High Resolution Earthquake Location in the New Madrid Seismic Zone

Meredith Dunn, Steve Horton, Heather DeShon, and Christine Powell

Earthquake relocations are being calculated in the New Madrid Seismic Zone (NMSZ) using the Double Difference algorithm developed by Waldhauser and Ellsworth (2000).  The New Madrid Seismic Zone is one of the few places on Earth where earthquakes occur frequently without the presence of a plate boundary.  Double-Difference relocations are especially useful in areas where the distance between neighboring events is small.    Our dataset consists of 1394 earthquakes recorded between 2000 and 2006.  The catalog data consists of approximately 280,000 P-wave observations and 195,000 S-wave observations from 197 stations.  The inversion of catalog data alone produces small changes in the original earthquake locations. Tests show that a high P to S velocity ratio (>3) in the shallow surface layer has little effect on locations when inverting synthetic data. Waveform cross-correlation travel time differences have been computed. The cross-correlation data and the catalog data are incorporated into the inversion using a variety of weighting schemes.  Results using both cross-correlation and catalog data exhibit a tightening of hypocenters within individual segments of the seismic zone.  The robustness of these results is being established through the use of additional synthetics. The Double-Difference tomography algorithm developed by Zhang (2003) will also be used with the catalog and cross-correlation data from this study to produce a three-dimensional P- and S-wave velocity model of the region.