1993 Fiscal Year Final Research Report Summary
A search for silent/slow earthquakes around Japan Islands
| Project/Area Number |
04640399
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
固体地球物理学
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| Research Institution | Toyama University |
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Principal Investigator |
KAWASAKI Ichiro Toyama Univ., Earth Sci., Prof., 理学部, 教授 (60115136)
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| Project Period (FY) |
1992 – 1993
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| Keywords | slow earthquake / extensometer / strain step / time constant / 1992 Sanriku earthquake / tsunami Earthquake / Bayseian model / BAYTAP-G |
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| Research Abstract |
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.
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