HR: 14:00h
AN: S53D-02    [Abstracts]
TI: Time-Reversal to Estimate Focal Depth for Local, Shallow Earthquakes in Southern California
AU: * Pearce, F
EM: fpearce@mit.edu
AF: Earth Resources Laboratory, MIT, 77 Massachusetts Ave., Cambridge, MA 02139,
AU: Lu, R
EM: lurr@mit.edu
AF: Earth Resources Laboratory, MIT, 77 Massachusetts Ave., Cambridge, MA 02139,
AU: Toksoz, N
EM: toksoz@mit.edu
AF: Earth Resources Laboratory, MIT, 77 Massachusetts Ave., Cambridge, MA 02139,
AB: Current approaches for focal depth estimation are typically based on travel times and result in large uncertainties primarily due to poor data coverage and inaccurate travel time picks. We propose an alternative method based on an adaptation of time-reversed acoustics (TRA). In the context of TRA theory, the autocorrelation of an earthquake recording can be thought of as the convolution of the source autocorrelation function with the autocorrelation of the Green's function describing propagation between source and receiver. Furthermore, the signal to noise ratio (S/N) of stationary phases in the Green's function may be improved by stacking the autocorrelations from many receivers. In this study, we employ such an approach to estimate the focal depth of shallow earthquakes based on the time lag between the direct P phase and pP converted phase, which is assumed to be stationary across the receiver array. Focal depth estimates are easily obtained by multiplying half the pP time lag by the average velocity above the earthquake. We apply this methodology to estimate focal depths for several local earthquakes in Southern California. Earthquake recordings were obtained from the Southern California Earthquake Center (SCEC) for events with accurate, independent estimates of focal depth below about 15 km, and local magnitudes between 4.0 and 6.0. We observe pP in the stacked autocorrelations that correspond to the focal depths listed in the SCEC catalog for earthquakes located throughout Southern California. The predictive capability of the method is limited by S/N, defined as the pP amplitude divided by the background noise level of the stacked correlation. By considering subsets of the Southern California array, we explore the sensitivity of the S/N on station density and location (i.e. epicentral distance & azimuth). We find S/N is generally better for subsets of receivers within regions with relatively simple geologic structure. We are currently developing an extension of this methodology using the time- frequency correlation function, which may significantly reduce the station coverage required for accurate focal depth estimation.
DE: 7205 Continental crust (1219)
DE: 7215 Earthquake source observations (1240)
DE: 7219 Seismic monitoring and test-ban treaty verification
SC: Seismology [S]
MN: 2007 Fall Meeting