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Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip


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 zone2,3,4,5,6,7. 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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Figure 1: Comparison of temporal distribution of tremor activity and low-frequency earthquakes (LFEs) in western Shikoku.
Figure 2: An extended period of tremor containing four identified LFEs.
Figure 3: Area map with events, stations and velocity nodes used in the inversion.
Figure 4: Seismic velocities and relocated hypocentres along cross-section A–A′.


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This material is based upon work supported by the National Science Foundation. We thank K. Obara for allowing us to use his determinations of daily tremor activity. Much of this work was conducted while D.R.S. was a participant in the Summer Institute in Japan programme, cosponsored by the Japan Society for Promotion of Science and the National Science Foundation. All data were obtained from the NIED Hi-net data server.

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Correspondence to David R. Shelly.

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Shelly, D., Beroza, G., Ide, S. et al. Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip. Nature 442, 188–191 (2006).

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