Examining the mechanical behavior and evolution of the southern San Andreas fault system through determination of late Quaternary slip rates and distinct element simulations

Sequence of incremental strain (top panels) and differential stress (bottom panels) distributions in a simulated two fault system, undergoing right lateral shear at a constant far-field strain rate, under a confining pressure of 30 MPa. Images record incremental slip over 0.45 m of relative wall displacement (0.075 \% shear strain), but are plotted at intervals of 4.5 m displacement. Red and blue colors denote right- and left-lateral shear strain, respectively, with color intensity scaled with slip magnitude. Right-lateral slip is localized onto the fault surfaces, but shows significant variations over time, and transfer of slip between fault strands. Blue "bursts" adjacent to the fault surfaces indicate earthquake-like slip events on the fault strand as locked zones are released, causing rebound of the adjacent wall rocks. Inclined zones of high differential stress (hotter colors) support compressive stresses within the domain, and correlate with locked zones along the faults. Stress releases correspond with fault rupture events (Images courtesy of J. Morgan and T. Fournier).

Project Summary To understand the processes and mechanisms driving fault growth and crustal deformation we combine (1) geologic field mapping of offset landforms, (2) Quaternary geochronology to obtain precise estimates of fault slip rates, and (3) distinct element models (DEM) to simulate the behavior and interactions of faults within the southern San Andreas fault system. The southern San Andreas fault zone of the SAFS is an ideal structure to investigate and model processes of crustal deformation because a high rate of strain is localized on a relatively simple set of faults, the Mission Creek and Banning faults. Furthermore, it is the most poorly understood section of the southern SAFS with respect to slip rate and timing of past large-magnitude earthquakes; therefore, its seismic hazard is difficult to quantify. Nonetheless, the seismic hazard that it presents is likely to be high since this section of the fault system is the only major section that has not ruptured in historic time. The last earthquake to rupture occurred over 300 years ago c. 1690. Accordingly, the long lapse time since the last surface rupture implies that this section of the SAFS is in the late phase of its earthquake cycle, and that strain accumulated over the past 300 years is likely to be relieved in a large-magnitude earthquake.

Tools LiDAR data, 10Be and U-series geochronology, and distinct element simulations

Geographic Location San Andreas fault zone in the Coachella Valley

Group Members Involved Kimberly Blisniuk <Email> <Personal Web Site>
Roland Bürgmann

Project Duration Jan 2012 to Jan 2014

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