chien S, Thompson D, Brown W, Oliver M, Boicourt W.  2010.  A Regional Slocum Glider Network in the Mid-Atlantic Bight Leverages Broad Community Engagement. Marine Technology Society Journal. 44:185-195. AbstractWebsite

Autonomous underwater gliders have proven to be a cost-effective technology for measuring the 3-D ocean and now represent a critical component during the design and implementation of the Mid-Atlantic Regional Ocean Observing System (MARCOOS), a Region of the U.S. Integrated Ocean Observing System. The gliders have been conducting regional surveys of the Mid-Atlantic (MA) Bight, and during the 3 years of MARCOOS, the glider fleet has conducted 22 missions spanning 10,867 km and collecting 62,824 vertical profiles of data. In addition to collecting regional data, the gliders have facilitated collaboration for partners outside of MARCOOS. The existence of the MA glider observatory provided a unique test bed for cyber-infrastructure tools being developed as part of the National Science Foundation's Ocean Observatory Initiative. This effort allowed the Ocean Observatory Initiative software to integrate the MARCOOS assets and provided a successful demonstration of an ocean sensor net. The hands-on experience of the MA glider technicians supported training and provided assistance of collaborators within the Caribbean Regional Association, also a region of the U.S. Integrated Ocean Observing System, to assess the efficacy of gliders to resolve internal waves. Finally, the glider fleet has enabled sensor development and testing in a cost-effective manner. Generally, new sensors were tested within the MARCOOS domain before they were deployed in more extreme locations throughout the world's oceans. On the basis of this experience, the goal of the MARCOOS glider team will be to expand the MA network in coming years. The potential of how an expanded network of gliders might serve national needs was illustrated during the 2010 Macondo Gulf of Mexico oil spill, where gliders from many institutions collected subsurface mesoscale data to support regional models and oil response planning. The experience gained over the last 5 years suggests that it is time to develop a national glider network.

Mellors, RJ, Camp VE, Vernon FL, Al-Amri AMS, Ghalib A.  1999.  Regional waveform propagation in the Arabian Peninsula. Journal of Geophysical Research-Solid Earth. 104:20221-20235.   10.1029/1999jb900187   AbstractWebsite

Regional waveform propagation is characterized in the Arabian Peninsula using data from a temporary network of broadband seismometers. Between November 1995 and March 1997, 332 regional (delta < 15 degrees) events were recorded from nine stations deployed across the Arabian Shield. Regional phase propagation was analyzed in two ways: by individual inspection of the waveforms and by stacking of waveforms. Inspection of the waveforms revealed consistent variations in individual seismograms according to the region of origin. Waveforms from events in the Gulf of Aqaba, northwest of the network, possess weak Pn, Pg, and Sn but show a prominent L-g phase. In contrast, clear Pn, Sn, and Lg are observed for events located in the Zagros, a region northeast of the network. Events near the Straits of Hormuz also display Pn and Sn but lack a strong high-frequency Lg. Southern Red Sea and African earthquakes have moderate-amplitude body phases with some Lg. For the stacks the data were high-pass filtered at 1 Hz, rectified, binned, and then stacked by time/distance or by time/slowness. The time/distance stacks show clear differences between regions that correspond to the variations observed in individual seismograms. The time/slowness stacks allow comparison of relative phase velocities and amplitudes. Pn velocity under the network was estimated to be 8.0 +/- 0.2 km/s, consistent with data from prior refraction profiles. The area of inefficient Pn and Sn propagation coincides with an area of Holocene volcanism and suggests that anomalous upper mantle underlies much of the Arabian Shield.

Mellors, RJ, Camp VE, Vernon FL, Al-Amri AMS, Gharib AA.  2000.  Regional waveform propagation in the Arabian peninsula (vol 104, pg 221, 1999). Journal of Geophysical Research-Solid Earth. 105:6305-6305.   10.1029/2000jb900002   Website
Sutherland, FH, Vernon FL, Orcutt JA, Collins JA, Stephen RA.  2004.  Results from OSNPE: Improved teleseismic earthquake detection at the seafloor. Bulletin of the Seismological Society of America. 94:1868-1878.   10.1785/012003088   AbstractWebsite

Earthquake data from three ocean seismic network (OSN) sensors, located (1) on the seafloor, (2) buried in seafloor sediments and (3) in a borehole, together with those from Hawaiian Island stations, were compared by calculating threshold-detection magnitudes for P-, S-, Rayleigh-, and Love-wave arrivals. Our results show that the borehole seismometer had noise levels similar to those of the Island stations and produced high-quality high- and low-frequency body- and surface-wave data. Shallow burial of the seismometer in the sediments had no effect on higher frequencies but significantly reduced low-frequency noise levels so that data for S and Rayleigh waves were of high quality. In fact, the buried seismometer was characterized by the lowest noise levels at very low frequencies (<20 mHz; Collins et al., 2001). The ocean-floor seismometer was consistently noisy, and the data produced were of lower quality. Both observed magnitudes and calculated threshold magnitudes were lower by more than an order of magnitude than those observed in previous studies. Results for short-period body waves at the borehole instrument in particular were much better than those that were previously found for any ocean-bottom recording.

Schreiber, KU, Hautmann JN, Velikoseltsev A, Wassermann J, Igel H, Otero J, Vernon F, Wells JPR.  2009.  Ring Laser Measurements of Ground Rotations for Seismology. Bulletin of the Seismological Society of America. 99:1190-1198.   10.1785/0120080171   AbstractWebsite

Since the discovery of the wave nature of light, optical interferometry has assumed an important place in high precision metrology. This is mostly due to the inherent high sensor resolution for operational wavelengths in the vicinity of several hundred nanometers. In this context, interferometers in the Michelson configuration are most prominently used in gravitational wave antennas, such as the large projects VIRGO, LIGO, TAMA, and GEO600. In the Sagnac configuration they are used for high resolution rotation monitoring such as the precise observation of Earth rotation. Modern large-scale ring lasers reach a sensitivity for the measurement of rotation of 1 prad/sec (with approximately 1 hr of averaging). Because of the comparatively short wavelengths employed, optical interferometers are extremely sensitive to small mechanical perturbations of the entire apparatus. These can be caused by deformations, thermal or mechanical stress, and instabilities in the alignment of the optical components at the level of about lambda/100. Ring lasers suitable for geophysical applications require a sensor resolution in the range of 10(-8) rad/sec and below. This demands a scale factor of the instrument that is only achievable with mechanical dimensions of the interferometer on the order of about 1 m(2) or larger. At the same time the necessary mechanical rigidity of the entire instrument has to be on the order of 5 nm. Currently, this has only been achieved with monolithic ring lasers made from blocks of Zerodur and installed in a temperature stabilized underground environment. However if long-term sensor stability is not required, compromises can be made and, in particular for studies of regional seismic events, it becomes feasible to build a heterolithic rotation sensor in a simpler and much cheaper way. Here, we report the design and first results from the GEOsensor, which has been specifically constructed for studies in rotational seismology. The sensor is operated at the Pi on Flat Seismological Observatory in Southern California.

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

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