1975 年 28 巻 3 号 p. 331-346
Five stations of the Tottori Microearthquake Observatory have supplied an amount of records of microearthquakes since June, 1965. They are very useful to investigate the characteristics of spatial and temporal distribution of microearthquakes. Using data from three stations MZT, OYT and IZT, a seismological bulletin has been compiled. It contains P, P-S and P-F times and directions of initial motions at three stations, the coordinates of foci calculated from data of three stations and magnitude of each shock determined from total duration (P-F time).
From June, 1965 to December, 1973, 7346 earthquake foci have been determined. Their epicenters have been plotted on seismicity maps. One of significant results from them is that the close relation between active faults and alignments of epicenters has been recognized.
Computed results of epicenters from data of the three stations have good accuracy comparing with those from five stations. Errors of hypocenters near to the network are less than 2km. Those of distant foci whose P-S times are larger than 10 seconds are less than 10km. Because of the important purpose to know the temporal distribution of microearthquakes, almost all data containing some reading values which do not have so good accuracy have been used for calculation.
Frequency distribution of magnitudes shows the normal activity in this region. It also shows that distant shocks whose magnitude are larger than 2.0 and epicentral distances are about 150km, are detected by this network of three stations. Hypocenters whose magnitudes are larger than 1.0 have been detected in the near region around the network.