Aftershock Characterization Using A Dense Array And Backprojection: A Quantitative Comparison Of Densely Spaced Geophones And Traditional Earthquake Arrays

John A. Hole*1, G. Didem Beskardes1,2, Qimin Wu1,3, Tyler W. Rasmussen1, Martin C. Chapman1, Kathy K. Davenport1,4, A. Christian Stanciu1,5, Larry D. Brown6, Diego A. Quiros6,7, Raymond M. Russo8

1Department of Geosciences, Virginia Tech, USA  (hole@vt.edu), 2now at: Geophysics Department, Sandia National Laboratories, USA, 3now at: ConocoPhillips School of Geology and Geophysics, University of Oklahoma, USA, 4now at: College of Earth, Ocean, & Atmospheric Sciences, Oregon State University, USA, 5now at: Department of Earth Sciences, University of Oregon, USA, 6Department of Earth & Atmospheric Sciences, Cornell University, USA, 7now at: Department of Geosciences, Baylor University, USA, 8Department of Geological Sciences, University of Florida, USA

After the 2011 Mw5.7 Virginia earthquake, a ~200-geophone array (AIDA) was deployed at 200-400 m spacing for 12 days. Backprojection was used to automatically detect and locate aftershocks. The AIDA backprojection aftershock catalog is complete to M(–0.5) and includes 1673 events. A network of 36 traditional stations was also deployed at 2-10 km spacing. The traditional network catalog is complete to M(–0.1) with 813 events. Only 494 of these, for a catalog completeness of M(+0.2), could be located with double-difference accuracy comparable to that of backprojection. The AIDA catalog observes the same seismicity pattern, but absolute uncertainty is reduced. Additional details illuminated by AIDA include: the main seismicity zone is not large faults but a tabular zone of many small faults; a broad zone of newly detected events lies above the main zone at shallow depth; a newly detected shallow cluster exists off the main zone; and the Gutenberg-Richterb-value decreases with depth. AIDA illustrates the benefits of dense arrays and backprojection for aftershock studies.

In 2012, coincident seismic surveys were acquired in Idaho. Broadband seismometers were deployed at quiet sites every 15 km. With similar effort per kilometre, 4.5-Hz geophones were deployed at roadside every 200 m. Nearby geophone and broadband seismograms and spectra were comparable down to <0.03 Hz for a M7.7 teleseism and down to <0.25 Hz for ambient noise. Inexpensive, rapidly deployed, passive seismometers produce good signal far below the corner frequency. Non-aliased array data provide back-azimuth and slowness information to enable wavefield imaging.

 

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