A property of minor shocks going ahead of large earthquakes



Introduction. Since about 1950, we have become aware that ‘twin earthquakes’ were occurring in and near the Kinki District, Central Japan, especially in the vicinity of the coastline of the Sea of Japan. They were tentatively called twin earthquakes, however, only because two major shocks occurred in a small area successively within a short time interval, say, several months or about a year.

The prominent one among them was the Tottori earthquake of magnitude 7.3, which occurred in September 1943 and was large enough to be felt all over Central Japan. Prior to this, major shocks of M:6.4 had taken place at the same area in March 1943. Another example of the twin earthquakes might be the couple of the Tango earthquake (M:7.4, 1927) and the Tajima earthquake (M:7.0, 1925), which occurred close to each other in the north of the Kinki District. To our regret, we have no recording of minor shocks that might have occurred previous to or between these major shocks because a network of observation stations with highly sensitive seismographs had not been set up as yet near the hypocenters.