TY - JOUR
T1 - Joint seismological–magnetotelluric investigation of shallow and implosive non-DC and DC earthquakes beneath the gravitationally unstable Heisei-Shinzan Lava Dome, Unzen Volcano, Japan
AU - Hashimoto, Tasuku M.
AU - Aizawa, Koki
AU - Hayashida, Yuto
AU - Yuasa, Yuhei
AU - Matsushima, Takeshi
AU - Yamamoto, Yuto
AU - Tsukamoto, Kaori
AU - Miyano, Kanta
AU - Matsumoto, Satoshi
AU - Shimizu, Hiroshi
N1 - Funding Information:
We thank the students of Kyushu University (Megumi Kamizono, Ayaho Mitsuoka, Yuta Kawamura, Naoto Maesako, Agnis Triahadini, and Dan Muramatsu) for their assistance during the field campaigns. We used seismic data from Nagasaki District, Japan Meteorological Agency (JMA), and Ministry of Land, Infrastructure, Transport, and Tourism (MLIT). The geomagnetic data used for the remote-reference processing during the magnetotelluric data analysis were provided by the Kakioka Geomagnetic Observatory, JMA. The comments from Y. Yukutake and an anonymous reviewer are appreciated for the improvement of the manuscript. This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (MEXT KAKENHI Grant Number JP15K17751 ), and its “ Earthquake and Volcano Hazards Observation and Research Program ”.
Funding Information:
We thank the students of Kyushu University (Megumi Kamizono, Ayaho Mitsuoka, Yuta Kawamura, Naoto Maesako, Agnis Triahadini, and Dan Muramatsu) for their assistance during the field campaigns. We used seismic data from Nagasaki District, Japan Meteorological Agency (JMA), and Ministry of Land, Infrastructure, Transport, and Tourism (MLIT). The geomagnetic data used for the remote-reference processing during the magnetotelluric data analysis were provided by the Kakioka Geomagnetic Observatory, JMA. The comments from Y. Yukutake and an anonymous reviewer are appreciated for the improvement of the manuscript. This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (MEXT KAKENHI Grant Number JP15K17751), and its “Earthquake and Volcano Hazards Observation and Research Program”. The hypocenter data is available on the SEVO website (http://www.sevo.kyushu-u.ac.jp/open_file/hypo_Hashimoto_et_al_2020_JVGR.txt). Other data are available from KA on reasonable request. Not applicable. Not applicable.
Publisher Copyright:
© 2020
PY - 2020/11/15
Y1 - 2020/11/15
N2 - Shallow, implosive non-double-couple (NDC) earthquakes have been detected beneath Unzen Volcano, Japan, by a routinely operated seismometer network. Here, we analyze the waveforms from 14 temporary seismic stations around Mt. Fugen (Fugendake) and Heisei-Shinzan Lava Dome, which formed during the 1990–1995 eruptions, to further investigate this shallow seismicity. We also estimate the subsurface electrical resistivity structure using broadband magnetotelluric observations and investigate its relationship to the shallow seismicity. We find that (1) implosive NDC earthquakes occur at the craters that were active during the 1990–1991 eruptions, which are currently buried by the approximately 200-m-thick Heisei-Shinzan Lava Dome; (2) the detected NDC earthquakes generally possess downward P-wave arrivals, although upward P-wave arrivals are observed at a few seismic stations; (3) the tensile axes of the NDC earthquakes show various slip directions; (4) repeated NDC earthquakes are in some cases detected within 20 s of the initial event; and (5) a high-resistivity zone exists beneath the NDC earthquakes. We conclude that the implosive NDC earthquakes were generated by the collapse of small, randomly oriented, vapor-filled voids at the two buried craters, with active volcanic conduits still present beneath these craters. The shallow DC earthquakes consist of two clusters that are located 2–3 km below the NDC earthquakes. The focal mechanisms of the shallower cluster (approximately 2 km depth) indicate various fault mechanism, whereas those of the deeper cluster (approximately 3 km depth) indicate strike-slip faulting. The top of the shallower cluster corresponds to the base of the conductive zone, which suggests that the DC earthquakes in the shallower cluster are generated in a zone of high pore pressure that is capped by a hydrothermally altered clay layer.
AB - Shallow, implosive non-double-couple (NDC) earthquakes have been detected beneath Unzen Volcano, Japan, by a routinely operated seismometer network. Here, we analyze the waveforms from 14 temporary seismic stations around Mt. Fugen (Fugendake) and Heisei-Shinzan Lava Dome, which formed during the 1990–1995 eruptions, to further investigate this shallow seismicity. We also estimate the subsurface electrical resistivity structure using broadband magnetotelluric observations and investigate its relationship to the shallow seismicity. We find that (1) implosive NDC earthquakes occur at the craters that were active during the 1990–1991 eruptions, which are currently buried by the approximately 200-m-thick Heisei-Shinzan Lava Dome; (2) the detected NDC earthquakes generally possess downward P-wave arrivals, although upward P-wave arrivals are observed at a few seismic stations; (3) the tensile axes of the NDC earthquakes show various slip directions; (4) repeated NDC earthquakes are in some cases detected within 20 s of the initial event; and (5) a high-resistivity zone exists beneath the NDC earthquakes. We conclude that the implosive NDC earthquakes were generated by the collapse of small, randomly oriented, vapor-filled voids at the two buried craters, with active volcanic conduits still present beneath these craters. The shallow DC earthquakes consist of two clusters that are located 2–3 km below the NDC earthquakes. The focal mechanisms of the shallower cluster (approximately 2 km depth) indicate various fault mechanism, whereas those of the deeper cluster (approximately 3 km depth) indicate strike-slip faulting. The top of the shallower cluster corresponds to the base of the conductive zone, which suggests that the DC earthquakes in the shallower cluster are generated in a zone of high pore pressure that is capped by a hydrothermally altered clay layer.
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U2 - 10.1016/j.jvolgeores.2020.107066
DO - 10.1016/j.jvolgeores.2020.107066
M3 - Article
AN - SCOPUS:85092060197
VL - 406
JO - Journal of Volcanology and Geothermal Research
JF - Journal of Volcanology and Geothermal Research
SN - 0377-0273
M1 - 107066
ER -