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The phreatic eruption is one of the volcanic eruption styles triggered by the rapid vaporization of heated fluids at shallow depth without the involvement of juvenile magma (e.g., Stix and deMoor [23]). 10.1186/s40623-021-01468-3 5 Kataoka KS, Tsunematsu K, Matsumoto T. Crisis hazard assessment for snow-related lahars from an unforeseen new vent eruption: the 2018 eruption of Kusatsu-Shirane volcano, Japan. Papers on the Kusatsu-Shirane volcano Kusatsu-Shirane volcano is an active dacite-andesite Quaternary volcano located in Gunma Prefecture, Central Japan. After the onset of the 2010 phreatic eruption of the Turrialba volcano, the underlying geothermal system changed as seen from the significant increase in the fumarole output at the Turrialba volcano. [Extracted from the article]

We estimate the mass and energy budgets for the 2018 phreatic eruption of Mt. Motoshirane on Kusatsu–Shirane volcano, Japan, based on data obtained from a network of eight tiltmeters and weather radar echoes. The tilt records can be explained by a subvertical crack model. Small craters that were formed by previous eruptions are aligned WNW–ESE, which is consistent with the strike of the crack modeled in this study. The direction of maximum compressive stress in this region is horizontal and oriented WNW–ESE, allowing fluid to intrude from depth through a crack with this orientation. Based on the crack model, hypocenter distribution, and MT resistivity structure, we infer that fluid from a hydrothermal reservoir at a depth of 2 km below Kusatsu–Shirane volcano has repeatedly ascended through a pre-existing subvertical crack. The inflation and deflation volumes during the 2018 eruption are estimated to have been 5.1 × 105 and 3.6 × 105 m3, respectively, meaning that 1.5 × 105 m3 of expanded volume formed underground. The total heat associated with the expanded volume is estimated to have been ≥ 1014 J, similar to or exceeding the annual heat released from Yugama Crater Lake of Mt. Shirane and that from the largest eruption during the past 130 year. Although the ejecta mass of the 2018 phreatic eruption was small, the eruption at Mt. Motoshirane was not negligible in terms of the energy budget of Kusatsu–Shirane volcano. A water mass of 0.1–2.0 × 107 kg was discharged as a volcanic cloud, based on weather radar echoes, which is smaller than the mass associated with the deflation. We suggest that underground water acted as a buffer against the sudden intrusion of hydrothermal fluids, absorbing some of the fluid that ascended through the crack. [ABSTRACT FROM AUTHOR]

Monitoring dynamics of volcanic eruptions with geophysics is challenging. In August and November 2012, two small eruptions from Mount Tongariro provided a unique opportunity to image subsurface changes caused by the eruptions. A detailed magnetotelluric survey of the Tongariro volcanic complex completed prior to the eruption (2008–2010) provides the preeruption structure of the magmatic system. A subset of the initial measurement locations was reoccupied in June 2013. Significant changes were observed in phase tensor data at sites close to the eruptive center. Although subsurface electrical resistivity changed, the geometry of the preeruptive reservoir did not. These subsurface resistivity variations are interpreted as being predominantly caused by interaction of partial melt and the overlying brine layer causing volume reduction of the brine layer through phreatic eruption. The ability to detect significant changes associated with the magma reservoir suggests that magnetotellurics can be a valuable volcano monitoring tool. Plain Language Summary: Preeruption and posteruption electromagnetic magnetotelluric measurements are used to determine the variation in subsurface electrical properties resulting from changes in the Tongariro volcanic center magma chamber associated with the 2012 eruptive cycle. The observed electrical property changes are related to the physical eruption properties (e.g., eruptive volume, style, and composition), revealing the state of the magmatic system both prior to and following the eruption. Knowing both the preeruption and posteruption states of the magmatic system and the surface eruptive properties enables reconstruction of the subsurface eruption mechanism. Successful identification of preeruption and posteruptive states of the volcano is evidence for the usefulness of continuous magnetotelluric monitoring of volcanoes to identify variations within magmatic systems that may be indicative of imminent eruption. Key Points: We identify significant temporal change in preeruption and posteruption magnetotelluric soundingsAn eruption mechanism model is developed consistent with both geophysical and geological observationsMagnetotellurics is a viable additional monitoring tool for active volcanic systems [ABSTRACT FROM AUTHOR]

The white line in c denotes the projection of fumarole zone F1 at 1400 m ASL. d The 3-D view of the three major conductors. The white, black and gray dots denote hypocenters before, during and after the 2014 unrest, respectively. Correction to: Anatomy of active volcanic edifice at the Kusatsu-Shirane volcano, Japan, by magnetotellurics: hydrothermal implications for volcanic unrests. [Extracted from the article]

