Earth Planets Space, Vol. 62 (No. 9), pp. 685-691, 2010
Takuto Maeda1, Kazushige Obara1, and Yohei Yukutake2
1National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba, Ibaraki 305-0006, Japan
2Hot Spring Research Institute, 586 Iriuda, Odawara, Kanagawa 250-0031, Japan
(Received April 5, 2010; Revised August 15, 2010; Accepted August 18, 2010; Online published December 13, 2010)
We found a recurring seismic velocity decrease associated with small earthquake swarms experienced in 2007 in a geothermal area in Kyushu, southwestern Japan, by analyzing long-term changes in the autocorrelation function (ACF) of seismic noise. The seismic velocity decrease appeared just after two major periods of earthquake activity began in June and October of 2007. In both instances, conditions returned to normal within a characteristic time period of 4 months. The observed size of the velocity changes agrees well with the magnitudes of the swarms. The lag-time dependence of ACF changes can be systematically explained by seismic velocity changes induced by fluid inclusion in a small, localized area deep within the hypocenter region.
Key words: Earthquake swarm, temporal change, seismic interferometry.