Measurements of total magnetic force and electric self-potential were performed at the joint observation of the explosion seismic experiment made at Ohtaki Village in Nagano Prefecture in Japan. The explosion was induced by 86 kg of gunpowder placed at a depth of 68 m. We observed an increase in total magnetic force of 3 nT at explosion time, at a point 33 m from the explosion point. Increase in the self-potential from 2 to 10 mV was also observed between the electrode couples, which were 60 m apart. These induced changes did not recover to their former levels during the hour following the explosion. The changes may have been caused by the physical stress and/or the fracture. Explanations of the mechanism for the large induced changes are attempted from the aspect of the movement of groundwater, change of remanent magnetization and the effect of steel casing. The area around the explosion experiment is the aftershocks region of the 1984 Western Nagano Prefecture Earthquake (M=6.8) and an active fault runs near the studied area (Kobayashi et al., 1985). The active fault zone has the specific structure in electromagnetic properties which may have enhanced the observed changes at the explosion.

Earth tidal strains observed concurrently at two stations in the western Chubu region have been analyzed to see if there are any temporal variations of their amplitude in relation to major earthquakes in the region.The tidal strain records obtained at Kamitakara during 7 years and at Inuyama during 2 years are bandpass-filtered, and correlated with the corresponding theoretical tidal function predicted for a solid earth, using least squares mainly in the time domain, to estimate the tidal admittance over the periods. The admittances determined for the two-component Kamitakara strains appear to show a consistent and gradual increase during 10-11 months prior to the central Gifu earthquake of 1969 (M=6.6), with a maximum variation reaching about 15%, and after the earthquake they seem to decrease and recover their normal value a half year later. No such significant variations as in this period can be found over the entire period. The resolved amplitudes of the semidiurnal M2 and diurnal O1 waves show similar trends. The results from the Inuyama observations involve somewhat larger fluctuations, but appear to reach a peak 3-4 months before the earthquake and then decrease to the time of its occurrence.Secular strains and tilts observed at the above two stations also indicate appreciable changes during the corresponding periods. If these temporal variations of the tidal strain amplitudes are not spurious but real phenomena, they might be interpreted as a manifestation of some precursory changes in crustal rigidity around the seismic source region.S 123

Earth tidal strains observed concurrently at two stations in the western Chubu region have been analyzed to see if there are any temporal variations of their amplitude in relation to major earthquakes in the region.The tidal strain records obtained at Kamitakara during 7 years and at Inuyama during 2 years are bandpass-filtered, and correlated with the corresponding theoretical tidal function predicted for a solid earth, using least squares mainly in the time domain, to estimate the tidal admittance over the periods. The admittances determined for the two-component Kamitakara strains appear to show a consistent and gradual increase during 10-11 months prior to the central Gifu earthquake of 1969 (M=6.6), with a maximum variation reaching about 15%, and after the earthquake they seem to decrease and recover their normal value a half year later. No such significant variations as in this period can be found over the entire period. The resolved amplitudes of the semidiurnal M2 and diurnal O1 waves show similar trends. The results from the Inuyama observations involve somewhat larger fluctuations, but appear to reach a peak 3-4 months before the earthquake and then decrease to the time of its occurrence. Secular strains and tilts observed at the above two stations also indicate appreciable changes during the corresponding periods. If these temporal variations of the tidal strain amplitudes are not spurious but real phenomena, they might be interpreted as a manifestation of some precursory changes in crustal rigidity around the seismic source region.