The two-dimensional electrical resistivity structure beneath Naruko volcano was determined using magnetotelluric soundings. The resulting model shows that a prominent conductor exists through the middle crust to the uppermost mantle beneath the volcano. The location of the conductor agrees closely with a seismic low-velocity zone. Low-frequency microearthquakes occur near the conductor around the Moho depth. The cutoff depth of crustal earthquakes is coincident with the upper boundary of the conductor, implying that the conductor has a temperature appreciably higher than 400?C. Furthermore, new helium isotope data from hot springs around the volcano were obtained. The spatial distribution of the observed 3He/4He ratios reveals the extent of mantle-derived materials beneath Naruko volcano. Consequently, it is apparent that the conductor determined beneath the volcano reflects the presence of high-temperature mantle-derived materials such as magmas and/or related fluids derived from active magmatism in the northeastern Japan subduction zone. [ABSTRACT FROM AUTHOR]

We aimed to perform three-dimensional imaging of the underlying geothermal system to a depth of 2 km using magnetotellurics (MT) at around the Yugama crater, the Kusatsu–Shirane Volcano, Japan, which is known to have frequent phreatic eruptions. We deployed 91 MT sites focusing around the peak area of 2 km × 2 km with typical spacings of 200 m. The full tensor impedances and the magnetic transfer functions were inverted, using an unstructured tetrahedral finite element code to include the topographic effect. The final model showed (1) low-permeability bell-shaped clay cap (C1) as the near-surface conductor, (2) brine reservoir as a deep conductor (C3) at a depth of 1.5 km from the surface, and (3) a vertical conductor (C2) connecting the deep conductor to the clay cap which implies an established fluid path. The columnar high-seismicity distribution to the east of the C2 conductor implies that the flushed vapor and magmatic gas was released from the brine reservoir by breaking the silica cap at the brittle–ductile transition. The past magnetization/demagnetization sources and the inflation source of the 2014 unrest are located just below the clay cap, consistent with the clay capped geothermal model underlain by brine reservoir. The resistivity model showed the architecture of the magmatic–hydrothermal system, which can explain the episodic volcanic unrest. [ABSTRACT FROM AUTHOR]

On 29 June 2015, a small phreatic eruption occurred at Hakone volcano, Central Japan, forming several vents in the Owakudani geothermal area on the northern slope of the central cones. Intense earthquake swarm activity and geodetic signals corresponding to the 2015 eruption were also observed within the Hakone caldera. To complement these observations and to characterise the shallow resistivity structure of Hakone caldera, we carried out a three-dimensional inversion of magnetotelluric measurement data acquired at 64 sites across the region. We utilised an unstructured tetrahedral mesh for the inversion code of the edge-based finite element method to account for the steep topography of the region during the inversion process. The main features of the best-fit three-dimensional model are a bell-shaped conductor, the bottom of which shows good agreement with the upper limit of seismicity, beneath the central cones and the Owakudani geothermal area, and several buried bowl-shaped conductive zones beneath the Gora and Kojiri areas. We infer that the main bell-shaped conductor represents a hydrothermally altered zone that acts as a cap or seal to resist the upwelling of volcanic fluids. Enhanced volcanic activity may cause volcanic fluids to pass through the resistive body surrounded by the altered zone and thus promote brittle failure within the resistive body. The overlapping locations of the bowl-shaped conductors, the buried caldera structures and the presence of sodium-chloride-rich hot springs indicate that the conductors represent porous media saturated by high-salinity hot spring waters. The linear clusters of earthquake swarms beneath the Kojiri area may indicate several weak zones that formed due to these structural contrasts. [ABSTRACT FROM AUTHOR]

Abstract: Continuous magnetotelluric (MT) measurements were conducted from May 2008 to July 2009 at Sakurajima, one of the most active volcanoes in Japan. Two observation sites were established at locations 3.3km east and 3km west–northwest of the summit crater. At both observation sites, the high-quality component of the impedance tensor (Zyx) showed variations in apparent resistivity of approximately ±20% and phase change of ±2°, which continued for 20–180days in the frequency range between 320 and 4Hz. The start of the period of changes in apparent resistivity approximately coincided with the start of uplift in the direction of the summit crater, as observed by a tiltmeter, which is one of the most reliable pieces of equipment with which to detect magma intrusion beneath a volcano. A 2D inversion of MT impedance suggests that the resistivity change occurred at a depth around sea level. One of the possible implications of the present finding is that the degassed volatiles migrated not only vertically through the conduit but also laterally through a fracture network, mixing with shallow groundwater beneath sea level and thereby causing the observed resistivity change. [ABSTRACT FROM AUTHOR]

