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Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Seismotectonics of the Rupture Zone of the 1964 MW=9.2 Great Alaska Earthquake

Michael A. Fisher1, Natalia Ratchkovski2, Ray E. Wells1, Donna M. Eberhart-Phillips3, Richard J. Blakely1, Ray Sliter4, and Eric L. Geist1
1 U.S. Geol Survey, 345 Middlefield Road, Menlo Park, CA 94025, [email protected]
2 Geophysical Institute, Univ of Alaska, P.O. Box 757320, Fairbanks, AK 99775
3 Institute Geol & Nuclear Sci, Private Bag 1930, Dunedin, New Zealand
4 USGS, 345 Middlefield Rd, Menlo Park, CA 94025

Hypocenters in the main rupture zone of the Mw=9.2 1964 Great Alaska Earthquake and deep-crustal seismic-reflection data were analyzed to show the relationship between the seismicity distribution and the deep structure of the subduction zone. More than 3700 earthquakes (M >=1.5) that occurred since 1988 below Prince William Sound, northern Gulf of Alaska, were relocated using the double-difference technique (Waldhauser and Ellsworth, 2000). Current seismicity is concentrated in the region where the greatest slip occurred during the 1964 earthquake. Focal mechanisms reveal two main styles of faulting. Below the northern part of the study area, predominately normal faulting occurs at 30-40 km depth, presumably in the downgoing plate. In contrast, below central Prince William Sound, mostly strike-slip faulting occurs at 10-20 km depth. Few underthrusting focal mechanisms are evident there, despite the subduction-zone setting. Depth-converted deep-crustal seismic-reflection data were collected primarily within Prince William Sound. A series of reflections from rocks 15-20 km deep correlates with a regional refractor interpreted as the top of Eocene(?) oceanic igneous rocks, which is the basement of the subducted Yakutat terrane (Brocher et al., 1994). Many hypocenters cluster vertically near this reflector, indicating that at least below Prince William Sound, earthquake stress tends to concentrate near this igneous basement. The 1964 earthquake probably nucleated along this basement top. The southwestern edge of the subducted Yakutat terrane is marked in seismic-reflection data by complex faulting, and southwestward across this terrane edge, hypocentral depths increase systematically from ~15 km to ~20 km. The deeper southwestern hypocenters probably occurred within the subducted Pacific plate. The main slip region of the 1964 earthquake lies within a gravity low, which is consistent with findings from around the Pacific Rim (Wells et al., 2003; Song and Simmons, 2003). Seismic reflection data, however, do not reveal a deep sedimentary basin or other likely cause for the low gravity values.

Posted with permission of The Geological Society of America; abstract also online ( © Copyright 2005 The Geological Society of America (GSA).