HR: 0800h
AN: S51E-1067    [Abstracts]
TI: Observations of deep intra-plate earthquakes along the western foothills of the Sierra Nevada
AU: * Gilbert, H
EM: hgilbert@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Street, Tucson, AZ 85721 United States
AU: Hurd, O
EM: ohurd@email.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Street, Tucson, AZ 85721 United States
AU: Jones, C H
EM: cjones@cires.colorado.edu
AF: University of Colorado, Department of Geological Sciences Campus Box 399, Boulder, CO 80309 United States
AU: Owens, T J
EM: owens@seis.sc.edu
AF: University of South Carolina, Department of Geological Sciences 701 Sumter Street, Columbia, SC 29208 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Street, Tucson, AZ 85721 United States
AB: Small anomalous deep crust earthquakes have been found in the lower crust beneath both Lake Tahoe and the western Sierra Nevada (SN) foothills with epicentral depths reaching beyond 30 km in both regions. Past investigators have proposed that the deep seismicity in the western foothills relates to ancient shear zones that accommodated extension resulting from batholith intrusion (Miller and Mooney, 1994, JGR). The concentration of the deep seismicity beneath Lake Tahoe has been found to align along an eastward dipping plane that may mark a zone of magmatic injection into the lower crust (Smith et al., 2004, Science). The currently deployed Sierra Nevada Earthscope Project (SNEP) array, which consists of over 40 broadband seismometers, that spans the central Sierra Nevada is ideally situated to study crustal and mantle structure of this region as well as local and regional seismicity. The design of this array focused on unraveling the extent to which the Sierra Nevada batholith lost its garnet-rich root, as has been proposed for the southern portion of the range, and may be continuing further north. Dynamics related to root removal may be responsible for disturbing the base of the crust and producing the weakened zones that are then filled by magmatic injection resulting in the earthquakes observed beneath Lake Tahoe. We have located several deep crustal earthquakes in the western foothills of the Sierra Nevada within the SNEP array from the initial data. The locations of these earthquakes have depths near 30 km and occur within a region where receiver functions possess very little converted energy off of the Moho and crustal thicknesses reach 40 km. The lack of Moho signal has been interpreted to result from a cusp of crustal material that has been entrained into the mantle by downward flow induced by the foundering root. Stresses induced by this same foundering material may also be responsible for producing earthquakes in the lower crust.
DE: 7205 Continental crust (1219)
DE: 7218 Lithosphere (1236)
SC: Seismology [S]
MN: Fall Meeting 2005