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Published Online November 22, 2007
Science DOI: 10.1126/science.1150558

Reports

Submitted on September 17, 2007
Accepted on November 13, 2007

Tidal Modulation of Nonvolcanic Tremor

Justin L. Rubinstein 1, Mario La Rocca 2, John E. Vidale 1, Kenneth C. Creager 1, Aaron G. Wech 1

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

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 moment-duration 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.




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Science. ISSN 0036-8075 (print), 1095-9203 (online)