A search for silent/slow earthquakes around Japan Islands

The purpose of this study is to find episodic aseismic faulting at the Japan trench.Subsequently to the July 18，1992，Sanriku-Oki earthquake(Ms6.9，39.42ºN，143.33°E)，a remarkable quasi-static phase appeared in the crustal strain records by quartz-pipe extensometers of a high resolution of-10^&lt；-11&gt；and low ground noises of-10^&lt；-9&gt；at ESASHI(39.15°N，141.34°E，r=174 km，phi=-99)，Iwate，Japan。We have decomposed the crustal strains into atmospheric pressure effects，tides and trends by the Baysian tidal analysis method of Ishiguro and Tamura(1985)and Tamura et al.(1991)。The EW component of the trend thus extracted displays a clear quasi-static exponential increase of-4Xl0^&lt；-8&gt；with a time constant of about one day.This is about four times greater than a strain step within about one hour subsequent to the seismic event。We have investigated the cause of the quasi-static phase to conclude that this was not due to local disturbances.A similar quasi-static phase of-15X10^&lt；-8&gt；was observed in the EW component of crustal strain records at MIYAKO(39.59°N，141.98°E，r=117 km，phi=-80°)of the Tohoku University。From the close similarity between the strain waveforms at ESASHI and MlYAKO，we can attribute the quasi-static 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 about60 km to the east from the NEIC epicenter and with a moment release of up to(1-4)Xl0^&lt；20&gt；N·m corresponding to that of M_w7.3-7.7 seismic event although the resolution is poor for small number of independent observations.Thus，the possibility is strengthened that the aseismic part of the relative plate motion in the south of the Sanriku region is released episodically with various time constants such as the ultra-slow earthquake.

The purpose of this study is to find episodic aseismic faulting at the Japan trench. Subsequently to the July 18, 1992, Sanriku-Oki earthquake (Ms6.9, 39.42ﾟN, 143.33ﾟE), a remarkable quasi-static phase appeared in the crustal strain records by quartz-pipe extensometers of a high resolution of -10^<-11> and low ground noises of -10^<-9> at ESASHI(39.15ﾟN, 141.34ﾟE, r=174km, phi=-99ﾟ), Iwate, Japan. We have decomposed the crustal strains into atmospheric pressure effects, tides and trends by the Baysian tidal analysis method of Ishiguro and Tamura(1985) and Tamura et al.(1991). The EW component of the trend thus extracted displays a clear quasi-static exponential increase of -4Xl0^<-8> with a time constant of about one day. This is about four times greater than a strain step within about one hour subsequent to the seismic event. We have investigated the cause of the quasi-static phase to conclude that this was not due to local disturbances.A similar quasi-static phase of -15X10^<-8> was observed in the EW component of crustal strain records at MIYAKO (39.59ﾟN, 141.98ﾟE, r=117km, phi=-80ﾟ) of the Tohoku University. From the close similarity between the strain waveforms at ESASHI and MlYAKO, we can attribute the quasi-static 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 60 km to the east from the NEIC epicenter and with a moment release of up to (1-4)Xl0^<20> N・m corresponding to that of M_w7.3-7.7 seismic event although the resolution is poor for small number of independent observations.Thus, the possibility is strengthened that the aseismic part of the relative plate motion in the south of the Sanriku region is released episodically with various time constants such as the ultra-slow earthquake.