Earth Planets Space, Vol. 58 (No. 2), pp. 173-179, 2006
Yuichi Kitagawa, Naoji Koizumi, Makoto Takahashi, Norio Matsumoto, and Tsutomu Sato
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Japan
(Received July 25, 2005; Revised December 2, 2005; Accepted December 3, 2005; Online published February 17, 2006)
Associated with the 2004 earthquake off the west coast of northern Sumatra, changes in groundwater levels or pressures were observed at many observation stations in Japan which are more than 5000 km from the hypocenter. At 38 of the 45 observation stations, there were changes in groundwater levels or pressures. At the 10 observation stations in which the Ishii-type borehole strain instruments were established, changes in crustal strains were also observed. A major part of the changes in crustal strains and groundwater levels or pressures were dynamic oscillations due to a seismic wave. At some stations, coseismic or postseismic rises or drops were also observed. At five stations where both crustal stain and groundwater levels or pressures were observed, postseismic changes in groundwater levels or pressures were consistent with coseismic static steps in crustal strains. At the other five stations, postseismic changes in groundwater levels or pressures did not agree with the coseismic static steps. At two stations of these five stations, it is anticipated that the pore water pressure change in each aquifer locally occurred independently of the change in crustal strain. At another station, postseismic changes in groundwater level possessed the same characteristics as a model removing the temporary deposition. At the last two stations, the causes of the changes are unknown.
Key words: Sumatra earthquake, groundwater level or pressure, crustal strain, oscillation due to a seismic wave, postseismic changes.