HR: 1340h
AN: T33A-1349    [Abstracts]
TI: Rapid variation in the upper mantle P velocity structure beneath Gulf California
AU: * Ni, S
EM: sdni@ustc.edu.cn
AF: Univ. Science & Technology of China, 96 Jinzhai Road, Hefei, Anh 230026 China
AU: * Ni, S
EM: sdni@ustc.edu.cn
AF: Seismological Laboratory, Caltech 252-21, Pasadena, CA 91125 United States
AU: Helmberger, D
EM: helm@gps.caltech.edu
AF: Seismological Laboratory, Caltech 252-21, Pasadena, CA 91125 United States
AB: Recent studies of upper mantle triplications involving large number of stations reveal a myriad of small-scale structures, ranging from deep low velocity zone just above the 410km discontinuity to breaking this sharp discontinuity into a combination of first-order and second-order changes. Here, we report on an even larger dataset involving over a thousand records containing a cluster of Mexican events at several different depths as recorded by CISN, IRIS, USGS networks and PASSCAL arrays. Absolute travel times of the various record sections seem to be reasonably well explained by recent surface wave tomography models, but are not very effective in predicting triplication behaviors. \par One of the sharpest anomalies occurs for record section recorded by CISN at distances between 19 and 30 degrees. At this range, the CD branch (410km arrival) and the EF branch (660km arrival) cross near 23 degree. However, about a third of the western portion of CISN array shows this cross-over occurring about a degree earlier and this edge is sharp (within 50km). The mid-point locations of these samples are at the eastern edge of the Gulf of California. Several neighboring events at different depths display similar patterns, but the pattern moves laterally for more distant events. \par Our preferred interpretation is that it is caused by lateral variations of the transition zone, either by about 10kms of topography on the 660km discontinuity (EF-changing) or a low velocity zone just above the 410km discontinuity (CD-changing).
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting