The nature of volcanic earthquake swarm preceding the 2000 flank eruption at Usu volcano, Hokkaido, Japan

The 2000 eruption of Usu volcano, Hokkaido, occurred on the northwestern flank of volcano on March 31 at 13:07 (Local time), after about 4 days of intensive seismic activity. We study the seismic process prior to the eruption using the routine earthquake catalogue of the Japan Meteorological Agency (JMA) and the digital seismograms of the short-period station NBBT (JMA). Earthquakes began beneath the summit and southern slope of the volcano. Then seismic foci clustered along two linear zones intersecting to the south of the initial epicentral zone. The majority of earthquakes occurred at depth within the range between 4 and 7 km. The magnitude range of located earthquakes varied between 0.8 and 4.4. The earthquake epicentres did not reach the sites of the forthcoming eruption; they stopped at a distance of about 1-3 km. The eruption began during the ceasing of seismic activity. A study of the nature of the swarm was performed for a sample of 303 earthquakes with a magnitude interval from 2.5 to 3.4 that were representative of the swarm. Visual analysis allowed separating the earthquakes into two groups: 286 volcano-tectonic (VT) earthquakes and 17 long-period (LP) earthquakes. The spectral analysis of S waves of 274 VT earthquakes in the frame of Brune's model showed that the VT earthquakes may be separated into two groups as characterized by high stress drops (HSD) and low stress drops (LSD). The swarm began with HSD earthquakes. The LSD and LP earthquakes were recorded about 30-40 hr later, simultaneously with the beginning of deformation of volcanic edifice, and accompanied the HSD earthquakes up to the beginning of eruption. We attribute the HSD earthquakes to the probable product of magmatic thermal fracturing of host rocks. The relatively long LSD ruptures may be attributed to the slips along the pre-existing fractures during magma movement.