HR: 11:35h
AN: G32A-06    [Abstracts]
TI: Creep on the San Andreas fault near San Juan Bautista and its relationship to large historic earthquakes
AU: * Johanson, I A
EM: ingrid@seismo.berkeley.edu
AF: UC Berkeley, Department of Earth and Planetary Science 307 McCone Hall, Berkeley, CA 94720 United States
AU: B\"{u}rgmann, R
EM: burgmann@seismo.berkeley.edu
AF: UC Berkeley, Department of Earth and Planetary Science 307 McCone Hall, Berkeley, CA 94720 United States
AB: The San Juan Bautista segment of the San Andreas fault is an area of moderate seismicity that forms the transition zone between the creeping section and the locked Santa Cruz segment. Here the San Andreas fault experiences active surface creep and transient slip events (slow earthquakes), which have been measured with creepmeters, strainmeters and short baseline trilateration arrays since the 1960s. The fault frictional properties that allow creep are thought to also prevent the nucleation and inhibit the rupture propagation of large earthquakes. In fact, the largest instrumentally recorded earthquakes on the San Juan Bautista segment have been between M5.3-5.5. However, between 1840 and 1899, historic records suggest six M$\geq$6 earthquakes ruptured the San Juan Bautista segment. These earthquakes occurred during a time of intense seismic activity in the Bay Area before the 1906 San Francisco earthquake, and the absence of more recent similar events may be attributable simply to the stress shadow produced by the 1906 quake. On the other hand, the San Juan Bautista segment experiences slip transients on time scales from days to years and it is possible that its rate and distribution of slip is highly time variable; for example, in relation to the regional stress state. We examine the current creep conditions on the San Juan Bautista segment using space geodetic data to determine whether they would allow a similar size and rate of earthquake production as seen in the historic record. We perform a joint inversion of GPS and InSAR data on distributed fault patches using an elastic half space model to determine the distribution of interseismic creep. We use GPS data from the B\={A}V\={U} dataset spanning 1994-2003. B\={A}V\={U} contains a combination of campaign and continuous data collected by several agencies and processed in a uniform manner using GAMIT/GLOBK. The InSAR data is taken from a stack of nine interferogram pairs made using ERS1 and ERS2 data collected between 1995-2001. We use a stack of small time span (less than 1.25 years) pairs because interferograms in this area become completely decorrelated when they span much more than two years. We choose the stacked interferograms such that the slave in one pair is the master in the next to remove the atmospheric errors from all but the very first and very last scenes. Our stack is then, in effect, a six year interferogram. Our model includes deep slip on the regional fault network, shallow creep on the nearby Calaveras fault and distributed creep on 3 $\times$ 5 km patches on the San Juan Bautista segment. We enforce a positivity constraint on all faults and Laplacian smoothing on the distributed segments. We also enforce a slip rate of 35 mm/yr on the creeping section to compensate for poor data coverage south of our study area. We find that below the shallow-most row of fault segments (3 km depth), the creep rate falls off considerably, causing a low slip zone at mid-seismogenic depths in the northern part of the San Juan Bautista segment that may represent the source region for some of the 19th century earthquakes. We consider further the earthquake potential of the low-slip zone and estimate a slip budget for the San Juan Bautista segment.
DE: 9350 North America
DE: 8150 Plate boundary--general (3040)
DE: 1206 Crustal movements--interplate (8155)
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting