HR: 09:45h
AN: S41B-07    [PDF]
TI: The Parkfield Stress Drop Controversy
AU: * Abercrombie, R E
EM: rea@bu.edu
AF: Boston University, Dept. Earth Science, 685 Commonwealth Ave., Boston, MA 02215 United States
AU: Nadeau, R M
EM: nadeau@seismo.berkeley.edu
AF: UC Berkeley, Berkeley Seismo Lab., 207 McCone Hall, Berkeley, CA 94720 United States
AB: Nadeau {\it et al.} (1995) found that the seismicity on the San Andreas fault at Parkfield is highly clustered. Individual clusters consist of a sequence of near periodically repeating small earthquakes of similar seismic moment. Nadeau and Johnston (1998) compared the moments and timing of these repeating earthquakes ($M_w < 2$), and some larger events, with the surface creep rate on the fault to estimate the slip and stress drop ($\Delta\sigma$). They obtained conventional values (0.1 to 10 MPa) for the larger earthquakes, but $\Delta\sigma$ increased with decreasing $M_w$ to extremely high values ($>$ 1000 MPa) for the small earthquakes ($M_w < 2$). Such values are just about physically possible, but they are much higher than those estimated by seismic methods for small earthquakes elsewhere (e. g. Abercrombie 1995). These controversial high $\Delta\sigma$ estimates have sparked a number of theoretical and laboratory studies (e. g. Sammis \& Rice 2000, Anooshepoor \& Brune 2001, Beeler 2001) aimed at investigating whether the observations could result from earthquakes with more normal $\Delta\sigma$ occurring within the unusual tectonic setting at Parkfield. Lane \& Nadeau (2000, 2002) considered whether localized patches of high $\Delta\sigma$ would be resolvable by standard seismic methods. However, to date nobody has used seismic methods to determine source parameters for these controversial small earthquakes at Parkfield. We use closely located earthquakes of different sizes (for example, the sub-clusters of cluster CL14, Nadeau {\it et al.}, 1995, $M_w-$0.2 to 1), recorded on the HRSN borehole network to analyse the source parameters. The smaller earthquakes are used as empirical Green's functions to resolve source processes of the larger events. Preliminary results from the earthquakes in cluster CL14 result in a source dimension of about 25 m and $\Delta\sigma$ of about 1 MPa for the $M_w$1 earthquakes, assuming that rupture velocity is the same as that for large earthquakes. We also resolve source-time functions for these earthquakes at most stations and so we can investigate the directivity and velocity of the rupture. Finally we compare the source parameter estimates from the seismic modeling, with those from recurrence and creep rate, and assess the validity of the various proposed models.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2003 Fall Meeting