HR: 1340h
AN: T33A-1147    [Abstracts]
TI: High-Resolution 3-D Regional Double-Difference Tomography of the Reno-Tahoe-Carson City Region
AU: * Preston, L
EM: preston@seismo.unr.edu
AF: University of Nevada Reno, Seismology MS/174, Reno, NV 89557, United States
AU: von Seggern, D
EM: vonseg@seismo.unr.edu
AF: University of Nevada Reno, Seismology MS/174, Reno, NV 89557, United States
AB: We have performed a high-resolution tomographic inversion using ~200,000 P and S absolute travel times from nearly 10,000 local sources recorded at the Nevada Seismological Laboratory from 1990-2006. An additional nearly 200,000 P- and S-wave high-quality cross-correlation differential travel times were also used in the inversion for double-difference relocation of earthquakes. Results indicate prominent low velocities in the Reno, Washoe Valley and Carson City basins. Shallow high velocities comprise the regions west of the high Sierra Nevada crest, whereas lower-than-average velocities are found in a broad region from Truckee to Reno to Carson Valley to just west of Lake Tahoe in the shallowest portion of the crust. These upper crustal features persist to about 5 km depth, where P velocities lose much of the correlations with surface features, averaging ~6 km/s and slowly increasing with depth. Although the bulk of seismicity lies shallower than 15 km depth, we have incorporated a unique deep sequence of events that occurred in 2003-2004 at 25-30 km depth under northern Lake Tahoe that has been previously interpreted as due to a magma intrusion from seismic and geodetic evidence. We image P velocities at 30 km depth in the low 7 km/s range, consistent with expected lower crustal velocities in the region. Vp/Vs is high within the basins, as expected in alluvial areas. Notably, we also image high Vp/Vs directly above the deep Tahoe sequence in the 16-23 km depth range. This latter observation may be due to the presence of fluids in the middle crust in this region. Low Vp/Vs primarily occurs under the high Sierra Nevada which could be indicative of their high silica content even at depth. Several seismicity lineaments are imaged in the high-precision double-difference earthquake locations. The 1995 Double Spring Flat (Mw 5.8) aftershock zone delineates multiple narrow zones of activity, possibly due to complex interactions among multiple faults that were activated as a result of this event. The deep Tahoe sequence is also sharply defined in volumetric extent; the subsequent shallower Tahoe sequence is imaged as a steeply SSE dipping lineament from ~9-15 km depth. Other features of interest include lineaments of seismicity near Carson City, northeast of Lake Tahoe to Antelope Valley, and northwest and south of Reno.
DE: 7203 Body waves
DE: 7205 Continental crust (1219)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7270 Tomography (6982, 8180)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting