This project will continue a long-term GPS-based study of the seismically hazardous Mexico subduction zone (MSZ), where rapid plate convergence rates, the proximity of the trench to the coast, short earthquake recurrence intervals (≈50-100 years), and nearly flat subduction afford exceptional opportunities to measure and model all phases of the subduction earthquake cycle. The proposed work moves beyond their previous work in this region in several important respects. All theMexican investigators who operate significant numbers of GPS stations in mainland Mexico have agreed to collaborate on an integrated study of the MSZ earthquake cycle, making this the first study to integrate GPS observations from the entire 1600-km-long MSZ, where all phases of the earthquake cycle have been recorded two or more times by GPS. They will use GIPSY?s newly available single-station ambiguity-resolution feature and JPL?s new satellite products (due in mid-2011) to reprocess the pooled GPS data from ∼80 continuous stations and more than 100 campaign sites. The expected reductions in the station coordinate noise will permit more detailed studies of episodic transient slip, interseismic strain, and afterslip than were previously possible. The researchers will test and apply a new inverse procedure to estimate and isolate the contributions of interseismic elastic strain, episodic transient slip, coseismic rupture, postseismic fault afterslip, and postseismic viscoelastic rebound to GPS station time series. Specific hypotheses regarding the stationarity and depth limits of transient slip will be tested and viscoelastic modeling will be done to understand present deformation in coastal and interior areas of Mexico.
In the Jalisco/Colima region of western Mexico, large subduction thrust earthquakes in 1995 and 2003, fault afterslip triggered by those earthquakes, and recently discovered nonvolcanic tremor offer an exceptional opportunity to determine rigorously where these three processes originate along the subduction interface and what they reveal about the factors that determine downdip changes in the frictional state along a subduction interface. GPS and seismic data from the Michoacan, Guerrero, and Oaxaca segments of the MSZ will be used to study how and whether episodic transient slip in the two regions is connected in space or time, and where this slip occurs relative to the seismogenic zones and nonvolcanic tremor in both regions. The same data will be used to determine the distribution of interseismic coupling and hence seismic hazard along these trench segments. GPS data from the Chiapas trench segment will be used to test for transient slip there and estimate the degree of interseismic locking, toward a better understanding of the cause of the Chiapas seismic gap. Useful output related to hazards will include estimates of interseismic locking rates along most of the earthquake-prone MSZ, assessment of whether transient slip in southern Mexico loads or unloads lower areas of the seismogenic zone, and continuous monitoring of deformation rates on and around Volcan Fuego de Colima, Mexico?s most active volcano. A sub-award to UNAVCO will enable the continued operation and maintenance of the 35 continuous GPS stations built by DeMets and hisMexican collaborators, which constitute one-third of Mexico?s GPS infrastructure. Finally, the project will fund 2 UW graduate students. Outreach presentations to students, faculty, and Mexican government officials will be done by the project participants.