HR: 0800h
AN: T51B-0538    [Abstracts]
TI: Tomography of the subducting Cocos plate in central Mexico: Images of a truncated slab
AU: * Husker, A L
EM: uskerhay@moho.ess.ucla.edu
AF: UCLA Dept of Earth and Space Sciences, 595 Charles Young Drive East 3806 Geology Building Box 951567, Los Angeles, CA 90095, United States
AU: Davis, P M
EM: pdavis@moho.ess.ucla.edu
AF: UCLA Dept of Earth and Space Sciences, 595 Charles Young Drive East 3806 Geology Building Box 951567, Los Angeles, CA 90095, United States
AB: The location of the subducting slab beneath Mexico City and its relation to the Trans-Mexican Volcanic Belt (TMVB) has been unknown because of the absence of deep seismicity that could be used to define the Wadati-Benioff zone. We used data from a temporary seismic network to locate the slab using seismic tomography. A break is seen in the Cocos plate under the TMVB. The break is seen with both P-wave and S-wave tomography and in a constrained tomographic inversion that finds parameters for a simple slab temperature model. The data used are 172 teleseismic earthquakes recorded by the Middle American Subduction Experiment (MASE). MASE was made up of 100 broadband seismometers spaced every 5 km running from Acapulco north through Mexico City almost to the Gulf Coast. In order to determine arrival time differences, Dt, across the array, waveforms were cross correlated. When Dt is plotted with respect to the latitude of the seismometer at which it was recorded, a Dt minimum (early arrivals) is seen near the TMVB. This minimum is shifted northward for back azimuths from the south, and southward for back azimuths from the north. The shift in the Dt minimum is indicative of a fast structure at depth. If there were no break in the slab, the localized minimum would not be seen. Tomography reveals an approximately 50-80 km thick slab diving into the mantle at about 75° to approximately 550 km depth and 375 km inland from Acapulco. We speculate the absence of deep earthquakes is due to low stresses in a young plate that has been truncated at depth.
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8180 Tomography (6982, 7270)
DE: 9350 North America
SC: Tectonophysics [T]
MN: 2007 Fall Meeting