Earthquake swarm activity with a low stress drop beneath Mt. Fuji, Japan: Indication of fluid-induced earthquakes?

An earthquake swarm activity took place beneath Mount Fuji, Japan with a depth of 20 km in the end of January, 2012. Japan Meteorological Agency (JMA) determined hypocenters of 76 earthquakes with M > 2 in the area within two days on January 28 and 29. Five of them had magnitudes greater than 4 and the largest one was 5.4. The swarm activity showed a circular migration from the epicenter of an earthquake with M4.9 at 0739am on January 28 (JST, 2239pm UT on January 27) with a speed of 2 to 5 km/day. We analyzed stress drops of 17 earthquakes with M > 3.5 that occurred from January, 2000 to June, 2012 in the area of the swarm activity. We calculated empirical Green's functions from waveforms of earthquakes with magnitudes of 3.0 to 3.2 and estimated stress drops of the earthquakes assuming that the source spectra can be expressed as the omega-squared model. We found that earthquakes of the swarm had smaller stress drops by an order of magnitude than the values of earthquakes that occurred in the same area before the swarm. This result would suggest that the fluid increased the pore pressure and triggered the swarm activity, which is consistent with the epicenter migration. Epicenters of hte 76 earthquakes determined by the Japan Meteorological Agency. Color indicates the origin time of earthquakes. Time zero corresponds to 6am on January 28 (JST). Stress drops of 17 earthquakes with magnitudes greater than 3.5.