Internal structure and weakening mechanisms of the San Andreas Fault Academic Article uri icon

abstract

  • New observations of the internal structure of the San Gabriel fault (SGF) are combined with previous characterizations of the Punchbowl fault (PF) to evaluated possible explanations for the low frictional strength and seismic characteristics of the San Andreas fault (SAF). The SGF and PF are ancient, large-displacement faults of the SAF system exhumed to depths of 2 to 5 km. These fault zones are internally zoned; the majority of slip was confined to the cores of principal faults, which typically consist of a narrow layer (less than tens of centimeters) of ultracataclasite within a zone of foliated cataclasite several meters thick. Each fault core is bounded by a zone of damaged hot rock of the order of 100 m thick. Orientations of subsidiary faults and other fabric elements imply that 1) the maximum principal stress was oriented at large angles to principal fault planes, 2) strain was partitioned between simple shear in the fault cores and nearly fault-normal contractions in the damaged zones and surrounding host rock, and 3) the principal faults were weak. The localized slip and fluid-saturated conditions are wholly compatible with additional dynamic weakening by thermal pressurization of fluids during large seismic slip events which can help explain both the low average strength of the San Andreas and seismogenic characteristics such as large stress relief. -from Authors

author list (cited authors)

  • Chester, F. M., Evans, J. P., & Biegel, R. L.

citation count

  • 698

publication date

  • January 1993