HR: 1340h
AN: T33B-0536    [Abstracts]
TI: Episodic tremor and slip and the alteration of the Juan de Fuca subduction interface, a feasibility study of time dependent teleseismic receiver functions.
AU: * Frassetto, A
EM: andyf@geo.arizona.edu
AF: Department of Geosciences University of Arizona, Gould-Simpson Building #77 1040 E 4th St., Tucson, AZ 85721 United States
AU: Gilbert, H
EM: hgilbert@geo.arizona.edu
AF: Department of Geosciences University of Arizona, Gould-Simpson Building #77 1040 E 4th St., Tucson, AZ 85721 United States
AU: Beck, S
EM: beck@geo.arizona.edu
AF: Department of Geosciences University of Arizona, Gould-Simpson Building #77 1040 E 4th St., Tucson, AZ 85721 United States
AU: Creager, K
EM: kcc@ess.washington.edu
AF: Department of Earth and Space Sciences University of Washington, 310 Condon Hall, Seattle, WA 98195 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: Department of Geosciences University of Arizona, Gould-Simpson Building #77 1040 E 4th St., Tucson, AZ 85721 United States
AU: Wech, A
EM: wech@u.washington.edu
AF: Department of Earth and Space Sciences University of Washington, 310 Condon Hall, Seattle, WA 98195 United States
AB: Slow-slip tremor events are detected at intervals of 13-16 months within the Cascadia subduction zone. These events, manifested by westward GPS displacement and low frequency seismic tremor, occur at depths of 25-45 km and emanate down-dip of the locked and seismogenic section of the plate interface. Characterizing the conditions by which these low frequency tremors initiate and propagate is fundamental to understanding subduction zone seismicity and tectonics. We utilize an extensive broadband seismic dataset recorded in northwestern Washington during the last five years to calculate P-wave receiver functions for eight stations from the Pacific Northwest Seismic Network. These sites are located in the Olympic Peninsula-Puget Sound region of the Cascade forearc above the approximate source region of the tremors. These data encompass the four most recently observed slow-slip tremors and provide a nearly continuous time series during that period. Investigating variations of the prominent seismic discontinuities as a function of time suggests a modification in amplitude and/or vertical distribution of the negative seismic impedance along the subduction interface prior to and during the slow-slip events. The receiver functions show the following discontinuity structure: (A) a narrow negative P-s at 1.5 seconds, (B) a positive P-s at 2-2.5 s, (C) a broad and negative P-s from ~3.5-4.5 s, and (D) a positive P-s at 5 s and below. We interpret (D) to represent subducted oceanic Moho. Discontinuity C, due to its negative polarity and comparable depth to similar features seen in other subduction zones, is interpreted to be the upper boundary of a zone containing the subducting and dehydrating oceanic crust and the overlying layer of hydrating and serpentinizing mantle. Prior to slow-slip events in 2001, 2002, and 2003 discontinuity C appears to grow in amplitude and reach shallower depths. During and after the event the amplitude and vertical extent of (C) appears to regain its initial state. We are currently augmenting this dataset by recording broadband data in a dense array located in the northern Olympic Peninsula, atop a major source region for the tremor. These five tightly spaced instruments were installed in August 2005 and will be able to provide the best receiver function images of the plate interface before, during, and after the slip event which appears to have begun on September 3, 2005, days before this abstract was submitted.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8104 Continental margins: convergent
DE: 8123 Dynamics: seismotectonics
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005