Back to Fall 1999 AGU abstract list.
Back to the News
|
Refraction/Reflection Tomography of Wide-Angle Seismic Data from the
Cascadia Subduction Zone Offshore the Olympic Peninsula, WA Ingo A Pecher 1 (+1-512-232-3203; ingo@ig.utexas.edu) Uri S ten Brink 2 Jie Zhang 3 1U. Texas, Inst. Geophys., 4412 Spicewood Springs Rd., Austin, TX 78731, United States 2U.S. Geol. Survey, Woods Hole Field Center, Woods Hole, MA 02543, United States 3GeoTomo LLC, 225 S. Boulder Rd., Suite 201, Louisville, CO 80027, United States Large pre-historic earthquakes were probably generated at the Cascadia subduction zone. The rupture characteristics of these earthquakes, however, are unknown. Detailed crustal structure, in particular along the plate boundary may help constrain some of these characteristics. We present results from an ocean bottom seismometer (OBS) line across the continental margin offshore the Olympic Peninsula, WA. The line was collected as part of a seismic reflection and OBS survey in 1996 by Geomar, Germany, and the USGS. We applied a combined refraction/reflection tomographic inversion technique to the OBS wide-angle arrivals. The tomography uses a graph method for forward modeling and a conjugate gradient approach for model update. The first arrival data are initially inverted and the resulting velocity model is used as a starting model to invert for secondary arrivals, such as reflections from the top and base of the oceanic crust. The model roughness as part of the objective function was allowed to increase during subsequent iterations. We used the final velocity model to convert a coincident normal-incidence seismic reflection profile to depth. In the sediments landward of the toe of the accretionary prism, we resolved increased velocities on the seaward side of thrust faults. A high-velocity zone (HVZ) with velocities of over 5 km/s is located above the oceanic crust about 60 km landward of the toe. The top of this zone coincides with discontinuous reflections in the normal-incidence profile. The HVZ may be terminated landward by a major thrust fault. A deep sedimentary basin with velocities of about 4 km/s down to 7 km depth is located landward of the HVZ. We were able to analyze arrivals from top and base of the subducted oceanic crust down to about 80 km east of the toe of the accretionary prism. Velocities in the upper oceanic crust increase landward of the toe possibly due to dewatering. A considerable thickening of the oceanic crust landward is not evident from our data. We will discuss these observations in light of the seismologic risk at the Cascadia subduction zone. |
Meeting: Meeting Section: Special Session: Index Terms: Theme: |
Last Modified: October 8, 1999
Comments: UTIG webmaster