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Testing Small-Aperture Array Analysis on Well-Located Earthquakes, and Application to the Location of Deep Tremor

Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Via Diocleziano, 328, 80124 Napoli, Italy mlarocca{at}ov.ingv.it galluzzo{at}ov.ingv.it

Steve Malone and Wendy McCausland^*

Department of Earth and Space Sciences, University of Washington, Box 351310, Office: ATG-226, Seattle, Washington 98195 steve{at}ess.washington.edu

Gilberto Saccorotti and Edoardo Del Pezzo

Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Via Diocleziano, 328, 80124 Napoli, Italy delpezzo{at}ov.ingv.it

^* Present address: Cascade Volcano Observatory, U.S. Geological Survey, 1300 Southeast Cardinal Street, Building 10, Suite 100, Vancouver, Washington, 98683-9589; wmccausland@usgs.gov.

Present address: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via U. della Faggiola, 32, 56126 Pisa, Italy; saccorotti@pi.ingv.it.

We have here analyzed local and regional earthquakes using array techniques with the double aim of quantifying the errors associated with the estimation of propagation parameters of seismic signals and testing the suitability of a probabilistic location method for the analysis of nonimpulsive signals. We have applied the zero-lag cross-correlation method to earthquakes recorded by three dense arrays in Puget Sound and Vancouver Island to estimate the slowness and back azimuth of direct P waves and S waves. The results are compared with the slowness and back azimuth computed from the source location obtained by the analysis of data recorded by the Pacific Northwest seismic network (PNSN). This comparison has allowed a quantification of the errors associated with the estimation of slowness and back azimuth obtained through the analysis of array data. The statistical analysis gives _BP=10° and _BS=8° as standard deviations for the back azimuth and and for the slowness results of the P and S phases, respectively. These values are consistent with the theoretical relationship between slowness and back azimuth and their uncertainties.

We have tested a probabilistic source location method on the local earthquakes based on the use of the slowness estimated for two or three arrays without taking into account travel-time information. Then we applied the probabilistic method to the deep, nonvolcanic tremor recorded by the arrays during July 2004. The results of the tremor location using the probabilistic method are in good agreement with those obtained by other techniques. The wide depth range, of between 10 and 70 km, and the source migration with time are evident in our results. The method is useful for locating the source of signals characterized by the absence of pickable seismic phases.