Study on the source rupture growth in earthquake swarms

我们在有地震群的Kouzu岛安装了宽带地震仪。将这一记录与区域宽带网络数据相结合，我们试图检验震源破裂的增长，特别是级联式破裂增长或小规模破裂合并成更大规模破裂。在项目期间，我们能够获得发生在Kouzu岛附近和伊藤附近的几个地震群的大量宽带记录。此外，我们还分析了发生在九州-琉球地区的6 ~ 7级大地震的震源破裂过程。将这些结果与Kouzu和Off-Ito事件的结果相结合，我们得出的结论是，地震群的强烈特征是各个震源区的互补性。这可能意味着在地震群中，单个震源之间的相互作用相当弱。这种性质可能反映了小型断层的大量分布和应力聚集的特征来源。对于这类课题，我们还在计划一个新的方案。得到的其他结论有：(1)应力降相对较小，但弥散性很好；(2)这种特征与地震群的频繁发生与否无关；(3)震源的大小分布呈现层次结构，上下级的长度尺度之比约为3。

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.