Slow slip along the plate interface at subduction zones can generate weak seismic tremor in a quasi-periodic process called episodic tremor and slip. This process differs in character from regular earthquake rupture and can release stresses that build up on the deep plate interface. Here we analyse the spatial and temporal evolution of the five largest episodic tremor and slip events between 2004 and 2009 in northern Washington on the Cascadia subduction zone. We find that the events are similar, but not identical because they initiate in different locations and propagate along the plate interface at different average speeds of 7 to 12 km per day. Our analysis reveals that tremor can migrate rapidly back, away from the region where tremor and slip are advancing, through parts of the plate interface that have just ruptured in the past three days. These rapid tremor reversals propagate backwards for tens of kilometres at speeds that are 20 to 40 times faster than the relatively slow, steady advance of episodic tremor and slip. Our observations suggest that once the plate interface is weakened by the initial advance of episodic tremor and slip, it allows stresses to induce slip more easily or fluid pressure waves to migrate back more rapidly, generating rapid tremor reversals.
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We are grateful for enlightening discussions with A. Rubin and J-P. Ampuero and helpful suggestions from D. Shelly for improving this manuscript. We thank H. Kao for supplying the tremor catalogue of the Pacific Geoscience Center for purposes of comparison. This work was supported by NSF grant EAR-0911759.
The authors declare no competing financial interests.
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Houston, H., Delbridge, B., Wech, A. et al. Rapid tremor reversals in Cascadia generated by a weakened plate interface. Nature Geosci 4, 404–409 (2011). https://doi.org/10.1038/ngeo1157
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