Shallow earthquakes are generally believed to be brittle fractures in a stressed medium with rupture velocity at a speed close to that for shear waves. We know, however, that the Earth allows failure over a wide range of timescales. Creep events occur on the San Andreas fault system, and for some earthquakes the high-speed rupture is accompanied by a slower process1-5. Here we report on episodic slow events which occurred before the Japan Sea earthquake of magnitude 7.7 on 26 May 1983. A Sacks-Evertson borehole strain meter6 90 km from the earthquake recorded about 100 aseismic strain events in a five-month period before and immediately following the earthquake; none has been detected after the large aftershocks. The observed signals are consistent with a precursory redistribution of stress through aseismic slip on a deep extension of the main-shock fault plane. Such episodic aseismic events may provide a mechanism for relatively rapid stress concentration before earthquakes.
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