17:15 〜 18:30
[MAG37-P03] Characteristics of seismic signals associated with the hydrothermal system beneath a giant submarine caldera
キーワード：火山活動、カルデラ噴火、海底観測
The submarine Kikai caldera is located approximately 50 km south from the Kyushu island and lies astride the volcanic front of the southwestern Japan. It has been recently revealed that giant rhyolite lava dome with a volume of 32 km3 is formed after the last super eruption with an emission of more than 170 km3 of tephra at 7.3 k.a. by multi-channel seismic reflection surveys by the Kobe University T/S Fukae Maru cruise (Tatsumi et al., 2018). We installed a seafloor seismic network composed of five ocean bottom seismometers (OBSs) on the rim of the submarine Kikai caldera from October 19, 2016 to March 14, 2017, which was the first seismic observation conducted in the vicinity of the giant submarine caldera. The seafloor seismic data observed a wide variety of signals associated with volcanic activities beneath the caldera.
In January and February of 2017, which was at around the middle of the observation period, earthquake swarm related to volcanic activity had been recorded at seismographs at Satsuma-Iwojima that is a volcanic island on the northwestern rim of the caldera (Monthly volcanic activity report by the Japan Meteorological Agency). Our OBSs clearly recorded the signals associated with those volcanic earthquakes with a dominant frequency of 6 Hz and with a duration of about 1 minute. Throughout the observation period, on the other hand, periodic signals associated with the volcanic tremors frequently observed at all stations. They had a dominant frequency of higher than 15 Hz and an interval of about 1 hour. The periodic and frequency-dependent amplitude modulation of volcanic tremors at intervals ranging from 46 to 50 minutes were observed by broadband seismic network around Satsuma-Iwojima, which are well explained by the model of a heated water pocket that pressure-controlled valve of the volcano (Ohminato, 2006). The observed signals at surrounding sites might be associated with the hydrothermal system beneath the submarine caldera.
In January and February of 2017, which was at around the middle of the observation period, earthquake swarm related to volcanic activity had been recorded at seismographs at Satsuma-Iwojima that is a volcanic island on the northwestern rim of the caldera (Monthly volcanic activity report by the Japan Meteorological Agency). Our OBSs clearly recorded the signals associated with those volcanic earthquakes with a dominant frequency of 6 Hz and with a duration of about 1 minute. Throughout the observation period, on the other hand, periodic signals associated with the volcanic tremors frequently observed at all stations. They had a dominant frequency of higher than 15 Hz and an interval of about 1 hour. The periodic and frequency-dependent amplitude modulation of volcanic tremors at intervals ranging from 46 to 50 minutes were observed by broadband seismic network around Satsuma-Iwojima, which are well explained by the model of a heated water pocket that pressure-controlled valve of the volcano (Ohminato, 2006). The observed signals at surrounding sites might be associated with the hydrothermal system beneath the submarine caldera.