HR: 15:25h
AN: T42C-08    [PDF]
TI: Stress interaction between subduction earthquakes and forearc strike-slip faults: Modeling and observations
AU: * Ten Brink, U S
EM: utenbrink@usgs.gov
AF: USGS, 384 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Lin, J
EM: jlin@whoi.edu
AF: WHOI, 384 Woods Hole Rd, Woods Hole, MA 02543 United States
AB: We explore the interaction between thrust events on the subduction interface and strike-slip faults within the forearc using 3-D models of static coulomb stress change. Strike-slip faults in the forearc often present significant seismic hazard because of their proximity to population centers. Static stress change models show a relationship between slip vector azimuth and the locations of induced stress on strike slip faults in the forearc. Stress is enhanced on faults that are located closer to the trench and is reduced on faults farther inland for increasing slip obliquity, which is defined as the angle between the actual slip vector of a subduction zone and the vector normal to the overall trend of the subduction trench. Significant contribution to the stress change on strike slip faults in the forearc comes from "unclamping" of the fault (reduction in normal stress) due to thrust motion on the subduction interface. We use these models to explain the contrasting deformation pattern between two adjacent segments of oblique subduction of the North American (NOAM) plate under the northern Caribbean. The abnormally-thick crust of the Bahamas subducts under the Hispaniola trench, whereas normal oceanic crust subducts under the Puerto Rico trench to the east. GPS measurements indicate that oblique convergence under Hispaniola is partitioned between dip-slip reverse motion on the subduction interface and left-lateral strike-slip on known faults in the forearc at a distance of 60 km or more from the trench. Slip of large earthquakes under Hispaniola is dominantly dip-slip reverse motion. In contrast, GPS measurements in the Puerto Rico segment indicate that oblique convergence is not partitioned. The velocity vector of Puerto Rico relative to NOAM is similar (within error) to that of the Caribbean plate relative to NOAM, and there is no internal deformation within Puerto Rico itself. Focal mechanisms of moderate earthquakes in the past 25 years show a slip direction which is subparallel to the highly oblique plate motion vector. The major left-lateral strike-slip fault in this segment is located at a distance of 10-15 km from the trench, cuts through the accretionary prism, and extends to the subduction interface at a depth of only 5 km. The fault curves inland as it approaches the Hispaniola segment, but does not connect with strike-slip faults on Hispaniola. The jump from a strike-slip fault near the trench in the Puerto Rico segment to an inland fault in Hispaniola is explained by different stress distributions in the forearc of the two segments due to the change from oblique slip on the subduction interface in Puerto Rico to dip slip in Hispaniola. The observations and modeling suggest that seismic hazard to Puerto Rico from oblique subduction and from subduction-induced forearc strike-slip earthquakes is much smaller than previously assumed, but hazard to Hispaniola remains high.
DE: 7223 Seismic hazard assessment and prediction
DE: 8102 Continental contractional orogenic belts
DE: 8123 Dynamics, seismotectonics
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting