Intellectural merit: Funds are requested for a joint geodetic and seismic study of the Oaxaca-Guerrero segment of the Middle America subduction zone, one of the few places on earth where land-based GPS and seismic observations can be used to study both the locked, seismogenic area of a subduction interface and region of deeper transitional slip, where episodic tremor and slip may originate. The principal scientific goal is to better understand the recently discovered phenomenon of episodic slip transients and its role in either relieving or increasing strain accumulating along the seismogenic portion of a subduction interface. A pilot study during the past year shows that such transients are occurring as frequently as every 12-24 months in this region, that some transients (1998 and 2004) originate far outside the Guerrero seismic gap, and that the transients may be accompanied by non-volcanic seismic tremors. Based on these previously unrecognized results, deployment for 24 months of an 8-station broadband seismic array and expansion of a regional continuous GPS array to include 4 critically located stations in a region where the transients appear to occur are planned. The new seismic array, which will span a broad gap between two dense seismic transects being installed as part of the already funded Caltech/UNAM Middle America Seismic Experiment, will allow detection and location of any episodic tremors in the study area. Continuous GPS measurements from the pre-existing GPS stations and nine new stations constructed by this group and collaborator Enrique Cabral of UNAM will provide critical complementary information about the source regions, magnitude, and timing of episodic slip. Data from an existing smaller-footprint 30-station continuous and campaign GPS array that DeMets and Cabral operate in Oaxaca above the Cocos-North America subduction interface will also be used to study in more detail deformation within a conspicuous seismic gap analogous to, but smaller than the well-known Guerrero seismic gap. The combined GPS and seismic arrays will be a powerful tool for studying subduction transients and non-volcanic tremor, and for imaging spatial and temporal variations in frictional coupling along the locked and transitional zones beneath our network. Brudzinski, who will be a new faculty member at Miami University of Ohio in August of 2005, will purchase the necessary seismic equipment with start-up funds from his faculty position and will have it prepared for a two-year deployment in early 2006. DeMets at UW-Madison will coordinate the GPS station installation with Dr. Cabral, as well as a modest expansion of the Oaxaca GPS network to improve coverage of the aforementioned seismic gap. The UW-Madison geophysics group has a long history of deploying seismic and GPS networks and will provide at no cost two expert technicians to help build and maintain the combined seismic and GPS network. Responsibilities for modeling of the geodetic and seismic observations will lie respectively with DeMets and Brudzinski. Broader impacts: Episodic tremor and slip are a newly observed phenomenon that appears to be related to many different aspects of earth science (seismology, tectonics, petrology, and geodynamics), with possible ramifications for understanding triggering of megathrust earthquakes. Students at Miami University will be offered a unique opportunity to participate in a field workshop course that will coincide with the initial seismic deployment, thereby exposing students to a geophysics field research. Broader impacts at UW-Madison will include education of Mexican graduate student Francisco Correa-Mora, who will do a substantial portion of his dissertation research on the mechanics of subduction along the Middle America trench. The availability of new continuous and campaign GPS and seismic data to the broader geophysical community will also extend the impact of this work beyond our group, particularly once future transient slip episodes and/or tremor sequences are recorded.
