Earth Planets Space, Vol. 60 (No. 11), pp. 1081-1086, 2008LETTER
Yusaku Ohta1, Satoshi Miura1, Takeshi Iinuma1, Kenji Tachibana1, Takeshi Matsushima2, Hiroaki Takahashi3, Takeshi Sagiya4, Takeo Ito4, Shin'ichi Miyazaki5, Ryosuke Doke6, Akira Takeuchi6, Kayo Miyao1, Akihiko Hirao2, Takahiro Maeda3, Teruhiro Yamaguchi3, Masamitsu Takada3, Makiko Iwakuni5, Tadafumi Ochi5, Irwan Meilano4, and Akira Hasegawa1
1Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
2Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, Shimabara 855-0843, Japan
3Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
4Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan
5Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
6Department of Earth Sciences, Toyama University, Toyama 930-8555, Japan
(Received December 4, 2007; Revised April 22, 2008; Accepted April 28, 2008; Online published November 18, 2008)
An intermediate-strength earthquake of magnitude Mj 6.8 occurred on July 16, 2007, centered beneath the Japan Sea a few kilometers offshore of Niigata Prefecture in central Japan. We constructed a dense GPS network to investigate postseismic deformation after this event, choosing our GPS sites carefully so as to complement the nationwide GPS GEONET array. Coseismic displacements caused by the mainshock detected at some GEONET sites were used to estimate coseismic fault parameters. The results indicate that the geodetic data can be explained by a combination of two rectangular faults dipping northwest and southeast. Minor but definite postseismic deformation was detected largely in the southern part of the dense network. The time series of site coordinates can be characterized by a logarithmic decay function, and the estimated time constant seems to be almost similar in range to that of the 2004 Mid-Niigata Prefecture Earthquake. We also found a possible site instability at 960566 (Izumo-zaki, GEONET) caused by a small, local landslide associated with the mainshock and therefore concluded that the data obtained at this site should not be used for coseismic or postseismic analysis.
Key words: Crustal deformation, coseismic slip, postseismic deformation, GPS, Niigata-Kobe Tectonic Zone.