AfaultmodelofanaftershockoccurredNovember8(MJMA5.docVIP

第３章：GPSを用いた地殻変動調査 3.2 A fault model of an aftershock (M5.9) on November 8 and postseismic deformation of the 2004 Niigata Chuetsu earthquake (M6.8) by a dense GPS observation Hiroaki Takahashi1, Takeshi Matsushima2, Teruyuki Kato3, Akira Takeuchi4, Teruhiro Yamaguchi1, Yuhki Kohno2, Takeshi Katagi2, Jun’ichi Fukuda3, Kazuya Hatamoto4, Ryousuke Doke4, Yuki Matsu’ura4 and Minoru Kasahara1 1Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sapporo, 060-0810, Japan 2Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, 855-0843, Japan 3Earthquake Research Institute, University of Tokyo, Tokyo, 113-0032, Japan 4Department of Earth Sciences, Toyama University, Toyama, 930-8550, Japan Abstract To investigate the postseismic crustal deformation associated with the 2004 Niigata-Chuetsu earthquake (M6.8), we newly started GPS observation to fill the gap of the nationwide GPS network. Our GPS sites were mainly distributed in the focal region in where there was no permanent GPS site, and succeeded to catch the postseismic deformation. Coseismic displacements of several aftershocks were clearly detected because of immediate observation. We could estimate a fault model of an aftershock (M5.9) on November 8 occurred just beneath our GPS network. Moreover, clear postseismic deformation, which could be characterized by logarithmic decay function, was observed. This signal might suggest possible afterslip. Our results indicated that dense GPS observation could give important and interesting data to clarify the properties of shallow inland middle-size earthquakes. Key word: The 2004 Niigata-chuetsu earthquake, GPS, postseismic deformation, fault model, afterslip 1. Introduction The Niigata-Chuetsu earthquake (M6.8) occurred at shallow depth (13km) on 26 October 2004 in Mid Niigata prefecture, central Japan. Focal mechanism estimated by Japan Meteorological Agency (JMA) indicated pure reverse faulting with