HR: 1330h
AN: S12A-0378    [PDF]
TI: Geophysical Investigation of the Denali Fault and Alaska Range Orogen Within the Aftershock Zone of the October-November, 2002, M=7.9 Denali Earthquake
AU: * Fisher, M A
EM: mfisher@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Glen, J M
EM:
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Ratchkovski, N A
EM:
AF: Institute of Geophysics, University of Alaska, P.O. Box 757320, Fairbanks, AK 99775 United States
AU: Pellerin, L M
EM:
AF: Green Engineering, 2215 Curtis St., Berkeley, CA 94702 United States
AU: Nokelberg, W J
EM:
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: The aftershock zone of the November 3, 2002, M-7.9 earthquake that ruptured along the right-slip Denali Fault in south-central Alaska has been investigated using gravity, magnetic, magnetotelluric, and deep-crustal, seismic-reflection data as well as outcrop geology and earthquake seismology. Strong seismic reflections from within the Alaska Range orogen north of the Denali Fault dip as steeply as 25$\deg$ north and extend to depths as great as 20 km. The most intriguing finding from geophysical data is that earthquake aftershocks occurred above a low-resistivity (less than about 10 ohm-m) rock body that lies mainly at depths greater than about 10 km. The low electrical resistivity might result from electrical conduction along carbon films in the Mesozoic flysch deposits that are truncated along the strike-slip Denali Fault. However, we prefer the interpretation that the low resistivity results from deep fluids, as suggested by the occurrence of the Denali bright spot, a strong and flat seismic reflection from midcrustal depths. Deep fluids may have weakened the middle and lower crust and helped localize seismicity. The Denali Fault developed during the Cenozoic along a Cretaceous suture zone that separates mainly Paleozoic continental rocks to the north from Mesozoic oceanic rocks to the south. Seismically reflective features within the Alaska Range orogen reveal relict, Cretaceous faults or shear zones that are essentially aseismic under today's stress regime. Faults that ruptured during the Denali earthquake are non-reflective, most likely because they dip steeply.
DE: 7205 Continental crust (1242)
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting