The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as Mw = 9 at an interval of several hundred years1,2,3. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data4,5,6. However, episodic surface displacements—in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface—are observed about every 14 months with an unusual tremor-like seismic signature7,8,9. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.
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We thank I. Al-Khoubbi, T. Christie, T. Claydon and R. Hall for their efforts in deploying and maintaining the seismic network; A. Bird, T. Mulder and W. Bentkowski for their assistance in seismic data processing; R. Baldwin for his software support; A. Calvert, R. Hyndman and K. Wang for stimulating discussion; and the POLARIS consortium for providing data from the BC POLARIS array. Most SSA computation is done on the Mercury cluster of the University of Victoria. This work is supported in part by a USGS NEHRP research grant.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Kao, H., Shan, SJ., Dragert, H. et al. A wide depth distribution of seismic tremors along the northern Cascadia margin. Nature 436, 841–844 (2005). https://doi.org/10.1038/nature03903
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