Abstract
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 fluids2,3,4,5,6, while its coincidence with geodetically observed slow-slip events at regular intervals7,8 has led others to consider slip on the plate interface as its cause7,8,9,10,11,12,13,14. 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 Japan3,4, 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 Mw = 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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Acknowledgements
H. Kao, G. Rogers and H. Dragert provided comments and data that assisted with the tremor source location. Comments from W. McCausland, N. Beeler and H. Houston improved this manuscript. The data used in this study come from the Canadian National Seismograph Network and are distributed freely by The Geological Survey of Canada. This work was inspired by conversations with T. Pratt.
Author Contributions J.L.R. and J.E.V. found and identified the tremor, J.G. and P.B. computed the fault plane stresses, K.C.C. located the tremor source, and J.L.R. performed the spectral analysis. J.L.R., with the help of all, integrated the results.
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Rubinstein, J., Vidale, J., Gomberg, J. et al. Non-volcanic tremor driven by large transient shear stresses. Nature 448, 579–582 (2007). https://doi.org/10.1038/nature06017
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DOI: https://doi.org/10.1038/nature06017
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