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Slip rates and rheology of the southern San Andreas-San Jacinto fault system from earthquake cycle models constrained by GPS and InSAR observations

Paul Lundgren(1), Eric Hetland(2), Kaj Johnson(3), Zhen Liu(1) and Eric Fielding(1)

(1) California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, United States
(2) California Institute of Technology, 1200 East California Blvd, Pasadena, CA 91125, United States
(3) Indiana University, 1001 East 10th Street, Bloomington, IN 47405, United States

Abstract

Estimates of fault slip rates across deforming plate margins from geodetic data are nonunique and depend upon assumptions in model parameters that may be poorly constrained and that trade-off with other parameters. Surface velocity profiles across active fault systems are a product of far-field loading, long-term slip rates and locking depths of the individual faults, the rheology of the crust and upper mantle, time since the last major earthquake on each fault, and the system’s seismic history. The southern San Andreas fault system (San Andreas, San Jacinto, and Elsinore faults, principally) has been the subject of a number of recent studies that seek to understand the effects of lateral variations in elastic and/or vertical variations in viscoelastic mechanical structure on estimates of fault slip rates as deduced from surface velocity measurements. There are known trade-offs in fault slip, mechanical structure, earthquake repeat interval, and the current relative time into the present earthquake cycle that make estimation of these parameters somewhat difficult, with differences between significantly different models resulting in rather small differences in the surface velocity profile. We apply two dimensional (2D) viscoelastic finite element models (FEM) of faults undergoing periodic earthquakes to explore the effects of rheology, inter-event time, and time into the earthquake cycle on estimates of long-term slip rates across the southern San Andreas fault system in southern California. Specifically we will explore a range of models incorporating lateral rheology (both elastic and time dependent) variations across this fault system that best fit the interferometric synthetic aperture radar (InSAR) mean line-of-sight (LOS) velocity field, and geodetic velocities from the SCEC CMM3 solution projected into the SAR LOS. The InSAR LOS velocities were calculated from a least-squares inversion of 61 interferograms from 33 SAR data images spanning the interval between the Landers (1992) and Hector Mine (1999) earthquakes. 2D viscoelastic FEM Green’s functions are calculated using the GeoFEST v. 4.5 software and/or a viscoelastic laterally heterogeneous boundary element software package. Green’s functions are calculated for a fully ‘spun-up’ edge driven earthquake cycle with an assumed fault locking depth and mechanical structure across the fault system. We use a Bayesian inversion method to solve for both fault slip on the San Jacinto and San Andreas faults and the most appropriate rheology across the fault system.

 

Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry