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Science 30 January 2009:
Vol. 323. no. 5914, pp. 620 - 623
DOI: 10.1126/science.1167112
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Reports

Cascadia Tremor Located Near Plate Interface Constrained by S Minus P Wave Times

Mario La Rocca,1* Kenneth C. Creager,2 Danilo Galluzzo,1 Steve Malone,2 John E. Vidale,2 Justin R. Sweet,2 Aaron G. Wech2

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 ±2 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.

1 Istituto Nazionale di Geofisica e Vulcanologia–Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy.
2 Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA.

* To whom correspondence should be addressed. E-mail: mlarocca{at}ov.ingv.it

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Imaging the source region of Cascadia tremor and intermediate-depth earthquakes.
G. A. Abers, L. S. MacKenzie, S. Rondenay, Z. Zhang, A. G. Wech, and K. C. Creager (2009)
Geology 37, 1119-1122
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Science. ISSN 0036-8075 (print), 1095-9203 (online)