Nobuo Matsushima

Degassing activity from Iwodake rhyolitic cone, Satsuma-Iwojima volcano, Japan: Formation of a new degassing vent, 1990–1999

Large changes in the surface manifestation of degassing activity were observed from 1990 to 1999 at the summit crater of Iwodake cone of Satsuma-Iwojima volcano. During this period, a new high-temperature fumarolic area formed in the center of the crater floor and became a degassing vent with a diameter of 40 m. Altered volcanic rocks were ejected during the course of vent formation. Although glass fragments were observed in the ejected ash, the glass comes from altered Iwodake rhyolite that covers the crater floor. The highest fumarolic temperature and equilibrium temperatures of volcanic gases had a maximum of about 900°C at the beginning of the vent formation. The flux of SO2, measured by COSPEC, varied from 300 to 700 ton/day and correlated directly with maximum fumarole temperature. During this period, open fractures formed along the southern rim of the crater almost contemporaneously with the vent formation and changes in the nature of fumarolic discharges. The continuous and intense degassing at Satsuma-Iwojima is likely caused by volatile transport from a deep magma chamber through a convecting magma column. An increase in the magma convection rate might have caused these large changes in surface manifestations, including increase in the SO2 flux and fumarolic temperatures, ground deformation, and the vent formation.

Wide‐band magnetotelluric measurements across the Taupo Volcanic Zone, New Zealand‐Preliminary results

The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by intensive geothermal activity and frequent rhyolitic volcanism. Sixteen wide-band (0.01-1,800 s) magnetotelluric soundings were measured along a 110 km-long profile approximately perpendicular to the strike of the TVZ. A model obtained from 2D inversion of the soundings shows two near-surface regions of high conductance which correspond to low density volcaniclastic sediments, up to 3 km thick, which infill a sequence of collapse calderas. At deeper levels (approximately 5-10 km) a resistive layer underlies the entire TVZ. Modelling shows other conductive zones occur beneath the TVZ, with the shallowest lying below the central part at a depth of 10 -15 km. Given the high heat flux and volcanic history of the TVZ, the high conductivity at depth may indicate the presence of connected melt. At greater depth (20-30 km) the upper mantle beneath the TVZ appears to be anomalously conductive, consistent with observed high seismic attenuation.

Mass and heat flux of volcanic gas discharging from the summit crater of Iwodake volcano, Satsuma-Iwojima, Japan, during 1996–1999

Abstract Volcanic gas of Iwodake has been discharged continuously from high-temperature (900°C maximum) fumaroles and a degassing vent which formed on the central floor of the summit crater after 1994. Although most of the high-temperature fumaroles located on the crater wall before 1991, many fumaroles appeared on the crater floor associated with the vent opening, suggesting a shift of thermal activity from the peripheral to the central crater. A large amount of volcanic gas has been discharged from the fumaroles and vent. Since heat has been transferred from ascending gas to the surrounding soil, a region showing a surface temperature anomaly has developed around the fumaroles and vent. To quantify Iwodake thermal activity, heat and volcanic gas mass fluxes were estimated during 1996–1999 using infrared thermal images, and plume velocity and temperature data which were observed by a pitot tube and thermocouple. The estimated gas mass flux was compared with the COSPEC data to investigate the accuracy of our estimation. The gas mass flux had been decreased from 230 kg/s in October 1996 to 30 kg/s in November 1999. Although the vent diameter had increased from 20 to 70 m during the same period, this mass flux variation had indicated the decline of degassing rate. The degassing depth was estimated from the volcanic gas mass flux and temperature. The depth showed a tendency of magma head migration to shallower depth during 1996–1999, which was consistent with the drastic change of the surface manifestation.

A resistivity cross-section of Usu volcano, Hokkaido, Japan, by audiomagnetotelluric soundings

We collected audio-magnetotelluric (AMT) data across Usu volcano, Hokkaido, Japan, which erupted in 1977 and is still active. We had a profile of 17 sites perpendicular to the regional tectonic strike, which crossed the 1977 cryptodome, Usu-Shinzan. Tensor-decomposed data were interpreted by a two-dimensional inversion. Outside the crater rim, the resistivity structure is simple. The resistive somma lava is underlain by a conductive substratum, implying altered Tertiary or Quaternary rocks. In the crater, there are two resistive bodies bisected by a vertical conductor, which corresponds to Usu-Shinzan fault, located at the foot of the uplift. The vertical conductor was not detected in the AMT sounding in 1985. One of the possible causes of the development of the vertical conductor is a cold water supply from the surface into the vapor dominant fracture zone. One of the resistive bodies is located beneath Usu-Shinzan and implies an intrusive magma body which caused the 1977 uplift.

