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Rebollar, CJ, Quintanar L, Castro RR, Day SM, Madrid J, Brune JN, Astiz L, Vernon F.  2001.  Source characteristics of a 5.5 magnitude earthquake that occurred in the transform fault system of the Delfin basin in the Gulf of California. Bulletin of the Seismological Society of America. 91:781-791.   10.1785/0120000077   AbstractWebsite

Portable and permanent broadband seismic stations in the neighborhood of the Gulf of California recorded a moment magnitude M-w 5.5 event on 26 November 1997. This is the first time that a moderate event located in the Gulf of California extensional province was well recorded by local broadband seismic stations. The event was located at 29.754 degrees N and 113.708 degrees W and at a focal depth of 5.0 km in the southeastern end of the transform fault that connects the lower and upper Delfin basins. The hypocentral location and the results of the wave modeling indicate that this is a complex event that originated in the pull-apart Delfin basin. The focal mechanism estimated from first motions (phi = 310 degrees, delta = 83 degrees, lambda = 97 degrees) and body-wave modeling of P waves in the frequency band 0.05-0.5 Hz suggests that the rupture started with dip-slip (reverse faulting) motion and ended releasing the bulk of energy through strike-slip motion. Synthetics of surface waves in the frequency band 0.050. 1 Hz were also calculated using a triangular source-time function of 3 sec. The best match between the synthetics and observed surface waves recorded at 90 km from the epicenter was obtained using a fault geometry defined by a strike of 330 degrees +/- 15, dip 85 +/- 5, and slip of 165 degrees +/- 15. The spectral analysis of the Lg phase recorded at stations in the Peninsular Ranges gives a seismic moment of 1.28 X 10(17) N m (1.28 X 10(24) dyne cm), a source radius of 6.3 km and a stress drop of 0.22 MPa (2.2 bar). The source parameters inferred with S-wave spectra and the same model (Brune, 1970) give similar values.

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.

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.