Relationship between Crustal Structure and Source Fault of 1997 Kagoshima Earthquake (M6.6), Japan, based on Relocated Hypocenters and Seismic Reflection Data
Abstract:The 1997 Kagoshima earthquake (M6.6) occurred in southern Kyushu, Japan, subsequently involving the largest aftershock (M6.4) two months later. The mainshock focal mechanism shows a lateral fault type with a tensional axis of NW-SE direction. This area is located in the Hokusatsu-bend area (Murata,1987), which is a regional bending structure of the Shimanto Supergroup strata, and furthermore, in the junction area of the south-western Japan and the Ryukyu arcs. However, the relationship between the crustal structure and the source fault still remains unknown. Here we approach the analyses of the aftershock relocation and the seismic reflection data, and clarify the existence of characteristic structure to control the fault rupture.
We have early aftershock data of the Kagoshima University and the catalog of NIED Hi-net, the nation-wide dense network established since 2000. Using a common velocity structure and a hypocenter determination method, we relocated the hypocenters of the both data. Also we have the data of the seismic survey, of which the survey line was designed in 2000, as crossing the near-vertical mainshock fault. We applied with the Multi Dip Reflection Surface method, resulting in clarifying the seismic image.
The comparison with the relocated hypocenters and the fault rupture (Horikawa, 2001) demonstrated the aftershock gap within the large slip area, while its surrounding area shows high seismicity. Interestingly, the high seismicity aftershock area includes some seismicity gaps like a band in places.
The seismic cross section shows two north dipping reflectors. The two reflectors become obscure around the source fault, where the aftershock seismicity is very low. This suggests that a north dipping layering structure may control seismic activity. It means that the subsurface structure related with the Hokusatsu-bend may influence on the rupture and its aftershock expansion.