2017 年 70 巻 p. 183-194
Seismic activity is spatially concentrated, defining earthquake clusters, and varies temporally with the occurrence of large earthquakes and earthquake swarms. However, earthquake clusters that occur in isolation from surrounding earthquakes are rarely observed. To investigate the seismic activity in a given area, it is preferable that an earthquake cluster is objectively extracted. In this study, we extracted earthquake clusters in and around the Japanese islands based on hypocenter information listed in the unified catalog of the Japan Meteorological Agency. As a result, we identified that earthquake clusters were efficiently extracted in nearly all areas in which hypocenters are well determined. The seismic characteristics of the earthquake clusters clarify the spatial-temporal heterogeneity of earthquake occurrence and illustrate the temporal changes in seismic activity in each area. Many repeating earthquakes are located in the Pacific Plate subducting from the Kuril-Japan Trench and Philippine Sea plate subducting from the Ryukyu Trench. Earthquake clusters with a high incidence of repeating earthquakes are concentrated near the deepest limit of the inter-plate coupling areas at the Pacific plate. In contrast, most earthquake clusters in regions of subduction-related inter-plate earthquakes are characterized as sequences of mainshock - aftershock type, owing to the occurrence of aftershocks following large earthquakes. Using this method of focusing on earthquake cluster, additional earthquake data are easily analyzed. Moreover, this approach is useful in investigating temporal changes in seismic activity and detecting previously unknown earthquake clusters. The intensive monitoring of earthquake clusters will provide a better understanding of the spatial-temporal changes in seismic activity throughout the Japanese islands.