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Detecting Ambient Tectonic Tremor in Southern California

Justin R. Brown, Susan E. Hough, & Jean-Paul Ampuero

Published December 2012, SCEC Contribution #1780

In every tectonic setting where it is observed, tremor proves difficult to detect due to its long durations and low amplitudes close to the noise band. This is particularly true in southern California where cultural noise sources are both spatially and temporally pervasive. Ambient tectonic tremor in circum-Pacific subduction zones was shown to consist of repeating low-frequency earthquakes and locates at the deep extent of the primary seismogenic zone in a region downdip of historical M 8+ earthquakes. The signals were also shown to have a distinct spectral shape (typically around 1-8 Hz) whose higher frequency falloff differs from that of local noise and ordinary earthquakes. In the strike-slip setting, families of deep ambient tremors near the Parkfield-Cholame segment of the San Andreas fault were considered proxies for deep fault slip. Whether ambient tremor occurs in a similar fashion- or even at all- in southern California remains unclear. We employ a combination of running autocorrelation and matched-filter techniques successfully used in both subduction and strike slip settings to detect and locate tremor/low-frequency earthquakes in southern California. In our case, however, detections are accepted or rejected by calculating their spectral shapes to assure the signals are distinct from local noise sources and earthquakes. We scan continuous seismic recordings of the Southern California Seismic Network near the San Jacinto fault from 2008-2012 to detect tremor-like signals, distinguish their spectral shape from other sources, and subsequently locate it. We target this area on account of the abundance of seismic stations including borehole stations. Also, since tremor (2-8 Hz) was triggered by the passing surface waves of the 2002 M 7.9 Denali earthquake in vicinity of the San Jacinto fault we expect an ambient signal to occur in the same area with a similar spectral shape. The application of a spectral discriminator is particularly needed in this region, where previous analysis identified recurrent transients with a spectrum peaked between 3 and 4 Hz associated with heavy train traffic along the Coachella Valley.

Brown, J. R., Hough, S. E., & Ampuero, J. (2012, 12). Detecting Ambient Tectonic Tremor in Southern California. Oral Presentation at AGU Fall Meeting 2012.