Akiko Takeo, Kiwamu Nishida, Hiroshi Aoyama, Motoko Ishise, Takeru Kai, Ryo Kurihara, Takuto Maeda, Yuta Mizutani, Yuki Nakashima, Shogo Nagahara, Xiaowen Wang, Lingling Ye, Takeshi Akuhara , Yosuke Aoki

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.