HR: 0830h
AN: S21D-0327    [PDF]
TI: Earthquake Relative Relocation Near the Mendocino Triple Junction
AU: * Evangelidis, C
EM: ce1@soc.soton.ac.uk
AF: SOC, School of Ocean and Earth Science, Southampton Oceanography Centre, University of Southampton, Southampton, SO14 3ZH United Kingdom
AU: Shearer, P M
EM: shearer@igpp.ucsd.edu
AF: IGPP, Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92037 United States
AB: We relocate 30664 events recorded from the Northern California Seismic Network (NCSN) between 1977 and 2002 near the Mendocino Triple Junction. Although absolute earthquake locations depend on the three--dimensional velocity structure of the Earth, relative event locations within seismicity clusters can be accurately improved through the use of source-specific station terms and waveform cross correlation. The Mendocino Triple Junction (MTJ) is formed where the Pacific, North American, and Juan de Fuca/Gorda plates interact. The area is one of the most seismically active regions in California. Active source seismic tomography of the MTJ 1993-94 PASSCAL wide--angle seismic experiment revealed a smooth three-dimensional velocity model of the area. Dividing the study area into clusters of similar seismicity, we relocate the regional earthquakes using local 1D velocity models extracted from the smooth 3D model. Using the appropriate velocity model for each region, P and S travel time tables are computed and compared with the picked arrivals from the NCSN. Source specific station terms (SSST) are obtained for each receiver--event pair as the weighted median of the residuals at the given station from a number of nearby events. An iterative approach that gradually reduces the number of nearby events ensures that events that belong to relatively small clusters are responsible for the final station corrections. The relative locations of $\sim$5000 events are further improved with waveform cross correlation of closely spaced events with similar waveforms. Results reveal a significant improvement in the relative locations of events, both laterally and in depth, within different seismicity clusters. Although the absolute location of each cluster is not fixed, events within the clusters tend to concentrate around known and apparent fault lines. The improved event locations give new insights into seismic areas within the North American plate, including the region north of Clear Lake, and offshore areas within the Gorda plate and the subduction zone.
UR: http://igpp.ucsd.edu/~christos/
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8010 Fractures and faults
DE: 9350 North America
SC: Seismology [S]
MN: 2003 Fall Meeting