HR: 14:40h
AN: S33C-05    [Abstracts]
TI: The effect of the 2004 M6.0 earthquake on small repeating earthquakes recorded downhole at Parkfield
AU: * Sonley, E
EM: esonley@bu.edu
AF: Department of Earth Sciences Boston University, 685 Commonwealth Ave., Boston, MA 02215 United States
AU: Abercrombie, R E
EM: rea@bu.edu
AF: Department of Earth Sciences Boston University, 685 Commonwealth Ave., Boston, MA 02215 United States
AB: We investigate the sources of the three clusters of repeating M2 earthquakes targeted by SAFOD. They repeat regularly before the M6.0 mainshock, and then more frequently immediately after. The seismic source parameters of all three clusters are similar to those of small earthquakes elsewhere. The spectra of the earthquakes in the two shallow clusters appear unaffected by the M6.0, but the deeper cluster shows small but systematic changes in frequency content. Progress in understanding the dynamics of earthquake rupture is currently limited by the large uncertainties in the observed source parameters. This is particularly a problem for small earthquakes, the source parameters of which are needed to understand fundamental processes of earthquake rupture at all scales. Parkfield represents an ideal setting to improve our knowledge of small earthquakes for a number of reasons: downhole, high frequency recording by the HRSN began in 1987 providing exceptional, long-term borehole azimuthal coverage; the instrumentation in the SAFOD pilot hole (PH) began in 2002 greatly increasing the high frequency bandwidth; and the presence of highly clustered seismicity, including many repeating events, enables the use of empirical Green's function (EGF) analysis. Smaller earthquakes in a cluster can be used to represent the earth's transfer function between source and observer. The transfer function can then be removed, in either the time or frequency domain, from a co-located, similar earthquake of larger magnitude. Preliminary work by Abercrombie and Nadeau (2004) and Imanishi (2004) found that the stress drops of small earthquakes in Parkfield, including the two SAFOD target earthquakes in October 2003 (M2 and M2.2) and those in another cluster (M1) are similar to the stress drops found by seismic methods for earthquakes elsewhere. These results contrast with the extremely high stress drops previously suggested for these small earthquakes based on the fault slip rate (Nadeau and Johnston, 1998). We use nearby M1 clusters to represent the Green's Function in order to better resolve static parameters and radiated energy for repeating earthquakes between 1990 and 2005 recorded down-hole on the HRSN, in Parkfield, CA.
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: Fall Meeting 2005