Seismic velocity decrease and recovery related to earthquake swarms in a geothermal area

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.