HR: 0800h
AN: S51E-1061    [Abstracts]
TI: Teleseismic travel times, the Isabella anomaly, and the missing Moho, from the Sierra Nevada EarthScope experiment
AU: * Thomas, A
S51E-1061 AF: Georgia Institute of Technology, Georgia Institute of Technology, Atlanta, GA 30332 United States
AU: Jones, C
EM: cjones@cires.colorado.edu
AF: University of Colorado, 2200 Colorado Ave. , Boulder, CO 80309 United States
AU: Reeg, H
EM: Heidi.Reeg@colorado.edu
AF: University of Colorado, 2200 Colorado Ave. , Boulder, CO 80309 United States
AU: Gilbert, H
EM: hgilbert@geo.arizona.edu
AF: University of Arizona, Department of Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E 4th St. , Tucson, AZ 85721 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: University of Arizona, Department of Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E 4th St. , Tucson, AZ 85721 United States
AU: Owens, T
EM: owens@seis.sc.edu
AF: University of South Carolina, Department of Geological Sciences University of South Carolina 701 Sumter St., Room EWSC 617 , Columbia, SC 29201 United States
AB: Seismic studies of the southern and central Sierra Nevada revealed a region within the western foothills where the teleseismic P to S Moho conversion is very weak or absent (Zandt et al., Nature, 2004; Burdick et al., this meeting). The conversion could be destroyed by a downward pointing cusp on the Moho produced as crust is entrained in the foundering of the garnet-rich Sierran lower crustal root. The first phase of the recently deployed Sierra Nevada Earthscope Project (SNEP) consists of an array of over 40 broadband seismometers spanning the central Sierra Nevada for a ~15-month period. Initial examination of the new teleseismic travel times from the SNEP stations reveal earlier P-wave arrivals above the "missing" Moho than in the surrounding area. These P-arrivals appear to be inconsistent with the concept of the downward cusp on the Moho: if there were a thicker crust where the Moho is "missing" then the P-wave arrivals in that area should be later than in surrounding areas. P-wave residuals, some of which are more than a second early, indicate that a subsurface high wavespeed body is instead present; these residuals are in part due to the previously described "Isabella anomaly" under the southwestern Sierra. By evaluating the P-wave arrival times from 30 different teleseisms to the SNEP,1997 Sierran Paradox, and 1988 Southern Sierra experiments, we can approximate the spatial geometry of the body or bodies as an elongated body plunging toward the southeast. A preliminary cross section of teleseismic P-residuals using events from the northwest and southeast suggests that the anomaly extends from less than 30 km to as much as 250 km depth. Quite possibly two anomalies exist: one near the Moho in the northwest and one closer to 200 km depth to the southeast. The shallow level of the upper, northwestern end of the body suggests that the "Moho hole" might instead reflect a region where high-wavespeed material exists in the crust, diminishing the contrast at the Moho and thus disrupting the Ps conversion, or that high-wavespeed material overwhelms the effect of thickened crust in this area. The lower bound on the bottom of the Isabella anomaly suggests that the entire foundering body remains in the uppermost mantle and that completed imaging from the combined 1988, 1997, and SNEP experiments will finally complete our picture of this enigmatic body.
DE: 1734 Seismology
SC: Seismology [S]
MN: Fall Meeting 2005