Seismic interferometry is a powerful tool to monitor the seismic velocity change associated with volcanic eruptions. For the monitoring, changes in seismic velocity with environmental origins (such as precipitation) are problematic. In order to model the environmental effects, we propose a new technique based on a state-space model. An extended Kalman filter estimates seismic velocity changes as state variables, with a first-order approximation of the stretching method. We apply this technique to three-component seismic records in order to detect the seismic velocity change associated with the Shinmoe-dake eruptions in 2011 and 2018. First, ambient noise cross correlations were calculated from May 2010 to April 2018. We also modeled seismic velocity changes resulting from precipitation and the 2016 Kumamoto earthquake, with exponential type responses. Most of the results show no significant changes associated with the eruptions, although gradual inflation of the magma reservoir preceded the 2011 eruption by 1 year. The observed low sensitivity to static stress changes suggests that the fraction of geofluid and crack density at about 1-km depth is small, and the crack shapes could be circular. Only one station pair west of the crater shows the significant drop associated with the eruption in 2011. The gradual drop of seismic velocity up to 0.05% preceded the eruption by 1 month. When the gradual drop began, volcanic tremors were activated at about 2-km depth. These observations suggest that the drop could be caused by damage accumulation due to vertical magma migration beneath the summit.

Summary To obtain an internal S-wave velocity structure, we conducted a passive seismic campaign with 21 1-Hz seismometers on and around the Showa-Shinzan lava dome, which emerged during the 1943–1945 eruption of Usu Volcano, Japan. Before the campaign, we calibrated seismometers and found slight phase-response differences between seismometers of less than 1–2 degrees. After the campaign, we extracted seismic wavefield by taking cross-correlations of vertical-component ambient noise records between seismic sites. We developed a new method to measure phase velocities of the Rayleigh wave automatically by assuming layered structure and finally obtained one-dimensional S-wave velocity models in summit, roof and base regions. The obtained S-wave velocity right beneath the intruded lava dome is higher than that in surrounding areas by a few tens of percent down to a few hundred meters below sea level, indicating narrow but deep existence of the root of the lava dome. The obtained S-wave velocity at depths shallower than ∼50 m inside the lava dome in the summit area was ∼1 km/s, significantly lower than that predicted from the density of  2.3 × 103 kg/m3 estimated in previous muon-radiography studies and a conventional scaling, indicating the effect of cracking in the lava dome.

To obtain an internal S-wave velocity structure, we conducted a passive seismic campaign with 21 1-Hz seismometers on and around the Showa-Shinzan lava dome, which emerged during the 1943–1945 eruption of Usu Volcano, Japan. Before the campaign, we calibrated seismometers and found slight phase-response differences between seismometers of less than 1–2 degrees. After the campaign, we extracted seismic wavefield by taking cross-correlations of vertical-component ambient noise records between seismic sites. We developed a new method to measure phase-velocities of the Rayleigh wave automatically by assuming layered structure and finally obtained one-dimensional S-wave velocity models in summit, roof and base regions. The obtained S-wave velocity right beneath the intruded lava dome is higher than that in surrounding areas by a few tens of percent down to a few hundred meters below sea level, indicating narrow but deep existence of the root of the lava dome. The obtained S-wave velocity at depths shallower than ~50 m inside the lava dome in the summit area was ~1 km/s, significantly lower than that predicted from the density of ~2.3 x 10^3 kg/m estimated in previous muon-radiography studies and a conventional scaling, indicating the effect of cracking in the lava dome.