The April 4, 2010, M7.2 El Mayor earthquake on the US-Mexico border was the largest earthquake to strike Southern California in the last 18 years. The main goals of this project are (1) to accurately capture the transient deformation following the earthquake using campaign GPS observations and (2) use precise geodetic observations, including continuous/campaign GPS and InSAR data to identify the physical mechanisms involved in postseismic relaxation. There is a vigorous debate about the nature of coupling between the upper seismogenic crust and the underlying ductile substrate. End-member models of interseismic deformation include ``bottom-up'' loading of crustal faults by localized shear zones in the lower crust (and perhaps upper mantle) and ``top-down'' loading of the viscous substrate by coseismic stress changes. A related question is what rheology best describes rocks below the brittle-ductile transition. Competing views include linear Maxwell, standard linear solid, bi-linear, powerlaw etc. rheologies. These questions have fundamental implications for our understanding of post-earthquake stress changes, delayed triggering of seismicity, and long-term strength of the continental lithosphere. Investigation of postseismic deformation due to the M7.2 El Mayor earthquake is offering new insights into these questions and allowing a robust test of the competing hypotheses.
