HR: 0800h
AN: S41A-0924    [Abstracts]
TI: A Structural Interpretation of Early Aftershock Double-Difference Event Locations from the 12/22/03 Mw 6.5 San Simeon Earthquake, Central California
AU: * Snyder, H A
EM: asnyder@usgs.gov
AF: U.S. Geological Survey - Parkfield, CA, P.O. Box 4958, Paso Robles, CA 93447 United States
AB: The San Simeon mainshock was located 11 km northeast of San Simeon on the California central coast. It nucleated at a depth of some 7 km, on a west-northwest-striking, moderately northeast-dipping thrust fault as determined by the focal mechanism, likely within the Oceanic-West Husuana Fault zone, but without any obvious surface expression in the immediate area. Aftershocks ranging from 2 to 9 km depths, migrated from the hypocenter along this trend to the southeast over some 30 km. Plan and sectional plots of double-difference locations for the first week after the event from Hardebeck et al. (2004) reveal three, distinct aftershock zones (intervals referenced southeast from the hypocenter): (1) a substantial zone extending from 0 to 13 km with a west-northwesterly-striking, northeast-dipping "hanging wall" pattern at 2 to 9 km depths, that is consistent with the mainshock focal mechanism, (2) a minor zone between 15 and 19 km at 2 to 4 km depths, that is of indeterminate orientation and (3) another substantial zone extending from 21 to 33 km with a north-northwesterly-striking vertical pattern at 2 to 8 km depths. The aftershock propagation may have been arrested on the southeast end by the zone 3 structure. Interestingly, the proposed truncating structure coincides with an 8 km-long area of northwest to north-northwesterly-striking, symmetric synclines with 4 km wavelengths mapped by Seiders (1982). Perhaps, the vertical feature formed along a synformal axial plane. Over the next several weeks after the mainshock, substantial seismic in-filling occurred among the three earlier, distinct aftershock zones.
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
SC: Seismology [S]
MN: 2004 AGU Fall Meeting