稠密余震観測データを用いたダブル・ディファレンス・トモグラフィー法による2004年新潟県中越地震の震源断層のイメージング

抄録

We obtained detailed aftershock distribution and three-dimensional seismic velocity structure in and around the focal area of the 2004 Mid Niigata Prefecture (Niigata-Chuetsu) earthquake (M6.8) by inverting travel time data from a dense observation network and surrounding permanent stations. We adopted double-difference tomography method for the inversions. The fault planes of the main shock and the largest aftershock, which are delineated by two parallel aftershock alignments, took place where seismic velocity changes abruptly at the boundary between the lower velocity hanging wall and the higher velocity footwall for both P- and S-waves. The seismic velocity structure changes along the strike of the faults. The major velocity boundary zone between the lower velocity hanging wall and the higher velocity footwall migrates to the west at a location near the central part of the focal area, where the main shock hypocenter is located. Some parts of the fault planes, e. g. around the hypocenter of the M6 aftershock on Oct. 27, are imaged as low velocity zones. Asperity (large coseismic slip area) of the main shock seems to correspond with a higher velocity portion along the fault plane.