抄録
Rokko-Tsurukabuto Station is located about 25 km from the epicenter of the 1995 Hyogoken- Nanbu Earthquake (Jan. 17, 1995: M=7.2) and is equipped with ten super-mvar bar extensometers set in observation galleries across and near the Otsuki fault, beneath which the rupture of the earthquake reached. Hourly ground strain data recorded from Jan. 1, 1992 to Dec. 31, 1994 (17 days before the earthquake occurrence) were analyzed by applying a Bayesian Tidal Analysis Program 'BAYTAP-G'. Because of the rather low signal to noise ratio around the tidal frequency at the station, we calculate tidal parameters for each year. Amplitudes and phases of M2 and O1 strains for 1992, 1993 and 1994 show no distinct temporal variations. The phases are generally stable all through the period, while M2 tidal amplitudes increase slightly outside the fracture zone and decrease near the fault and within the fracture zone. This tendency may be related to the occurrence of the earthquake or local temporal change of rock properties. Strain observations at the depth are urgently required to monitor critical strain conditions before the occurrence of major earthquakes.
