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Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA.
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- PublicationRestrictedShort‐term and long‐term tremor migration patterns of the Cascadia 2004 tremor and slow slip episode using small aperture seismic arrays(2010)
; ; ; ; ;McCausland, W. A.; U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington, USA. ;Creager, K. C.; Department of Earth and Space Sciences, University of Washington Seattle, Washington, USA. ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Malone, S. D.; Department of Earth and Space Sciences, University of Washington Seattle, Washington, USA.; ; ; Tectonic tremor has been recorded at many subduction zones, including the Nankai, Cascadia, Mexican, and Alaskan subduction zones. This study, the first to use small aperture seismic arrays to track tremor, deployed three small aperture seismic arrays along the Cascadia subduction zone during a tremor and slow slip episode in July 2004. The tremor was active during virtually all (up to 99%) minutes of the analyzed tremor episode using 5 min sample windows. Individual wave phases were tracked across the arrays and used to derive slowness vectors. These were compared with slowness vectors computed from a standard layered Earth model to derive tremor locations. Locations were stable within a volume roughly 250 km2 in epicenter and 20 km in depth for hours to days before moving to a new volume. The migration between volumes was not smooth, and the movement of the sources within the volume followed no specific pattern. Overall migration speeds along the strike of the subduction zone were between 5 and 15 km/d; smaller scale migration speeds between volumes reached speeds up to 2 km/min. Uncertainties in the best locations were 5 km in epicenter and 10 km in depth. For this data set and processing methodology, tremor does not locate predominately on the primary subduction interface. Our favored model for the generation of tectonic tremor signals is that the tremor is triggered by stress and fluid pressure changes caused by slow slip and is composed, at least in part, of low‐frequency earthquakes broadly distributed in location132 22 - PublicationRestrictedCascadia Tremor Located Near Plate Interface Constrained by S Minus P Wave Times(2009-01-30)
; ; ; ; ; ; ; ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Creager, K. C.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA. ;Galluzzo, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Malone, S.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA. ;Vidale, J. E.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA. ;Sweet, R.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA. ;Wech, G.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA.; ; ; ; ; ; Nonvolcanic tremor is difficult to locate because it does not produce impulsive phases identifiable across a seismic network. An alternative approach to identifying specific phases is to measure the lag between the S and P waves. We cross-correlate vertical and horizontal seismograms to reveal signals common to both, but with the horizontal delayed with respect to the vertical. This lagged correlation represents the time interval between vertical compressional waves and horizontal shear waves. Measurements of this interval, combined with location techniques, resolve the depth of tremor sources within T2 kilometers. For recent Cascadia tremor, the sources locate near or on the subducting slab interface. Strong correlations and steady S-P time differences imply that tremor consists of radiation from repeating sources.300 31 - PublicationRestrictedTidal Modulation of Nonvolcanic Tremor(2007-11-22)
; ; ; ; ; ;Rubinstein, J. L.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Vidale, J. E.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA ;Creager, K. C.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA ;Wech, A. G.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA; ; ; ; Episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of Japan and Cascadia. In Cascadia, such episodes typically last a few weeks, and differ from “normal” earthquakes in their source location and momentduration scaling. The three most recent episodes in the Puget Sound/Southern Vancouver Island portion of the Cascadia subduction zone have been exceptionally well recorded. In each episode, we see clear pulsing of tremor activity with periods of 12.4 and 24-25 hours, the same as the principal lunar and lunisolar tides. This indicates that the small stresses associated with the solid-earth and ocean tides influence the genesis of tremor much more effectively than they do “normal” earthquakes. Because the lithostatic stresses are 105 times larger than those associated with the tides, we argue that tremor occurs on very weak faults.284 27