Collaborative Research: Continuation of a GPS Study of the Subduction Earthquake Cycle: Jalisco, Mexico

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Funds for a four-year, third and final phase of a GPS-based study of theColima-Jalisco region of western Mexico are requested. Since early 1995,two large magnitude earthquakes (M=8.0 and M=7.6) have ruptured the MiddleAmerica subduction interface directly offshore from a 30-station GPS network,most recently in January of 2003. These earthquakes have afforded an exceptionalopportunity to study the coseismic and postseismic effects of largesubduction earthquakes, including the gradual decay of postseismic faultafterslip and viscoelastic flow and re-emergence of interseismic elasticstrain accumulation. Studying these earthquakes and their after effectshave numerous intellectual merits, outlined below. Specific goals forthe proposed project are as follows: (1) Continue annual measurements of theregional velocity field to characterize spatial variations in viscoelasticflow and afterslip triggered by the Mw=8.0 Oct. 9, 1995 Colima-Jalisco andM_w=7.6, 1.22.2003 Tecoman earthquakes, (2) Construct a six-station continuousGPS network to better characterize the temporal evolution of these viscoelasticand afterslip responses and better separate them from the underlying linearelastic strain accumulation. Continuous measurements will enable determine ofwhether the Cocos/Rivera subduction interfaces accommodate downdip transientaseismic slip events similar to those recently described for Cascadia, Japan,and southern Mexico. (3) Complete modeling of the coseismic and post-seismiceffects 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, andsensitivity analysis for our assumed elastic constants, layer thickness,and subduction interface geometry. (4) Employ measured variations inthe direction and magnitude of interseismic strain accumulation toboth estimate the degree of locking along the Rivera and Cocos platesubduction interfaces and relate onshore strain accumulation tothe oceanic plate motion. (5) Determine how the 1995 Colima-Jalisco earthquakeand subsequent processes altered stress levels in the adjacent Tecoman earthquakerupture zone. (6) Extract information about upper plate deformation associatedwith the continental rifts in our field area from GPS, seismic, and ShuttleRadar Topographic Mission data. As has been the case in previous stages ofthis project, the present phase of the project will have numerous broader impacts.Three Mexican scientists will participate as official collaborators: Osvaldo Sanchezof UNAM, Bertha Marquez of the Universidad de Guadalajara, and Gabriel Reyes ofthe University of Colima, as well as Mexican undergraduate and graduate studentsfrom three universities. In addition, support is requested for a UW graduate studentto 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 newcontinuous sites will constitute a useful legacy for other investigators, particularlyfor detecting and characterizing transient signals related to aseismic slip alongthe subduction interface.