Earth Planets Space, Vol. 62 (No. 12), pp. 905-913, 2010
Yoshiyuki Tanaka1, Aitaro Kato1, Takayuki Sugano1, Guangyu Fu2, Xinlin Zhang1, Masato Furuya3, Wenke Sun4, Shuhei Okubo1, Shigeo Matsumoto5, Masaki Honda6, Yasuhiro Sugawara6, Isao Ueda6, Masaaki Kusaka6, and Misao Ishihara6
1Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
2Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, China
3Department of Natural History Sciences, Graduate School of Science, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810 Japan
4College of Earth Science, Graduate University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing, 100049, China
5Drilling Technology Department, Sumiko Consultants Co., Ltd., 9-7, Ikenohata 2-Chome, Taito-Ku, Tokyo 110-0008, Japan
6Department of Geodesy, Geospatial Information Authority of Japan, 1 Kitasato, Tsukuba, Ibaraki 305-0811, Japan
(Received July 8, 2010; Revised October 15, 2010; Accepted November 10, 2010; Online published February 3, 2011)
Slow-slip events (SSEs) have been observed in many plate-boundary zones along the circum-Pacific seismic belt. Previous studies have revealed that high-pressure fluids supplied from the subducted oceanic plate can generate SSEs. However, the behavior of these fluids during an SSE has not been fully elucidated. This paper discusses possible fluid migration along the plate boundary on the basis of spatiotemporal gravity changes observed by absolute and relative gravimeters during a long-term SSE in the Tokai district, Japan. Relative-gravity data are sometimes unreliable because of limited observation accuracies and possible noise produced by groundwater. Nevertheless, the observed gravity changes show a systematic pattern of spatial changes over the slow-slip area. This pattern can be explained by a poroelastic model assuming fluid migration along the plate interface, for which an inversion indicates a permeability of about 10-15 m2. This lies within the range of permeability values inferred by other studies in slow-slip areas. Long-term SSEs have occurred repeatedly in the Tokai district. If the permeability remains greater than 10-15 m2 during a future SSE, it will be possible to detect fluid migration by improving the observation accuracy to the 1-μGal level and accurately evaluating groundwater-related noise.
Key words: Slow earthquake, slow slip, gravity, fluid, poroelasticity, permeability, subduction zone, crustal deformation.