HR: 12:00h
AN: T21E-07    [PDF]
TI: Detail Structure of San Andreas Fault Jog in Cholame Valley From Small-Scale Topography and Sesmic Data
AU: * Korneev, V
EM: vakorneev@lbl.gov
AF: Lawrence Berkeley National Laboratory, 90-1116, 1 Cyclotron Rd.,, Berkeley, CA 94720 United States
AB: The significance of the Parkfield area is in it being a transition zone between creeping NW and locked SE sections of San Andreas Fault (SAF), which makes it a most likely spot defining future seismic activity of the region. The detailed studies of seismicity generated by Parkfield 1996 M6 event revealed very high seismic activity in Cholame valley region SE of the Parkfield and ruptures along Cholame valley tracing the fault along the surface. In the middle of the valley SAF has about 1.5 km jog to SW after which it continues to go in SE staying parallel to the original direction. The jog geometry had no surface evidence and was determined on the basis of epicenter locations of multiple events in this area. Seismic and theoretical studies (Hanna, et. Al. 1972; Eaton et. Al. 1970; Harris and Segall, 1986) suggested a complex structure of the jog area, being and interacting segment of two parts of SAF. Such segments usually represent wide zones with multiple fractures oriented at 45 degrees with respect to main fault orientation and which connect isolated segments of a fault. The corresponding micro-fracturing was observed in multiple locations of Cholame valley. Extent and mechanical properties of interacting segments have strong impact on accumulated strain release. The fault structure studies in Cholame valley are mostly based on seismic information since valley sediments cover bedrock and do not allow accurate fault mapping. Nevertheless we have found that low amplitude topography features of the valley apparently affected by bedrock surface geometry. The fault can be traced along both sides through an existing point of connection of jog with SW strand of SAF. Also there is a wide shear zone of interaction between upper and lower parts of SAF. Multiple NS oriented faults which make 45-degree angles with traces of SAF are quite visible. Image features also suggest a straight continuation of NW part of the SAF in SE direction after passing the jog. In the same time the NW straight extension of SE part of the SAF connects it with mapped part of the South-West Fracture Zone (SWFZ). This result suggests the Cholame valley fault structure represent two faults SAF and SWFZ, which were separate at some point in history but began interacting through development of zone of shear faulting in the middle of the valley leaving some of their old parts temporarily inactive.
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
DE: 7230 Seismicity and seismotectonics
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting