Observation of Microearthquakes in Mino District in Gifu Prefecture, Central Japan

Since 1956, observation of microearthquakes have been carried out at several stations in Mino area in Gifu Prefecture. The observation was continued for about three months at every station, except the Sinpukuzi station where the observation was made for more than one year. The ground unrest is about 10μ kines in velocity amplitude at all stations, and earthquakes having the magnitudes from -2 to 2 are observed. The frequency of occurrence of earthquake is about 8 per day and is almost constant throughout a year in the vicinity of Gifu City. The magnitude-frequency relation of these earthquakes is well consistent with the following relation which was obtained for moderate earthquakes around Gifu City: logn=-1.69+0.74(8-M) for r≤100km, dM=1, per year. These observations of microearthquakes were made by means of velocity seismographs. Using the maximum velocity amplitude and the hypocentral distances of earthquakes, the magnitudes of earthquake were determined by the following equation: M=1.25logV(kine)+2.5logr(km)+1.0, which is compatible with the Tsuboi's equation based on the maximum displacement amplitude and the epicentral distance. According to the results of simultaneous observation with the Wiechert's displacement seismograph, this equation is found to be useful also for determinig the M of "bigger" earthquakes. The energy E of each microearthquake was evaluated from its seismogram. The magnitude M(E) which is calculated with the Gutenberg-Richter's equation, M=(logE-11.8)/1.5, is consistent with the magnitude, M(υ, r) calculated by the above equation. The geographical distribution of microearthquakes was determined by simultaneous observation at three stations. Both P and S phases of the microearthquakes within a range of r≤100km around these three stations are clear and the accurate determination of the foci was possible. They occur at the depth of about 20km and 50-60km. The sequential occurrence is recognized. The geographical distribution of these microearthquakes is concentrated in the vicinity of the seismological stations. The fact is more clearly illustrated by the frequency curve of P-S interval time and that of magnitude. The smaller the magnitudes of group of the observed microearthquakes, the nearer is the location of their foci.