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Toward a Unified View of Tremor Distribution in Space and Time

GHOSH, A., University of Washington, Seattle WA USA,; VIDALE, J.E., University of Washington, Seattle WA USA, ; SWEET, J.R., University of Washington, Seattle WA USA, ; CREAGER, K.C., University of Washington, Seattle WA USA, ; WECH, A.G., University of Washington, Seattle WA USA, ; HOUSTON, H., University of Washington, Seattle WA USA,

The recent discovery of slow slip and nonvolcanic tremor (NVT) challenges our understanding of fault dynamics. Episodic tremor and slip (ETS) refers to the remarkably periodic slow-slip events and associated NVT activity in Cascadia that recurs every 14.5 months or so. ETS events occur in the transition zone, and each such event transfers stress to the updip locked part of the subduction fault, making it a step closer to the big megathrust earthquake. Hence, it is important to comprehend the physical processes, and state of stress that control this phenomenon. We captured the May 2008 ETS event in Washington, USA, with a small-aperture dense seismic array. We developed and applied a novel beam-backprojection method to detect and locate tremor. This technique reveals four times more tremor duration during weak tremor episode, and gives unprecedented resolution in relative tremor location, compared to an envelope cross-correlation method. We track tremor minute-by-minute using the beam-projection method, and map spatiotemporal tremor distribution over various time scales. Over the short time scale of several minutes, tremor shows rapid, continuous, slip-parallel migration with a velocity of ~50 km/hr. Over time scale of several hours, slip-parallel tremor bands sweep Cascadia along-strike from south to north with a velocity of ~10 km/day. Finally, over time scale of several days, tremor activity develops distinct moment patches that coincide with geodetic slip patch on the plate interface. We integrate these varied and intriguing observations over different time scales, strive to explain their relationship, explore possible physical models, and present a unified picture of tremor distribution in space and time.


Last Modified: 2011 Aug 10