Funds for a four-year, third and final phase of a GPS-based study of the Colima-Jalisco region of western Mexico are requested. Since early 1995, two large magnitude earthquakes (M=8.0 and M=7.6) have ruptured the Middle America subduction interface directly offshore from a 30-station GPS network, most recently in January of 2003. These earthquakes have afforded an exceptional opportunity to study the coseismic and postseismic effects of large subduction earthquakes, including the gradual decay of postseismic fault afterslip and viscoelastic flow and re-emergence of interseismic elastic strain accumulation. Studying these earthquakes and their after effects have numerous intellectual merits, outlined below. Specific goals for the proposed project are as follows: (1) Continue annual measurements of the regional velocity field to characterize spatial variations in viscoelastic flow and afterslip triggered by the Mw=8.0 Oct. 9, 1995 Colima-Jalisco and M_w=7.6, 1.22.2003 Tecoman earthquakes, (2) Construct a six-station continuous GPS network to better characterize the temporal evolution of these viscoelastic and afterslip responses and better separate them from the underlying linear elastic strain accumulation. Continuous measurements will enable determine of whether the Cocos/Rivera subduction interfaces accommodate downdip transient aseismic slip events similar to those recently described for Cascadia, Japan, and southern Mexico. (3) Complete modeling of the coseismic and post-seismic effects of the 2003 Tecoman earthquake using existing and new observations, including a joint inversion of the geodetic and seismic constraints, collaborative analysis of local seismic data for aftershock relocations, and sensitivity analysis for our assumed elastic constants, layer thickness, and subduction interface geometry. (4) Employ measured variations in the direction and magnitude of interseismic strain accumulation to both estimate the degree of locking along the Rivera and Cocos plate subduction interfaces and relate onshore strain accumulation to the oceanic plate motion. (5) Determine how the 1995 Colima-Jalisco earthquake and subsequent processes altered stress levels in the adjacent Tecoman earthquake rupture zone. (6) Extract information about upper plate deformation associated with the continental rifts in our field area from GPS, seismic, and Shuttle Radar Topographic Mission data. As has been the case in previous stages of this project, the present phase of the project will have numerous broader impacts. Three Mexican scientists will participate as official collaborators: Osvaldo Sanchez of UNAM, Bertha Marquez of the Universidad de Guadalajara, and Gabriel Reyes of the University of Colima, as well as Mexican undergraduate and graduate students from three universities. In addition, support is requested for a UW graduate student to continue his finite element modeling of the geodetic data for his dissertation, a Caltech graduate student who will work with Gabriel Reyes on local seismic data, and two REU undergraduates who will work under Stock's guidance. Data from the new continuous sites will constitute a useful legacy for other investigators, particularly for detecting and characterizing transient signals related to aseismic slip along the subduction interface.
