HR: 1330h
AN: T32B-0933    [PDF]
TI: Low Velocity Zone atop the 410 km Seismic Discontinuity in the Northwestern US
AU: * Song, T A
EM: alex@gps.caltech.edu
AF: Seismological Laboratory, Division of Geological and Planetary Science, Caltech, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Helmberger, D V
EM: helm@gps.caltech.edu
AF: Seismological Laboratory, Division of Geological and Planetary Science, Caltech, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Grand, S P
EM: steveg@speer.geo.utxc.edu
AF: Department of Geological Sciences, University of Texas at Austin, 4412 Spicewood Springs Rd, Austin, 78749 United States
AB: Waveform modelling at triplication distances of 15-23 degrees indicates a low velocity zone (LVZ, shear wave velocity drop of 5 %) atop the 410 km seismic discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range. The LVZ has thickness varying from 20 to 70 km and has sharp western and southern boundaries. We did not find such anomaly from paths sampling offshore Washington-Oregon coast, the central Basin and Range and the Gulf of California. The spatial extent of the LVZ coincides with anomalous receiver function determinations and the region with distinct high alumina olivine tholeiite (HAOT) volcanisms. We interpret the LVZ directly above the 410 km seismic discontinuity as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior Farallon plate subduction and back-arc extension. We also investigate P-wave triplications with exactly the same ray paths that are used to constrain the LVZ. Preliminary results show probably less variations in P-wave velocity at depths near the 410 km seismic discontinuity. Further detail modelling will help constrain the P-wave velocity and the Vp/Vs ratio in the LVZ.
DE: 3924 High-pressure behavior
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 9350 North America
SC: Tectonophysics [T]
MN: 2003 Fall Meeting