It has been suggested that a large amount of crustal fluid is trapped at a supercritical state within intrusive rocks beneath volcanoes or calderas near the mountain ranges of northeastern Japan. If we could extract and use these crustal fluids, we could expect to achieve a high level of energy productivity. We have collated field data on high-temperature geothermal areas of the world, used these to produce simple models of their geothermal systems, and then explored their features in terms of the amount of potential power generation. For example, a potential of around 0.1 GW per reservoir over 30 years is expected in northeastern Japan if we consider supercritical reservoirs extending to a 5 km depth. [ABSTRACT FROM AUTHOR]

This study clarifies the hydrothermal system of Jigokudani Valley near Mt. Tateyama volcano in Japan by using a combination of audio-frequency magnetotelluric (AMT) survey and hot-spring water analysis in order to assess the potential of future phreatic eruptions in the area. Repeated phreatic eruptions in the area about 40,000 years ago produced the current valley morphology, which is now an active solfatara field dotted with hot springs and fumaroles indicative of a well-developed hydrothermal system. The three-dimensional (3D) resistivity structure of the hydrothermal system was modeled by using the results of an AMT survey conducted at 25 locations across the valley in 2013–2014. The model suggests the presence of a near-surface highly conductive layer of < 50 m in thickness across the entire valley, which is interpreted as a cap rock layer. Immediately below the cap rock is a relatively resistive body interpreted as a gas reservoir. Field measurements of temperature, pH, and electrical conductivity (EC) were taken at various hot springs across the valley, and 12 samples of hot-spring waters were analyzed for major ion chemistry and H 2 O isotopic ratios. All hot-spring waters had low pH and could be categorized into three types on the basis of the Cl − /SO 4 2 − concentration ratio, with all falling largely on a mixing line between magmatic fluids and local meteoric water (LMW). The geochemical analysis suggests that the hydrothermal system includes a two-phase zone of vapor–liquid. A comparison of the resistivity structure and the geochemically inferred structure suggests that a hydrothermal reservoir is present at a depth of approximately 500 m, from which hot-spring water differentiates into the three observed types. The two-phase zone appears to be located immediately beneath the cap rock structure. These findings suggest that the hydrothermal system of Jigokudani Valley exhibits a number of factors that could trigger a future phreatic eruption. [ABSTRACT FROM AUTHOR]

The Kusatsu-Shirane volcano is a Quaternary andesitic-to-dacitic active volcano located on the central Honshu Arc. The volcano is known to have had repeated phreatic eruptions in last 200 years. A number of geochemical and geophysical studies have been conducted around the Yugama crater lake, the focus of current volcanic activity. However, the 2018 unexpected phreatic eruption occurred at Mt. Motoshirane, a different pyroclastic cone from that which hosts Yugama. There were also frequent magmatic eruptions until 1500 years ago at this cone and the magma produced at that time has not yet cooled and solidified. A better understanding of the magmatic hydrothermal system of this volcano requires structural information gathered over a larger area, and to greater depths, than has been achieved to date. Here, we describe the three-dimensional (3-D) electrical resistivity structure around Mt. Motoshirane down to a depth of ~10 km using broadband magnetotelluric (MT) data (320–0.0005 Hz) collected in 2015 and 2016. The 3-D resistivity structure model shows an extensive low-resistivity layer at depths of 1–3.5 km beneath the summit. However, structures characteristic of the presence of magma are not observed beneath this layer. It could be too deep or too small to be detected. Combining the new data with results of previous geochemical and geophysical studies, we interpret the conductor as a hydrothermal fluid reservoir that supplies fluids to the crater lake and to hot springs on the eastern and western flanks of the volcano. The existence of a fluid reservoir that extends from the vicinity of the Yugama crater lake to the subsurface beneath an inactive pyroclastic cone that has produced repeated magmatic eruptions in the past suggests that a large-scale magmatic hydrothermal system is developing beneath the volcano. • We conducted broadband magnetotelluric (MT) measurements in Kusatsu-Shirane volcano. • A 3-D electrical resistivity structure was inferred from the MT data. • We found an extensive low-resistivity layer at depths of 1–3 km in the summit area. • Conductor was interpreted as the source of crater-lake water and hot springs. [ABSTRACT FROM AUTHOR]