Earth Planets Space, Vol. 57 (No. 4), pp. 309-314, 2005LETTER
Yasuhiro Asai1, Makoto Okubo1, Hiroshi Ishii1, Harumi Aoki1, Tsuneo Yamauchi2, Yuuichi Kitagawa3, and Naoji Koizumi3
1Tono Research Institute of Earthquake Science, Association for the Development of Earthquake Prediction, 1-63 Akeyo-cho Yamanouchi, Mizunami, Gifu 509-6132, Japan
2Nagoya University, Furou-cho Chikusa-ku Nagoya 464-8602, Japan
3Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan
(Received November 30, 2004; Revised March 25, 2005; Accepted March 28, 2005)
The 2004 off the Kii peninsula earthquakes (Mj7.1 and Mj7.4) occurred at the Nankai trough, on 5 September 2004. Clear strain-steps associated with these earthquakes were observed with Ishii-type borehole strainmeters and quartz-tube extensometers in the Tokai and Kinki districts. We investigated the spatial and depth distribution of the observed principal strain changes and compared the observed strain-steps and theoretical calculations at all observatories. The following results were obtained: the observed strain-steps at all observatories are generally consistent with the polarities of the theoretical values, and the observed strain-step increases with depth at the same place. As for the Togari site, the following relationships are obtained: the strain-step and the tidal strains increase with depth and increasing of the modulus of elasticity, namely, hardness of rock. We consider that the geological structure around the observatory may cause a modification of the strain field.
Key words: The 2004 off the Kii peninsula earthquakes, strain-step, principal strain, Ishii-type borehole strainmeter, quartz-tube extensometer, modification of strain field.