HR: 08:50h
AN: T51E-04    [Abstracts]
TI: Earthquakes and crustal structure beneath the central Cascadia continental margin
AU: * Trehu, A M
EM: trehu@coas.oregonstate.edu
AF: Oregon State Un., College of Oceanic and Atmospheric Science, Admin Bld 104, Corvallis, OR 97331, United States
AU: Braunmiller, J
EM: jochen.braunmiller@sed.ethz.ch
AF: Oregon State Un., College of Oceanic and Atmospheric Science, Admin Bld 104, Corvallis, OR 97331, United States
AU: Nabelek, J L
EM: nabelek@coas.oregonstate.edu
AF: Oregon State Un., College of Oceanic and Atmospheric Science, Admin Bld 104, Corvallis, OR 97331, United States
AB: In the summer of 2004, two clusters of "repeating" earthquakes occurred beneath the continental shelf of the central Cascadia subduction zone near 44.5N, 124.5W where the subduction megathrust is thought to be locked or transitional. The largest event in each cluster reached moment magnitude M=4.8-4.9. Seismicity has continued since with small (M<3) earthquakes occurring in each cluster on August 23-25, 2007. Moment tensor analysis for the main shock in each cluster indicates a 6-15 degree eastward dipping fault plane, consistent with the plate boundary dip of ~12 degrees. One cluster is serendipitously occurring on a transect along which crustal structure is well known from active source seismic experiments, and raytracing through this crustal model to match observed relative arrival times of secondary phases indicates a source depth of 16 ± 1 km, within 1 km of the plate boundary. This segment of the forearc also displays several characteristics indicative of along-strike and down-dip variations in plate coupling including: a subducted ridge on the downgoing plate; a "bright spot" on the plate boundary at a depth of ~15-20 km; an along-strike change in the gravity field and basement depth; a transition in plate coupling indicated by inversion of GPS data; geologic indications of active folding in the upper plate; and anomalous deformation in the adjacent oceanic plate. On the other hand, no obvious correlation with ETS in this region is observed. In September, 2007, we deployed an array of ocean bottom seismometers to record microseismicity and distinguish among several possible models for the physical properties of the megathrust.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting