Slip characteristics of San Andreas Fault transition zone segments

Transition zones are areas of mixed behavior that divide areas of velocity strengthening and velocity weakening frictional parameters. Their slip characteristics have implications for the underlying mechanism for interseismic creep, the relationship between aseismic slip and earthquakes, and the seismic potential of the transition zones. Two transition zones on the San Andreas fault in California, USA are included in this work; the San Juan Bautista and the Parkfield segments. They are analyzed in three phases of the earthquake cycle; the interseismic, coseismic and postseismic.; The San Juan Bautista segment currently undergoes only moderate seismicity. However, six M≥6 earthquakes occurred near the SJB segment between 1840 and 1899. A joint inversion of Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) measurements was performed to determine its current rate and distribution of interseismic creep. The model resolves two low-slip asperities surrounded by creep, indicating that its behavior arises from the heterogeneous distribution of fault frictional properties.; InSAR and GPS data were also used to constrain models of coseismic and post-seismic slip in the 2004 Parkfield earthquake. The models indicate that coseismic and postseismic slip occurred in separate regions of the fault, suggesting that the distribution of frictional parameters on the fault exerted some control over the size of the earthquake. The postseismic model included nearly equal amounts of slip as the coseismic, suggesting that this is an important method of relieving stress along areas of the fault that slip aseismically and that these areas may not participate in earthquakes.; The sensitivity of the Parkfield segment to outside stresses was also explored. Static stress changes from the 2003 San Simeon earthquake encouraged right-lateral strike slip on the Parkfield segment. While there is no clear correlation between the distribution of slip in the 2004 Parkfield earthquake and stress changes from the San Simeon earthquake; the 2004 event's hypocenter occurred in an area that experienced increased shear stress. Small stress changes from San Simeon postseismic slip also correlate with the 2004 event's hypocenter, suggesting that the Parkfield segment may have responded very sensitively to the small stress changes imparted by the previous events.