In this study the contribution of post-critical reflections at the oceanic Moho to ground motions from mega-thrust events in the Cascadia subduction zone is examined. The hypothesis to be tested is that the Moho post-critical reflection is a primary component of the S-wave-field at large epicentral distances from a subduction zone thrust earthquake. Pseudo 3-D modeling using P-SV pseudo-spectral synthetic seismograms and ray tracing amplitude calculations are employed. Double couple line sources are initiated within a structural model for the Cascadia subduction zone that incorporates an updated Juan de Fuca slab geometry. Areas in which the oceanic Moho post-critical reflection is a primary component of the seismic wave-field are defined as a function of landward extent of rupture for the next mega-thrust event. For rupture extending landward of the down-dip limit of the seismogenic zone, Moho post-critical reflections create a broad maximum in peak ground motions at locations that fall within the Pacific Northwest urban corridor [Portland, Seattle. Vancouver. Victoria]. At these locations, ground motions from sources with dominant frequencies between 1 Hz and 3 Hz can be a factor up to 1.5 greater than those modeled in the absence of the oceanic Moho post-critical reflection. A second maximum is associated with the presence of forearc mantle serpentinization, which affords a route for post-critical reflections at the oceanic Moho to escape upwards toward the Earth's surface rather than being trapped in the oceanic crust wave-guide. The forearc mantle wedge acts to concentrate seismic energy within a narrow region at locations, just landward of major population centers.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/1872 |
| Date | 18 November 2009 |
| Creators | McNeill, Andrew Floyd |
| Contributors | Spence, George D., Rogers, Garry Colin |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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