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Scanning San Andreas Fault near Parkfield with a mini seismic array

Abhijit Ghosh

Published September 2013, SCEC Contribution #1946

Parkfield segment of the San Andreas Fault (SAF) and surrounding area host a unique blend of abundant regular earthquakes and tremor. It presents an opportunity to study a wide spectrum of the fault slip and explore possible connection between the end members of this spectrum. Substantial tremor activity has been reported at the southeastern edge of the patch that repeatedly breaks to produce damaging magnitude 6 earthquakes. I design and install a mini seismic array near Parkfield about 8 kms southwest of the surface trace of the SAF. The array consists of 18 3-component seismic stations with about 300 meters station spacing. The array, with an aperture of ~1 km, is located at Shandon and currently fully operational. So far, I analyze one month of continuous data and preliminary results indicate that the Shandon array is recording seismicity of the surrounding faults with high sensitivity. Application of a beam-backprojection algorithm [Ghosh et al., 2009] detects about ~300 minutes of tremor activity in a month using only the Shandon array (no network station is used). This is about 5 times more duration of tremor activity compared to a standard tremor detection technique [Nadeau and Guilhem, 2009] using data from existing seismic network that includes High-Resolution Seismic Network borehole stations and newly installed TremorScope stations. The array analyses show slowness parameters consistent with the majority of tremor being located at the southeastern and northwestern edge of the Parkfield rupture patch. Both are previously identified tremor sources, with the southeastern source near Cholame being the more active one by far. I found, however, that both sources are producing comparable duration of tremor activity. In addition, it appears that the array detects tremor that is not located at the SAF. Further investigation is being done to ascertain this possible tremor source. Overall, only a month of preliminary analyses using a solo array shows a significant improvement in tremor detection compared to a standard method using existing seismic network. The Shandon array is producing an enhance image of the base of the seismogenic zone where tremor occurs and detecting possible new sources. A higher level of tremor detection combined with a more complete and detail spatiotemporal distribution of tremor would lead to a better understanding of the dynamics of fault slip and possible interaction between fast and slow earthquakes.

Ghosh, A. (2013, 9). Scanning San Andreas Fault near Parkfield with a mini seismic array. Poster Presentation at SCEC Annual Meeting 2013.