えびの高原の地震活動

抄録

The seismic activity in and near the Ebino area is discussed based on the data obtained at the Kirisima Volcano Observatory (KVO), a branch station of Earthquake Research Institute, between May of 1964 and December 1971. A test for the goodness of fit to a Poisson distribution is tried with χ²-distribution for the daily frequency of earthquakes in each year. The occurrence of earthquakes during the period of 1964 to 1967 prior to the Ebino earthquakes in 1968 does not fit to a Poisson distribution. But earthquakes during the period of 1969 to 1971 was found to fit to a Poisson distribution. The variation of Ishimoto-Iida’s coefficient m was examined. The m values were found to be 1.8-2.0 which are not so large as the m values for shallow volcanic earthquakes. A significant difference of a values with time was not recognized. The energy release was calculated. The order of annual energy release before and after the Ebino earthquke swarm in 1968 are 10¹³ erg and 10¹² erg respectively. Two earthquake swarms in March, 1967 and in November, 1970 were occurred near Mt. Kurino-dake which is located at the west side of the Ebino area. The relation between the energy of an earthquake and the strain energy stored within a confined volume is given by the equation E=1/2 pμ x²ν. If this formla is applied to a case of earthquake swarm, the strain stored before the swarm could be roughly obtained provided that the conversion factor p, and the strained volume ν being equal to the both earthquake swarms. The order of magnitude of strain which was calculated along the above method was 10^-6 for the swarm in 1967 and was 10^-7 for that in 1971. These values of strain are reasonable as the strain stored up in low seismic acitivity field like southern Kyushu.