HR: 16:35h
AN: S54A-03    [Abstracts]
TI: Seismicity and structure of the central Cascadia shallow megathrust
AU: * Trehu, A
EM: trehu@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University Ocean Admin Bld 104, Corvallis, OR 97331-5503 United States
AU: Nabelek, J
EM: nabelek@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University Ocean Admin Bld 104, Corvallis, OR 97331-5503 United States
AU: Braunmiller, J
EM: j. braunmiller@sed.ethz.ch
AF: Swiss Seismological Service, ETH-Hoenggerberg, Zurich, CH-8093 Switzerland
AB: That the Cascadia subduction zone ruptures in occasional large earthquakes on the megathrust is now generally accepted. Recent paleoseismic work indicates that while the average rupture interval is ~560 years, the most common time period between events is 200-400 years, and the most recent event occurred on January 26, 1700. Instrumental seismicity, however, has not been documented for most of the shallow, nominally locked, part of the megathrust. One exception is a section of the continental margin offshore central Oregon where several small earthquakes are detected each year by the Pacific Northwest Seismic Network. During July and August, 2004, the region experienced two moderate and widely felt earthquakes with moment magnitude 4.9 and 4.8, respectively. Both events appear to have been thrust earthquakes with a shallow east-dipping nodal plane consistent with rupture along the megathrust. Crustal models based on active source seismic and gravity data indicate the presence of a buried basement ridge that abuts against crystalline rocks of the subduction backstop, which is composed of an anomalously thick oceanic accreted terrane in this region. We compare the seismicity and structure in this region to seismicity and structure in the shallow part of other subduction zones that have experienced large earthquakes in historic time and discuss the impact of upper and lower plate structure on subduction zone earthquakes. A better understanding of plate interactions here is needed to determine whether this is a likely nucleation point for the next great Cascadia earthquake.
DE: 7215 Earthquake source observations (1240)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Seismology [S]
MN: Fall Meeting 2005