HR: 16:30h
AN: S54A-03    [Abstracts]
TI: Seismicity Patterns Before the Mw 6.5 San Simeon Earthquake of 22 December 2003
AU: * McLaren, M K
EM: mkm2@pge.com
AF: Pacific Gas and Electric, 245 Market Street Mail code N4C, San Francisco, CA 94105 United States
AB: The Mw 6.5 San Simeon Earthquake of 22 December 2003 was the largest earthquake to occur in the south-central coastal region of California since the 1952 ML 6.2 Bryson earthquake. In response to this earthquake we have updated our catalog to study pre-mainshock seismicity patterns and monthly rates. The catalog consists of 2800 earthquakes recorded in the region from Oct 1987 to 19 Dec 2003 by the PG&E and USGS networks. Seismicity patterns over the 17-year period have been relatively consistent, generally coinciding with faults and areas of tectonic uplift, including the offshore regions N and S of the Santa Maria Basin, and onshore within the uplifting blocks of the Los Osos structural domain (LOD) and along the NW trending San Lucia Range. Seismically quiescent areas are the offshore Santa Maria Basin, onshore Santa Maria Valley and smaller onshore regions N and E of Estero Bay. Events constrained to horizontal and vertical errors less than 2 and 5 km show the depth of the seismogenic zone to be 12 to 15 km throughout the region, but nearly 20 km at the NW end of the Santa Lucia Range. Focal mechanisms show predominantly reverse and reverse oblique faulting within the Santa Lucia Range, along the uplifting structural blocks of the LOD, and the offshore region west of San Simeon. Right lateral strike-slip mechanisms are predominantly along the Hosgri fault zone and to the east in Estero Bay. Seismicity patterns along the Santa Lucia Range show three main spatial clusters extending SE from N of San Simeon, near Ragged Point to where the Oceanic and Nacimiento faults converge. The San Simeon mainshock and aftershock locations are consistent with previous seismicity. The mainshock hypocenter is at the NW end of the middle cluster at 11.3 km depth, near the base of the seismogenic zone. The aftershocks occur in two main clusters, one nearly coincident with the mainshock and the middle cluster and other coincident with the NW half of the SE cluster. Variations in the seismicity patterns prior to the San Simeon earthquake were: 1) a linear NW trend of 11 M2 and smaller events that occurred NW of the mainshock between Apr 1997 and Aug 2003, 2) a drop in the seismicity rate between about Mar 1999 and Mar 2000 to about 5 events per month, and 3) an increase (43 events) in Sept 2003, which was substantially above the 11 events per month average. Most of the Sept 2003 events are scattered across the northern part of the study area, including the Santa Lucia Range and are not localized near the mainshock or aftershock region. No foreshocks were observed in the catalog.
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting