Fine structure of aftershock distribution of the 1997 Northwestern Kagoshima Earthquakes with a three-dimensional velocity model

In 1997, two earthquakes (M6.5 and M6.3) occurred in the northwestern part of Kagoshima Prefecture, Japan. We carried out temporary seismic observatio n to obtain the detailed aftershock distribution. We constructed a 3-D P wa ve velocity model by inverting the travel times of aftershocks observed at 14 seismic stations in and around the focal area and relocated more than 14 ,000 aftershocks with the 3-D velocity model. The general features of the aftershock distribution are as follows: (1) Aft ershocks of the first main shock (M6.5) are distributed with a strike of ne arly E-W (N 100 degrees E) in a vertical plane with a horizontal length of 21 km and a depth range of 2 to 9 km; (2) The second main shock (M6.3) has an 'L'-shaped aftershock distribution: one plane strikes nearly E-W, which is parallel to the aftershock zone of the first main shock, and the other i s a conjugate plane; (3) An obvious seismicity gap of about 3 km wide is fo und between the aftershock zones striking E-W for the first and second main shocks; (4) The aftershock activities are generally low around the hypocen ters of the two main shocks. Our results show that most of aftershocks occurred not in high or low veloc ity zones, but in intermediate velocity areas. Several vertical linear dist ributions of aftershocks are also confirmed in the two focal zones striking E-W. These peculiar distributions suggest that the aftershock activity is affected by the underground structural boundaries.