地震に先駆するP波速度変化地域の検出

抄録

About 400 shallow earthquakes in central Japan occurring during 1967-1974 have been relocated using data supplied by the Japan Meteorological Agency and several university seismic stations. About 4200 P-residuals are obtained in the relocation. The residual is approximately normally distributed with a mean of 0.0sec and a standard deviation of 0.59sec. Therefore, the probability that a residual exceeds 0.4sec is 0.25. If the actual travel-time for paths crossing the focal region of an impending earthquake is increased by 0.4sec, the probability that an observed residual for one of these paths exceeds 0.4sec will be 0.50. Let R denotes the ratio of the number of paths with residuals larger than 0.4sec to the total number of the paths crossing a certain region. The R-values for the focal regions of the central Gifu earthquake of 1969 (M=6.6), the Izu-hanto-oki earthquake of 1974 (M=6.9), and other 24 earthquakes of smaller magnitudes during some time-intervals before the occurrence of them have been determined to be about 0.5 or more. These values suggest the decrease in P-velocity before the earthquakes. A map has been made showing the distribution of R-values in 204 areas of 0.2°×0.2° in central Japan. Significantly high R-values are found in the areas containing the focal regions of the above-mentioned two earthquakes in the maps covering certain periods before the earthquakes. However, there are many other areas of high R-values, which are not connected with the occurrence of large earthquakes until now. Most of these areas may correspond to inherent low-velocity regions in the crust.