Abstract
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.
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