PRECURSORY AND COSEISMIC CHANGES IN GROUND RESISTIVITY

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Abstract
A resistivity variometer of unusually high sensitivity has been in operation at a crustal movement observatory about 60km south of Tokyo since 1968. The variometer records changes in ground resistivity caused by crustal strain in association with tidal loading. When a large earthquake occurs, the variometer often records a step-wise change. Steps are observed for earthquakes having a magnitude of 8 or thereabout even if the epicentral distance exceeds 1, 000km. It is also the case for earthquakes of magnitude about 5 or more in the Tokyo area. Detailed investigations indicate that the variometer seems likely to detect extremely small coseismic strains in the form of resistivity steps. The resistivity steps are often preceded by a precursory change having a precursor time of a few hours. A Weibull distribution analysis of 21 examples brings out that the characteristic parameter controlling the distribution of precursor time is more or less the same as that for a group of short-term precursors such as land deformation, tilt and strain, and underground water. This kind of resistivity change, that may probably reflect an occurrence of premonitory rupture in the focal region, seems to have an important bearing on short-term earthquake prediction.

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