HR: 11:35h
AN: T12C-06    [Abstracts]
TI: Strong Tidal Modulation of Non-Volcanic Tremor in Cascadia
AU: * Creager, K C
EM: kcc@ess.washington.edu
AF: University of Washington, Earth and Space Sciences, Seattle, WA 98195-1310, United States
AU: Rubinstein, J L
EM: justin@ess.washington.edu
AF: University of Washington, Earth and Space Sciences, Seattle, WA 98195-1310, United States
AU: LaRocca, M
EM: mlarocca@ov.ingv.it
AF: Osservatorio Vesuviano, INGV, Via Diocleziano 328, Naples, 80124, Italy
AU: Vidale, J E
EM: john_vidale@mac.com
AF: University of Washington, Earth and Space Sciences, Seattle, WA 98195-1310, United States
AU: Wech, A G
EM: wech@u.washington.edu
AF: University of Washington, Earth and Space Sciences, Seattle, WA 98195-1310, United States
AU: Sweet, J R
EM: jrsweet@u.washington.edu
AF: University of Washington, Earth and Space Sciences, Seattle, WA 98195-1310, United States
AB: The July 2004, September 2005 and January 2007 episodic tremor and slip events in the Puget Sound/Vancouver Island ETS source region, were exceptionally well recorded by temporary deployments of small-aperture seismic arrays. Analysis of stacked, filtered envelope functions across these arrays shows a clear pulsing of tremor activity with strong, isolated peaks at periods of 12.4 and 24-25 hours, coincident with the periods of the principle lunar and lunisolar tides. The amplitudes of these peaks rise above noise levels, measured at times without active tremor, by factors of 2 to 20. During each ETS episode the amplitude of tremor is modulated by ±30% at the 12.4-hour period. The strong dependence of tremor amplitude on the tides indicates that the small stresses associated with the solid earth and ocean tides influence the genesis of tremor much more than they do "normal" earthquakes at comparable depths. Because variations in calculated tidal stresses are approximately 10⁵ times smaller than the lithostatic loads compressing deep faults, tremor likely occurs on very low-stress faults, perhaps in the presence of nearly lithostatic pore-fluid pressures. Preliminary modeling of variations in shear and normal stresses acting on the interface between the subducting Juan de Fuca and the overriding North American plates, indicates that the most important contributions to these stresses are from water tides in the inland waterways of the Straits of Juan de Fuca, Puget Sound and Georgia Straits, while the Pacific Ocean and solid earth tides are generally less important. Each of these ETS events had durations of 2-3 weeks and occurred in regions near inland waterways that produce tidal variations in stress.
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
DE: 8164 Stresses: crust and lithosphere
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting