Observations and models of postseismic deformation: Constraints on the ductile strength of continental lithosphere

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Large shallow earthquakes generate sudden stress perturbations in the ambient rocks that may induce a ductile response of the lower crust and the uppermost mantle. Much improved global imaging of surface deformation due to the existing Synthetic Aperture Radar satellites (in particular, the L-band ALOS mission) allows us to measure the delayed postseismic response of the crust and upper mantle to the coseismically induced stress changes in areas of recent large earthquakes (most of which occurred in remote and poorly accessible regions). Measurements of postseismic transients bear on a long-standing debate on the effective rheology of the lithosphere below the brittle-ductile transition. Commonly considered models of postseismic transients include enhanced creep on a seismic rupture or its extension below the brittle-ductile transition (the so-called afterslip), poro-elastic rebound of the fluid-saturated crust, and visco-elastic relaxation of the lower crust or upper mantle. A robust discrimination (or some assessment of relative importance) of these mechanisms is often hindered by similarities in the predicted patterns of surface displacements. Such discrimination is nevertheless critical for our understanding of post-earthquake stress changes, delayed triggering of seismicity, and long-term strength of the ductile part of the continental lithosphere. We will use space geodetic observations and a new modeling approach based on elastic solutions for equivalent body forces in a spectral domain to study postseismic deformation due to three major recent earthquakes from diverse tectonic settings: the 2003 M7.2 Altai (Russia) earthquake, the 2006 M7.0 Mozambique earthquake, and the 2008 M7.9 Sichuan (China) earthquake. Data that will be used to constrain our models will primarily come from the ALOS mission of the Japanese Space Agency. The chosen events are characterized by different mechanisms (strike-slip, normal, and mixed thrust/strike-slip, respectively) which may greatly help with identifying a dominant relaxation mechanism and deducing the effective long-term strength of a ductile portion of the lithosphere in the corresponding locations. Detailed observations of post-seismic deformation in diverse tectonic settings will provide new constraints on the effective mechanical thickness and strength of the continental lithosphere. Such constraints will improve our understanding of continental tectonics. Physically-based models of post-seismic deformation might help forecast the evolution of stress and strain in the Earth crust following large earthquakes, and provide a useful input for seismic hazard estimates. Computer codes developed under the auspices of this project will be made available to a wider research community. Data and modeling results will be used in two graduate classes taught at SIO/UCSD.