Multiple earthquake events at moderate-to-large magnitudes in Japan.

A systematic study of multiple earthquake events to be found in the Japan Meteorological Agency catalogue, 1926-1979, has been made by restricting the data to magnitudes M≥5.0, and identifying multiple events by the three-in-one-day rule. Of the 74 events identified, 33 are classed as mainshock events, 29 as swarms, and 12 as multiplets. The locations of many mainshock events have been close to those of swarms (although no swarms have been located in western Honshu), and this may indicate the occurrence of local changes in the mechanical condition of the rock, and in the complexity of the stress field, with time. The results encourage the search for a possible precursory relationship between swarms and mainshock events in Japan such as has been proposed for New Zealand.