17:15 〜 18:30
[SVC29-P09] Behavior of magmatic hydrothermal system of Kusatsu-Shirane Volcano inferred from numerical simulations
キーワード：草津白根、マグマ熱水系、比抵抗構造、数値シミュレーション
Kusatsu-Shirane Volcano (KSV) is an active volcano known for its significant hydrothermal activity and phreatic eruptions in recent years. According to geological studies, frequent magmatic eruptions were estimated at Mt. Motoshirane, which constitutes the southern part of KSV, until 1500 years ago. Therefore, it is expected that the magma produced at that time has not yet cooled and solidified.
To infer the extent of the magmatic-hydrothermal system of KSV, broadband magnetotelluric observations covering the entire region of KSV were conducted from 2015 to2020. The study found a remarkable conductive zone extending from a depth of ~1.5 km below the summit area to a depth of ~10 km northwest of the KSV. The lower part (5−10 km deep) of this conductor showed moderately low resistivities (~50 Ωm) and the upper part (1.5−5 km deep) showed extremely low resistivities (< ~1 Ωm)(Matsunaga et al., 2020). The lower half of the conductor can be explained as a zone containing hydrous silicate melt, because it exhibited low resistivities on the order of 10 Ωm (Pommier et al., 2011). Based on previous geochemical studies and the seismicity, we interpreted the upper half of this conductor as a fluid-rich zone strongly affected by magmatic volatiles.
Since the hypocenters of volcanic earthquakes are distributed from the top of this conductor to the summit area, it is likely that these conductive zones are involved in the various volcanic activities observed in KSV. Therefore, it is crucial to consult the behavior of hydrothermal fluids to understand the magmatic hydrothermal system of KSV. However, it is still difficult to estimate the exact position and volume of molten rocks because the MT data collected has low resolution and sensitivity, particularly in the lower part. Since the geothermal and geochemical processes observed near the surface are strongly influenced by the behavior of shallow hydrothermal systems, it is also difficult to infer the deep processes of volcanoes which have well-developed hydrothermal systems like KSV.
One possible way to estimate the thermal state and process inside a conductor is to numerically simulate the movement of geothermal fluids using the observed data and the inferred subsurface structure as constraints. To construct a hydrothermal fluid circulation model of KSV, we use a multi-phase and multi-component TOUGH3 model (Jung et al., 2017). In the presentation, we will show some of the simulation results.
To infer the extent of the magmatic-hydrothermal system of KSV, broadband magnetotelluric observations covering the entire region of KSV were conducted from 2015 to2020. The study found a remarkable conductive zone extending from a depth of ~1.5 km below the summit area to a depth of ~10 km northwest of the KSV. The lower part (5−10 km deep) of this conductor showed moderately low resistivities (~50 Ωm) and the upper part (1.5−5 km deep) showed extremely low resistivities (< ~1 Ωm)(Matsunaga et al., 2020). The lower half of the conductor can be explained as a zone containing hydrous silicate melt, because it exhibited low resistivities on the order of 10 Ωm (Pommier et al., 2011). Based on previous geochemical studies and the seismicity, we interpreted the upper half of this conductor as a fluid-rich zone strongly affected by magmatic volatiles.
Since the hypocenters of volcanic earthquakes are distributed from the top of this conductor to the summit area, it is likely that these conductive zones are involved in the various volcanic activities observed in KSV. Therefore, it is crucial to consult the behavior of hydrothermal fluids to understand the magmatic hydrothermal system of KSV. However, it is still difficult to estimate the exact position and volume of molten rocks because the MT data collected has low resolution and sensitivity, particularly in the lower part. Since the geothermal and geochemical processes observed near the surface are strongly influenced by the behavior of shallow hydrothermal systems, it is also difficult to infer the deep processes of volcanoes which have well-developed hydrothermal systems like KSV.
One possible way to estimate the thermal state and process inside a conductor is to numerically simulate the movement of geothermal fluids using the observed data and the inferred subsurface structure as constraints. To construct a hydrothermal fluid circulation model of KSV, we use a multi-phase and multi-component TOUGH3 model (Jung et al., 2017). In the presentation, we will show some of the simulation results.