日露共同地震観測による2007年8月2日に発生したサハリン南西沖の地震(M_ 6.4)の余震活動

On August 2, 2007, an MJMA 6.4 moderate earthquake occurred off the southwest coast of Sakhalin Island, far eastern Russia. In the city of Nevelsk, situated just in the aftershock region, two people were killed and more than 240 buildings were damaged (we call this event as the 2007 Nevelsk earthquake). We have operated a temporal seismological network in southern Sakhalin consisted in ten stations with short-period sensors and 16-bit recorders since 2003. This dense network well recorded this earthquake and the following aftershock sequence. Initial aftershock hypocenters were calculated using a 1-D P-wave velocity structure that was estimated from 746 P-wave arrival times, including data from the aftershocks. To obtain a more precise aftershock distribution, we applied a double-difference hypocenter determination method. The well-determined aftershock hypocenters shows the following features; (1) in the northern and central parts of aftershock region, epicenters were distributed in the sea along the coast with NE-SW direction, (2) while in the southern part of aftershock region trend of epicenters changed toward land with NNW-SSE direction. This southern most part intersects the city of Nevelsk where severe damage occurred. On August 17, 2006, the 2006 Gomozavodskoe earthquake (MJMA 5.9) occurred near the southeastward aftershock region of the 2007 Nevelsk earthquake. The 2006 earthquake sequence was occurred deeper area as compared to the 2007 event. We examine the change in Coulomb failure function produced by the 2006 event and conclude that the 2006 event has less influence on occurrence of the 2007 event.