HR: 0830h
AN: G31B-0706    [PDF]
TI: Analysis of data from multiple geodetic networks for a spatio-temporal fault slip history at Parkfield, CA during the 1990s
AU: * Murray, J R
EM: jrmurray@pangea.stanford.edu
AF: Stanford University, 397 Panama Mall, Stanford, CA 94305 United States
AU: Segall, P
EM: segall@pangea.stanford.edu
AF: Stanford University, 397 Panama Mall, Stanford, CA 94305 United States
AB: Transitions from creeping to locked portions of the San Andreas fault have been seen to experience transient deformation. One such transition zone occurs at Parkfield, CA, a locale well-known for its history of M6 earthquakes. Despite a prediction that another M6 earthquake would take place in 1988 $\pm$ 5 years, the most recent such event at Parkfield was in 1966. In the early 1990s, however, rate changes were observed in data from the permanent two-color geodimeter network and tensor strain meters. There was a general increase in seismic moment release, and three M$\sim$4.5 earthquakes nucleated near the inferred hypocenter of previous Parkfield M6 earthquakes. More recently a concurrent magnetic field anomaly was identified. These observations have been interpreted to result from a transient increase in fault slip-rate on the San Andreas. However, to date analyses of the geodetic data have been limited to forward models and inversions for instantaneous rate changes. Here we present results of a time-dependent inversion using data from the permanent two-color network, two portable two-color networks, and GPS surveys in the Parkfield area. Our analysis uses an efficient nonlinear Kalman filtering technique well-suited to inferring slip-rate changes from data contaminated by random walk and white noise. This method exploits the fact that, unlike local benchmark noise, a deformation signal is spatially coherent. We have also explored means for simultaneously estimating the seasonal noise that is common in geodetic time-series. This type of analysis enables estimation of fault slip at every epoch for which there are data, resulting in a time-history of slip rather than a single rate change. Results to date show an increase in slip-rate on the upper $\sim$6 km of the fault near Middle Mountain between 1993 and 1995 followed by a decrease to the background rate by 1998. The Parkfield two-color network, established in 1984, provides a long baseline of temporally dense measurements. Thus, the analysis methods we discuss are applicable to modeling data from continuous GPS networks like PBO. While large deformation events are easily observed; here we demonstrate the potential for highlighting subtle slip-rate transients with continuous geodetic monitoring.
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
DE: 3210 Modeling
SC: Geodesy [G]
MN: 2003 Fall Meeting