HR: 08:15h
AN: T11F-02 INVITED    [Abstracts]
TI: 35 Cascadia Episodic Tremor and Slip events observed on GPS, seismic, and strain/tiltmeter arrays
AU: * Melbourne, T I
EM: tim@geology.cwu.edu
AF: Central Washington University, 400 E University Way, Ellensburg, WA 98926, United States
AU: Aguiar, A C
EM: aguiara@geology.cwu.edu
AF: Central Washington University, 400 E University Way, Ellensburg, WA 98926, United States
AU: Santillan, V M
EM: marcelo@geology.cwu.edu
AF: Central Washington University, 400 E University Way, Ellensburg, WA 98926, United States
AU: Szeliga, W
EM: walter@geology.cwu.edu
AF: Central Washington University, 400 E University Way, Ellensburg, WA 98926, United States
AU: Szeliga, W
EM: walter@geology.cwu.edu
AF: University of Colorado, Boulder, Dept of Earth Sciences, Boulder, CO 98242, United States
AU: Miller, M
EM: meghan@geology.cwu.edu
AF: Central Washington University, 400 E University Way, Ellensburg, WA 98926, United States
AB: Several rapidly expanding GPS networks along the greater Cascadia forearc have enabled identification of 36 isolated Episodic Tremor and Slip (ETS) events since 1997, including two in 2007. ETS events are observed throughout the forearc, from northern California to southwestern British Columbia, with station density generally increasing towards the north. Events located in well-instrumented regions can be tracked as they migrate laterally north-south along the plate boundary, but increasing station density has resolved many smaller transients that could not previously be confidently identified. At the specific latitude of the northern Washington State and southwestern British Columbia, the 14-month average recurrence interval still holds true, 5 events after first recognition. Elsewhere, this periodicity is not observed. Along central Vancouver Island, a host of smaller events distinct from the 14-month recurrence occur with an aperiodic fashion. Sporadic smaller events also appear throughout the subduction zone to the south, including within the region known for the 14-month periodicity. In southern Washington State, some of the largest transient displacements are observed, but lack any obvious periodicity in their recurrence. Along central Oregon, an 18-month recurrence is evident, while in northern California (Yreka) the 11-month periodicity previously documented still holds true. We invert estimated GPS offsets for the largest 14 events using non-negative thrust faulting along a plate interface divided into roughly 500 subfaults. Those events have equivalent moment magnitudes ranging from 6.3 (smallest resolvable with GPS) to 6.8, and typically 2-3 cm of slip. The largest spatial extent of the events resolved to date is just under 500 km along strike, and maximum duration is seven weeks, which lies in marked contrast to other subduction zones. Averaged over many ETS events, the upper limit of transient slip in the vicinity of Seattle, WA lies just west of the heavily urbanized Puget Sound region, suggesting that the lower limit of megathrust seismic rupture may extend much closer to this area than previously thought. During the time window of 2005-2007.2, a systematic quantification of seismic tremor observed both during and outside of ETS events has demonstrated a very linear relationship between tremor duration and inverted GPS equivalent moment. Moreover, no GPS transients during this time period are observed in the absence of tremor. This suggests that the GPS-measured transient deformation may be the integrated near-field offsets resulting from many small, discrete seismic slip events (meaning aseismic, transient creep is not needed to explain the GPS signal). This hypothesis is supported from inferences about tremor source characteristics drawn from borehole seismic spectral features and tremor locations determined from surface arrays: the observed quantity, timing and location of tremor largely explains the ETS signal as observed on surface and borehole strain- and tiltmeter arrays. Together, these data suggest that Cascadia, and presumably other subduction zone ETS events, are comprised largely of a form of microseismic clustering unique to subduction zones and may in fact have little aseismic creep associated with them.
DE: 7209 Earthquake dynamics (1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting