Study on the source rupture growth in earthquake swarms

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We installed broadband seismographs in an area with earthquake swarms : Kouzu island. Combined this record with the regional broadband network data, we tried to examine the growth of source rupture, especially the cascade-type of rupture growth or the coalescence of small scale ruptures into a larger scale rupture.During the term of project, we were able to get a lot of broadband records for several earthquake swarms which occcurred near Kouzu island and off Ito. In additon, we analyzed the source rupture process of larger (Ms 6 to 7 class) earthquakes with swarm-like nature which occurred in Kyushu-Ryukyu district. Combining these results with those obtained for Kouzu and Off-Ito events, we reached the conclusion that an earthquake swarm is strongly characterised by the complementarity of individual source areas. This may imply that the interaction between individual seismic sources is rather weak in earthquake swarms. Such a nature probably reflects the bulk distribution of small-scale faults and the characteristic source of stress accumulation. We sill plan a new proposal for this kind of subject in future.Other remarks we obtained are (1) the stress drop is relatively small but very dispersive, (2) this characteristics does not depend on whether the earthquake swarm frequently happens or it rarely happens, and (3) the size distribution of seismic sources reveals a hierarchical structure, the ratio of length-scale between lower and higher classes being about three.