Seismicity rate immediately before and after main shock rupture from high‐frequency waveforms in Japan

We analyze seismicity rate immediately before and after 82 main shocks with the magnitudes ranging from 3 to 5 using waveforms recorded by the Hi‐net borehole array in Japan. By scrutinizing high‐frequency signals, we detect ∼5 times as many aftershocks in the first 200 s as in the Japan Meteorological Agency catalogue. After correcting for the changing completeness level immediately after the main shock, the aftershock rate shows a crossover from a slower decay with an Omori's law exponent p = 0.58 ± 0.08 between 20 and 900 s after the main shock to a faster decay with p = 0.92 ± 0.04 after 900 s. The foreshock seismicity rate follows an inverse Omori's law with p = 0.73 ± 0.08 from several tens of days up to several hundred seconds before the main shock. The seismicity rate in the 200 s immediately before the main shock appears steady with p = 0.35 ± 0.50. These observations can be explained by the epidemic‐type aftershock sequence (ETAS) model, and the rate‐and‐state model for a heterogeneous stress field on the main shock rupture plane. Alternatively, nonseismic stress changes near the source region, such as episodic aseismic slip, or pore fluid pressure fluctuations, may be invoked to explain the observation of small p values immediately before and after the main shock.