[SSS17-P09] Implosion earthquakes at very shallow level beneath lava dome, Unzen Volcano, Japan
Keywords:Unzen Volcano, implosion earthquake, volcanic earthquake, resistivity structure, closing crack, lava dome
In order to explore the shallow earthquake activity beneath Unzen volcano, we established 12 seismic stations around Mt. Fugen and Heisei-Shinzan lava dome in August, 2018 (plus 2 stations at Heisei-Shinzen lava dome since April, 2019). In addition, broad-band magnetotelluric (MT) observation was conducted in October and November, 2018 to investigate the contribution of fluids to the earthquakes. From these observations, we found (1) the hypocenters of very shallow earthquakes are relocated 200m depth, which correspond to the boundary between Mt. Fugen and Heisei-Shinzan lava dome and also correspond to the conduits of the 1991-1995 eruptions, (2) very shallow earthquakes not only shows downward P-wave arrivals but shows upward P-wave arrivals at a few seismic stations, (3) very shallow earthquakes sometimes occur repeatedly within 20 seconds, (4) Tensile axes of very shallow earthquakes shows various directions, (5) the zone below very shallow earthquakes shows significantly high resistivity. We conclude that the very shallow earthquakes were generated by the collapse of small voids filled with gas and air at the boundary between Mt. Fugen and Heisei-Shinzan lava dome. We speculate that the recent deflation around the volcanic conduits may generate the voids for the very shallow earthquakes.