Kilometer-Scale Fault Zone Structure and Kinematics Along the San Andreas Fault near Parkfield, California

Arizona State University, Scottsdale AZ

Investigators

Abstract

Earthquakes are an increasing threat to societies globally because of continued urban development into earthquake-prone areas, yet our understanding of earthquake behavior is limited. Fundamental understanding of faulting requires detailed description of the geometry of rock bodies and structures and their motions within and adjacent to fault zones. This project is providing such an explanation for the heavily instrumented and geophysically-imaged Parkfield section of the San Andreas Fault in south-central California.by defining the positions, attitudes, and characteristics of the major rock bodies along the San Andreas Fault, and determining the rates of active deformation associated with the principal structures of the fault zone. The detailed geologic and geomorphic data sets and geometric and kinematic models are valuable for addressing questions related to landscape development along strike-slip fault zones, the motions of blocks in the upper crust within and adjacent to strike-slip fault zones, and the material properties as defined by geologic history within and adjacent to this important fault zone. In addition, the data and inferences are essential for a complete understanding of the results from the EarthScope element: San Andreas Fault Observatory at Depth (SAFOD). Geologic and geomorphic mapping (1:6000 scale or greater) in an area within a 10 km radius of SAFOD is being performed. At a lower but still relatively detailed scale (1:24000 or greater), selected portions of the Parkfield region are being mapped to enhance interpretations of the structural geology, geomorphic, and geochronologic data. An apatite fission track study on samples from the Tertiary sedimentary rocks and the granitic rocks permits inference of the exhumation and rock uplift history for the area. The geomorphic analyses help document the geometries and rates of active geologic structures in the area, including the main San Andreas Fault, subsidiary strike-slip faults, and nearby dip-slip faults and folds. Such structures may be identified by their disruption of landforms and young deposits, and their geometries inferred by interpretation of the areal distribution of deformation associated with them. These investigations include establishing a terrace chronosequence for the major drainage system of the area, determining rates of deformation along the active tectonic structures, and improving the paleoseismic record. Rates of transport associated with landslides are being compared with exhumation rates inferred for the same blocks based on the apatite fission track thermochronology. The geologic and geomorphic mapping results and remotely sensed imagery will be compiled in a digital geospatial data system.