HR: 1340h
AN: T33A-1339    [Abstracts]
TI: Waveform modeling of upper mantle shear wave triplications in the western United States
AU: * Song, T A
EM: alex@gps.caltech.edu
AF: Seismo Lab, Division of Geological and PLanetary Science, Caltech, 252-21, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Helmberger, D V
EM: helm@gps.caltech.edu
AF: Seismo Lab, Division of Geological and PLanetary Science, Caltech, 252-21, 1200 E. California Blvd., Pasadena, CA 91125 United States
AB: Previous study (Song at al. 2004, Nature) has shown rapid variation of S wave triplication associated with possible low velocity zone atop the 410 seismic discontinuity beneath the Basin and Range in the western United States. The results are based on anomalous differential time between the CD (bottoming below the 410 seismic discontinuity) and the shallower AB-(bottoming around 200-300 km) triplication branches, along with anomalous receiver function estimate. To take into account variations in shallow mantle structure, we make velocity crosssections from recent regional tomographic images and compute 2-D synthetics. Here, we report on waveform modeling (2D) of record sections sampling the western edge of this structure from events offshore Washington-Oregon. Broadband data recorded by TriNet (12 events), Berkeley network (6 events) and NARS array (3 events) are used to study the western edge of this LVZ structure. Typically, at distances of 14-17 degrees, the S wave triplication recorded by TriNet exhibits rapid changes in both the AB-branch and the CD-branch when raypaths travel across the California-Nevada border. In particular, the CD-branch recorded by stations near the California-Nevada border arrive late for about 3-4 secs with larger amplitude than that recorded by stations close to the coastal California. Rapid azimuthal variations in the delay of CD branches are observed from record sections in multiple events.Such anomalous delay, however, is not observed for paths travelling offshore California-Oregon. Our preliminary result indicates a low velocity zone (5% reduction in shear velocity) atop the 410 seismic discontinuity beneath the western Basin and Range, with dimension of about 300-500 km long and 25 km thick. This low velocity anomaly extends to shallower depth ($\sim$ 250 km) beneath the southern part of the Western Great Basin with reduced amplitude (2%). Given the rapid variation in the S-wave triplication data, we attribute the low velocity anomaly to silicate melts, which are probably produced by the dehydration melting across the 410 seismic discontinuity.It may have strong implications on the water content in the mantle transition zone, melting processes and geochemistry.
DE: 7260 Theory and modeling
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting