Seismic resistivity changes observed at Aburatsubo, central Japan, revisited

Earth's resistivity variation has been continuously recorded in a vault at Aburatsubo, central Japan, since 1967. In the previous works, tidal, coseismic, and precursory resistivity variations were reported, which were explained by strain-induced changes. Based on the recent data obtained by a digital recorder and laboratory experiments newly conducted on the rock's electrical properties, we have reexamined the nature of the seismic resistivity changes. New findings so far obtained from in-situ observation are as follows. (1) A coseismic step takes place on arrival of the S-wave rather than that of the P-wave. (2) The polarity of the coseismic step shows no correlation to the source mechanism of the earthquake, but shows a clear seasonal variation; i.e. positive steps appear in summer but are negative in winter. (3) There is a seasonal variation in the Earth's resistivity with minimum and maximum in October and in April, respectively. The polarity of coseismic resistivity steps has a good correlation with the rate of seasonal variation of the ground resistivity itself. Combining these observations with results of recently made laboratory experiments, we present a tentative model for coseismic resistivity step generation which proposes mixing of temperature stratification in the shallow ground due to ground oscillation by seismic S-waves.