Spatio-temporal analysis of multiplets in the aftershocks of the 2005 West Off Fukuoka earthquake

On March 20, 2005, a damaging earthquake (M6.4), West Off Fukuoka earthquake, occurred in the Genkai Sea, Japan. The main shock was followed by more than 1,000 aftershocks, including a M6.0 event. Uehira et al.(2006) reported that some of the aftershocks occurred on a different fault to the main shock. Migration of the hypocenters was also observed. In this study we re-analyzed the aftershocks of this earthquake with a combination of multiplet analysis and conventional methods to better understand the physics behind the recorded behavior of the aftershocks. Multiplet clustering identified 643 events (50 clusters) as multiplets. The hypocenter distribution of these multiplets by double difference (DD) showed that most of the multiplet clusters occurred at the edge of the seismic cloud of aftershock locations. This is a similar result to findings with the data set collected after 1995 Kobe earthquake (Asanuma et al., 2005). The present work has also revealed that the activity of the multiplets remained relatively high up to 50 days after the main shock. We have characterized each multiplet cluster in terms of fault plane solution, the spatial distribution of the coherence within a multiplet cluster, ΔCFF, and the spatio-temporal variation of the source radius. From these analyses the following observations can be made: (a) in most of the multiplet clusters, the source radii of the multiplets overlap, (b) progressive extension of the seismic structure was observed for some multiplet clusters, (c) most of the FPS of the multiplets were similar to that of the main shock, and (d) the occurrence of some multiplets is correlated to the change of ΔCFF around the hypocenter. We compiled these features and investigated physical models to interpret the observed behaviors of the multiplet clusters. From this we conclude that the multiplet clusters at the edge of the seismic cloud on the same fracture as the main shock can be modeled by repeated slip controlled by small asperities in the plastic or post-seismic deformation area. This suggests that these multiplet clusters lie within the area of stress release of the main shock. We also have identified multiplets that are distributed more widely over the aftershock seismic cloud defining not any obvious seismic structures. However the similarities of their waveforms and source mechanisms to those associated with the main shock leads us to suggest that they occurred on independent fractures that are distributed within the larger fault system.