Multiparametric observations are commonly acquired around volcanoes to monitor volcanic activity, with seismic motion and ground deformation being the primary geophysical observations that are obtained around well-monitored active volcanoes. Infrasound, which is an acoustic wave that propagates through the air at frequencies below 20 Hz, has recently been added as a volcanic monitoring tool owing to its ability to record the occurrence and explosivity of eruptions. The increasing implementation of multiparametric observations in volcanic monitoring networks has led to the discovery that different types of data are coupled in terms of volcanic activity. For example, Ichihara et al. (2012) showed that infrasound and vertical ground motion were coupled during volcanic activity via a cross-correlation analysis of infrasound and vertical ground velocity data at Asama and Shinmoe-dake volcanoes in Japan. Infrasound-vertical ground motion coupling has been further confirmed via similar cross-correlation analysis (Matoza & Fee, 2014;Muramatsu et al., 2022;Yukutake et al., 2018). These data suggest that the infrasound signal produced by the volcanic activity vibrates the ground surface and subsequently generates seismic motion. Matoza et al. (2019) analyzed geophysical observations during the explosive eruptions at Popocatépetl volcano in Mexico, and reported that the high-amplitude initial phase of the infrasound waveform is inversely correlated with the vertical displacement waveform recorded by a broadband seismometer. These studies have clearly demonstrated that the propagation of the infrasound wave emitted by volcanic eruptions generates vertical ground motion.Clear coupling between the observed ground motion and electric field has been reported at a number of volcanoes. Collocated seismometer and telluric stations have shown that the ground shaking owing to seismic wave propagation can induce electric-field variations (e.g.,
Summary Unzen volcano, located on Shimabara Peninsula, Nagasaki, Japan, is an active volcano that has been intensively monitored since 1989, one year before the most recent eruption in 1990–1995. Previous earthquake and surface deformation studies have revealed that magma is transported obliquely from a magma reservoir beneath Tachibana Bay, to the west of Shimabara Peninsula. Here we conduct broadband magnetotelluric (MT) surveys at 99 sites around Shimabara Peninsula to investigate the crustal structure beneath Unzen volcano that is related to magma migration. A three-dimensional resistivity model that is constructed from 25 broadband MT sites and 45 telluric sites shows a broad high-resistivity zone beneath Shimabara Peninsula and low-resistivity zones to the west and east of the peninsula. An unexpected observation is the spatial alignment of the high-resistivity zone with a seismic low-velocity zone at 3–15 km depth. Quantitative analysis indicates this high-resistivity zone contains <0.7% melt under the assumption that the melt is stored in a good porosity network, while <11% melt in relatively poor pore network. We infer this high-resistivity, low-velocity zone to be a highly crystallized mush zone with low permeability. The hypocenters and pressure sources of the 1990–1995 eruption are distributed along the boundary between the high- and low-resistivity zones beneath the western part of the peninsula. We therefore conclude that the magma migrated along a structural boundary that possessed a relatively high permeability. Previous studies have suggested that eruptible magma is usually transported vertically upward through the center of the mush zone, whereas the present results reveal that magma can be transported along the upper boundary of a highly crystallized mush zone.
<p>Unzen Volcano is located in Shimabara Peninsula, Nagasaki, Japan. After 198 years of dormancy, the volcano erupted throughout 1990-1995 and resulted the emergence of new lava dome called Heisei-Shinzan. Following the eruption, numerous studies have been intensively conducted in Unzen volcano to assess the eruption mechanism and the magma plumbing system. Regarding to the magmatic system, the most preferred model is that the primary supply of magma is stored beneath Chijiwa bay. This magma chamber is located about 15 km west of the active dome at vertical depth approximately 15 km, and followed by subordinate shallower magma chambers beneath the volcano (e.g. Nakamura 1995; Kohno et al 2008). Upon the eruption, the magma ascended obliquely towards the summit in east direction (e.g. Umakoshi et al 2001). However, how main magma chamber&#160; and shallower chambers are connected to the summit via oblique pathway is poorly imaged in terms of structure.<br>As widely known, Magnetotelluric method is highly sensitive to low resistivity zone caused by interconnected fluids. Low resistivity zone detected in the volcanic area usually can be interpreted as hydrothermal/magmatic fluid and or magma chamber containing partial melt (e.g. Aizawa et al 2014; Hill et al 2015). Thus, by using broadband Magnetotelluric method, we aim to investigate resistivity structure of Unzen volcano associated with magmatic system and its controlling structure (e.g. pathway and faults).<br>Although the shallow structures around Unzen volcano are estimated by the 2017-2019 campaigns (Triahadini et al 2019; Hashimoto et al 2020), we are unable to image deeper structure around the proposed location of magma chambers and magma pathway. To achieve our goals, during November-December 2020, we installed 35 new sites to cover whole area in Shimabara Peninsula. In total, deployed 99 Magnetotelluric stations covering Unzen volcano and Shimabara Peninsula. On this meeting, we would like to present our resistivity structure derived from all dataset.</p>
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