HR: 1340h
AN: S53A-0187    [Abstracts]
TI: Source processes of the 1949 Olympia, Washington and other Cascadia intraslab earthquakes
AU: * Ichinose, G A
EM: gene_ichinose@urscorp.com
AF: URS, 566 El Dorado Street 2nd Floor, Pasadena, CA 91101 United States
AU: Somerville, P G
EM: paul_somerville@urscorp.com
AF: URS, 566 El Dorado Street 2nd Floor, Pasadena, CA 91101 United States
AU: Thio, H
EM: hong_kie_thio@urscorp.com
AF: URS, 566 El Dorado Street 2nd Floor, Pasadena, CA 91101 United States
AB: Cascadia intraslab earthquakes have occurred frequently including in 1949 (M 7), 1965 (M 6.9), and 2001 (M$_{w}$ 6.8) and the hazard they pose is equal to other seismic sources for time scales relevant to retrofitting (50%PE in 75 yrs). Reexamination of historical earthquakes and analysis of broadband seismograms from new earthquakes improves strong motion prediction more than stochastic scenarios and provides a better understanding in how stress is released in the subducting slab relative to external tectonic forces and local dehydration. Reanalysis of the 1949 Olympia earthquake using digitized seismograms collected by Barker and Langston [1987], Wiest et al. [2004], long-period WWSSN data from College, Alaska, Bogot\'{a}, Columbia, and Pasadena, California, has yielded a normal-slip mechanism that fits all the available data with $\sim$ E-W extensional-axis. This mechanism better resembles those from previous Puget Sound intraslab earthquakes. The earthquake ruptured southward along a plane with 170\deg strike and 70\deg dip. The hypocenter depth is 60 km with a total M$_{o}$ of 1.9\times10$^{26}$ dyne-cm (M$_{w}$ 6.78). We inverted teleseismic body waves to determine the mechanism and rupture pattern in space and time using a time-domain iterative inversion process. We limited the number of unknowns by assuming only 2 subevents and performed a grid search on the remaining variables including hypocenter depth, rupture direction, subevent rise time and rupture velocity. A rise time of 3 s best fits the frequency content of the waveforms but there is no resolution of rupture velocity with this dataset so we assume 3.5 km/s. The total moment is typically lower because of stricter model assumptions and the use of band limited data. The amplitudes from the Pasadena 1-90 s Benioff records indicate a higher M$_{w}$ $\sim$ 6.8 when compared to those from the 2001 Nisqually earthquake convolved with a similar response. We show from similar analyses of this and other intraslab earthquakes that they have significantly smaller asperity areas relative to other types of earthquakes with the same seismic moment originally identified by Asano et al. [2003] and Ichinose et al. [2004].
DE: 7230 Seismicity and seismotectonics
DE: 7209 Earthquake dynamics and mechanics
DE: 7212 Earthquake ground motions and engineering
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting