HR: 08:50h
AN: T11E-04 INVITED     [Abstracts]
TI: The Influence of Fluids on Seismogenic Zone Processes in Northern Costa Rica
AU: * Schwartz, S Y
EM: sschwartz@es.ucsc.edu
AF: Earth Sciences Dept. and IGPP, University of California, Santa Cruz, Santa Cruz, CA 95064
AU: DeShon, H R
EM: hdeshon@geology.wisc.edu
AF: Dept. of Geology and Geophysics, University of Wisconsin, Madison, Madison, WI 53706
AU: Brown, K M
EM: kmbrown@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093
AB: Fluids have been increasingly identified as important in several seismogenic processes within subduction zones. They have been implicated in controlling the up and down dip frictional stability transitions as well as in the occurrence of episodic creep and tremor. The 1999-2001 Costa Rica Seismogenic Zone Experiment (CRSEIZE) collected abundant seismic, geodetic and fluid flow data in northern Costa Rica that allows the role of fluids in seismogenic processes to be investigated. Images of the up and down dip limits of geodetic locking and microseismicity indicate that the GPS defined locked region extends to shallower depth than the microseismicity. We interpret the up dip edge of geodetic locking as the frictional stability transition from stable sliding to stick-slip behavior. The location of this transition agrees well with models of pore pressure dissipation and of the 100-150°C isotherms. The deeper onset of interplate seismicity likely results from fault weakening caused by an increase in pore fluid pressure from basalt dehydration reactions and/or decreased permeability in the upper plate. Low P and S wave velocities in the shallowest mantle wedge are consistent with 10-20% serpentinization so contact with the plate interface here may be responsible for the down dip limit of the seismogenic zone. Continuous measurements of fluid flow at three flow meters located 30 km apart reveal periods of transient correlated flow that correspond with periods of increased seismic amplitudes on co-located OBS. We believe that repeated plate boundary creep events generate the observed signals with a poro-elastic stress/strain field simultaneously forcing flow through fracture networks in the forearc and oceanic basement rocks and inducing diffuse flow through the shallow sediments. The former generates the seismic tremor-like noise recorded by the OBS and the latter generates the flow transients recorded by the flow meters.
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8045 Role of fluids
DE: 8150 Plate boundary: general (3040)
SC: Tectonophysics [T]
MN: Fall Meeting 2005