1. Department of Geophysics, 397 Panama Mall, Stanford University, Stanford, California 94305-2215, USA
2. Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
3. Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan

Correspondence to: David R. Shelly¹ Correspondence and requests for materials should be addressed to D.R.S. (Email: dshelly@pangea.stanford.edu).

Abstract

Non-volcanic seismic tremor was discovered in the Nankai trough subduction zone in southwest Japan¹ and subsequently identified in the Cascadia subduction zone². In both locations, tremor is observed to coincide temporally with large, slow slip events on the plate interface downdip of the seismogenic zone^{2, 3, 4, 5, 6, 7}. The relationship between tremor and aseismic slip remains uncertain, however, largely owing to difficulty in constraining the source depth of tremor. In southwest Japan, a high quality borehole seismic network allows identification of coherent S-wave (and sometimes P-wave) arrivals within the tremor, whose sources are classified as low-frequency earthquakes. As low-frequency earthquakes comprise at least a portion of tremor, understanding their mechanism is critical to understanding tremor as a whole. Here, we provide strong evidence that these earthquakes occur on the plate interface, coincident with the inferred zone of slow slip. The locations and characteristics of these events suggest that they are generated by shear slip during otherwise aseismic transients, rather than by fluid flow. High pore-fluid pressure in the immediate vicinity, as implied by our estimates of seismic P- and S-wave speeds, may act to promote this transient mode of failure. Low-frequency earthquakes could potentially contribute to seismic hazard forecasting by providing a new means to monitor slow slip at depth.

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