HR: 09:15h
AN: S41D-06    [Abstracts]
TI: Distinct Geodetic and Seismic Up-dip Limits to the Northern Costa Rica Seismogenic Zone: Evidence for Two Mechanical Transitions
AU: * Schwartz, S Y
EM: sschwartz@es.ucsc.edu
AF: Earth Sciences Dept. and IGPP, University of California, Santa Cruz, Santa Cruz, CA 95064 United States
AU: DeShon, H R
EM: hdeshon@es.ucsc.edu
AF: Earth Sciences Dept. and IGPP, University of California, Santa Cruz, Santa Cruz, CA 95064 United States
AB: Results from CRSEIZE (Costa Rica Seismogenic Zone Experiment), a joint seismic and geodetic experiment designed to elucidate seismogenic processes at the southern end of the Middle America Trench, indicate distinct patterns of geodetic strain accumulation and interplate microseismicity in northern Costa Rica. Geodetic inversions for interseismic slip on the plate interface reveal locking equal to $\sim$75% of the plate velocity between 8-15 km depth below sea-level while shallow interplate microearthquakes begin deeper, between 15-18 km below sea-level, in a region that appears to be freely slipping. Interplate microseismicity terminates at $\sim$30 km depth along the up-dip edge of a second patch of partial geodetic locking that represents $\sim$30-40% of the plate velocity. The strike parallel alternation of geodetically locked regions with no microseismicity and freely slipping regions with abundant microseismicity suggests that down-dip increases in temperature and pressure may generate two distinct transitions in mechanical behavior along the plate interface. We interpret the up-dip limit of the shallow geodetically locked patch as the frictional stability transition from stable sliding to stick-slip behavior. The shallow transition correlates with models of the 100$^{o}$C isotherm. The up-dip limit of interplate seismicity and the freely slipping zone most likely correspond to a change in mechanical properties along the plate interface; increased pore fluid pressure resulting from basalt dehydration reactions and/or decreased permeability in the upper plate may lead to fault weakening. Modeled temperatures at the deeper transition to the onset of seismicity are estimated at 200-250$^{o}$C, in the range where low-grade metamorphic reactions potentially release 0.3-3 wt % water and experimental data reveal significant decreases in permeability for a variety of rock types.
DE: 8045 Role of fluids
DE: 7209 Earthquake dynamics and mechanics
DE: 1242 Seismic deformations (7205)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting