The 1992 Sanriku-Oki, Japan, Ultra-Slow Earthquake.

We report the detection of a large ultra-slow earthquake. Subsequent to the July 18, 1992, Sanriku-Oki, Japan, earthquake (Ms6.9, 39.42°N, 143.33°E), a clear post-seismic phase appeared in the crustal strains recorded by quartz-tube extensometers at Esashi (Δ=174 km, φ = -99°, from the NEIC CMT source). We have decomposed the crustal strains into atmospheric pressure effects, tides and trends by the Bayesian tidal analysis method. The EW component of the trend thus extracted displays a clear quasi-static increase of -4 x 10-8 with a time constant of about one day. This is about four times greater than the co-seismic strain step. We have investigated the cause of the post-seismic phase to conclude that this was not due to local disturbances. A similar post-seismic phase of -15 x 10-8 was also observed in the EW component of crustal strain records at Miyako (Δ =117 km, φ = - 80° ), Tohoku University. From the close similarity between the post-seismic waveforms at Esashi and Miyako, we attribute the post-seismic phase to the ultra-slow faulting subsequent to the seismic event. We have inverted the strain steps to obtain a solution of an interplate low-angle thrusting located about 54 km to the east from the NEIC CMT epicenter with a moment release of up to (1-4) x 1020 m, which corresponds to that of a MW7.3-7.7 seismic event.