4:45 PM - 5:00 PM
[SVC28-18] Resistivity structure of Mt. Motoshirane, Japan, inferred from an audio-frequency magnetotelluric survey
Keywords:Mt. Motoshirane, resistivity structure, audio-frequency magnetotelluric method, phreatic eruption, hydrothermal system
Mt. Motoshirane is situated in the northwestern part of Gunma prefecture and is one of the pyroclastic cones of the Kusatsu-Shirane volcano. Its location is about 2 km south from Mt. Shirane, which hosts the Yugama crater. In 2018, some eruptions occurred from the Kagamiike-kita crater of Mt. Motoshirane. The eruptions were sudden phreatic explosions that resulted in casualties.
Since the most activities had occurred in the Yugama crater and its vicinity, few studies were conducted at Mt. Motoshirane. A broadband magnetotelluric (MT) survey carried out before the 2018 eruption revealed a low resistivity zone at a depth of ~2 km, but shallow structures, which include the explosion depths, were not well constrained due to the low-resolution at shallow depths (Matsunaga et al., 2020). X-ray diffraction analysis of the ash sampled of the 2018 eruption suggested that the source depth of the eruption reached the basement rocks (Yaguchi et al., 2019). Geological studies inferred that the basement rocks are exposed around the Kusatsu-Shirane volcano (Hayakawa, 1983). However, there is no information on the depth of the basement rocks below Mt. Motoshirane.
In this study, we conducted an audio-frequency magnetotelluric (AMT) survey to reveal the shallow structures around the craters erupted in 2018 and to clarify the shallow hydrothermal system of Mt. Motoshirane. The survey was conducted from September to October 2020, and five components of the electromagnetic field were measured at a total of 30 locations around the craters of the 2018 eruption. The distribution of apparent resistivity was calculated using det (Re Z), one of the rotational invariants of the impedance tensor, and tended to show high resistivities in the high frequencies and low resistivities in the low frequencies. In particular, some apparent resistivity maps at low frequencies showed low resistivity around the craters of the 2018 eruption.
We used a three-dimensional (3-D) inversion code using tetrahedral meshes (Usui, 2015; Usui et al., 2017) to accurately express the steep topography around the surveyed area. The obtained model was characterized by features similar to the apparent resistivity distribution: high resistivities in the shallow part of the pyroclastic cone and low resistivities in the deep part. To confirm the fitness to the data, we compared the model response function with the observed data using the phase tensor and the induction arrows. The calculated and observed data were consistent at most areas. However, a large difference between them was found on the east flank of the Mt. Motoshirane, which may have been caused by noise originating from man-made objects. We should further examine the data carefully.
In the presentation, we will show the inferred shallow structure of Mt. Motoshirane and interpret it based on the previous studies.
Since the most activities had occurred in the Yugama crater and its vicinity, few studies were conducted at Mt. Motoshirane. A broadband magnetotelluric (MT) survey carried out before the 2018 eruption revealed a low resistivity zone at a depth of ~2 km, but shallow structures, which include the explosion depths, were not well constrained due to the low-resolution at shallow depths (Matsunaga et al., 2020). X-ray diffraction analysis of the ash sampled of the 2018 eruption suggested that the source depth of the eruption reached the basement rocks (Yaguchi et al., 2019). Geological studies inferred that the basement rocks are exposed around the Kusatsu-Shirane volcano (Hayakawa, 1983). However, there is no information on the depth of the basement rocks below Mt. Motoshirane.
In this study, we conducted an audio-frequency magnetotelluric (AMT) survey to reveal the shallow structures around the craters erupted in 2018 and to clarify the shallow hydrothermal system of Mt. Motoshirane. The survey was conducted from September to October 2020, and five components of the electromagnetic field were measured at a total of 30 locations around the craters of the 2018 eruption. The distribution of apparent resistivity was calculated using det (Re Z), one of the rotational invariants of the impedance tensor, and tended to show high resistivities in the high frequencies and low resistivities in the low frequencies. In particular, some apparent resistivity maps at low frequencies showed low resistivity around the craters of the 2018 eruption.
We used a three-dimensional (3-D) inversion code using tetrahedral meshes (Usui, 2015; Usui et al., 2017) to accurately express the steep topography around the surveyed area. The obtained model was characterized by features similar to the apparent resistivity distribution: high resistivities in the shallow part of the pyroclastic cone and low resistivities in the deep part. To confirm the fitness to the data, we compared the model response function with the observed data using the phase tensor and the induction arrows. The calculated and observed data were consistent at most areas. However, a large difference between them was found on the east flank of the Mt. Motoshirane, which may have been caused by noise originating from man-made objects. We should further examine the data carefully.
In the presentation, we will show the inferred shallow structure of Mt. Motoshirane and interpret it based on the previous studies.