1. Department of Earth and Space Science,
2. United States Geological Survey, University of Washington, Box 351310, Seattle, Washington, 98195, USA.

Correspondence to: Justin L. Rubinstein¹ Correspondence and requests for materials should be addressed to J.L.R. (Email: justin@ess.washington.edu).

Non-impulsive seismic radiation or 'tremor' has long been observed at volcanoes¹ and more recently around subduction zones². Although the number of observations of non-volcanic tremor is steadily increasing, the causative mechanism remains unclear. Some have attributed non-volcanic tremor to the movement of fluids^{2, 3, 4, 5, 6}, while its coincidence with geodetically observed slow-slip events at regular intervals^{7, 8} has led others to consider slip on the plate interface as its cause^{7, 8, 9, 10, 11, 12, 13, 14}. Low-frequency earthquakes in Japan, which are believed to make up at least part of non-volcanic tremor⁹, have focal mechanisms¹⁰ and locations¹¹ that are consistent with tremor being generated by shear slip on the subduction interface. In Cascadia, however, tremor locations appear to be more distributed in depth than in Japan^{3, 4}, making them harder to reconcile with a plate interface shear-slip model. Here we identify bursts of tremor that radiated from the Cascadia subduction zone near Vancouver Island, Canada, during the strongest shaking from the moment magnitude M_w = 7.8, 2002 Denali, Alaska, earthquake. Tremor occurs when the Love wave displacements are to the southwest (the direction of plate convergence of the overriding plate), implying that the Love waves trigger the tremor. We show that these displacements correspond to shear stresses of approximately 40 kPa on the plate interface, which suggests that the effective stress on the plate interface is very low. These observations indicate that tremor and possibly slow slip can be instantaneously induced by shear stress increases on the subduction interface—effectively a frictional failure response to the driving stress.

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