Mathematical simulation of magma-hydrothermal activity associated with the 1977 eruption of Usu volcano

During the 1977 eruption of Usu volcano, magma was emplaced at shallow crust. This intrusion induced fumarole activity immediately after the eruption. Based on the repeated thermal observations, the amount of heat discharged by this thermal activity is estimated to be 2 × 1017J. The corresponding volume of the intrusion is 6 × 107m3. The inferred intrusion volume is comparable to the volume of the resistive block beneath the major faults formed by this eruption, which was interpreted as a cooled intrusion on the basis of recently conducted MT surveys. The heat discharge rate is a surface boundary condition for an underlying magma hydrothermal system. A mathematical simulation, which accounts for multiphase mass and heat transport within a porous media, is conducted to reproduce the thermal activity of Usu volcano. The simulation incorporates the supply of latent heat by solidifying magma and heat transfer by degassing. Permeability conditions are important factors to fit the simulated heat discharge rate with the observation. Increased permeability of surrounding formations causes early appearance and high amplitude of surface heat discharge rate and reduced permeability causes opposite effect. The intrusion permeability has a strong influence on the surface thermal activity. High permeability is needed for the early appearance of the surface heat discharge rate, although it results in the maximum intrusion temperature that is much lower than the observed fumarolic temperature. To avoid the inconsistency, temperature dependent permeability is used in the simulation. The inside of the intrusion with the temperature dependent permeability that has low initial values and the high permeable margin are supposed to satisfy the conditions of the observed surface heat discharge rate and fumarolic temperature.

Crustal magma pathway beneath Aso caldera inferred from three‐dimensional electrical resistivity structure

At Naka-dake cone, Aso caldera, Japan, volcanic activity is raised cyclically, an example of which was a phreatomagmatic eruption in September 2015. Using a three-dimensional model of electrical resistivity, we identify a magma pathway from a series of northward dipping conductive anomalies in the upper crust beneath the caldera. Our resistivity model was created from magnetotelluric measurements conducted in November–December 2015; thus, it provides the latest information about magma reservoir geometry beneath the caldera. The center of the conductive anomalies shifts from the north of Naka-dake at depths >10 km toward Naka-dake, along with a decrease in anomaly depths. The melt fraction is estimated at 13–15% at ~2 km depth. Moreover, these anomalies are spatially correlated with the locations of earthquake clusters, which are distributed within resistive blocks on the conductive anomalies in the northwest of Naka-dake but distributed at the resistive sides of resistivity boundaries in the northeast.

Permeability-control on volcanic hydrothermal system: case study for Mt. Tokachidake, Japan, based on numerical simulation and field observation

We investigate a volcanic hydrothermal system by using numerical simulation with three key observables as reference: the magnetic total field, vent temperature, and heat flux. We model the shallow hydrothermal system of Mt. Tokachidake, central Hokkaido, Japan, as a case study. At this volcano, continuous demagnetization has been observed since at least 2008, suggesting heat accumulation beneath the active crater area. The surficial thermal manifestation has been waning since 2000. We perform numerical simulations of heat and mass flow within a modeled edifice at various conditions and calculate associated magnetic total field changes due to the thermomagnetic effect. We focus on the system’s response for up to a decade after permeability is reduced at a certain depth in the modeled conduit. Our numerical simulations reveal that (1) conduit obstruction (i.e., permeability reduction in the conduit) tends to bring about a decrease in vent temperature and heat flux, as well as heat accumulation below the level of the obstruction, (2) the recorded changes cannot be consistently explained by changing heat supply from depth, and (3) caprock structure plays a key role in controlling the location of heating and pressurization. Although conduit obstruction may be caused by either physical or chemical processes in general, the latter seems more likely in the case of Mt. Tokachidake.Graphical abstract.

Contention between supply of hydrothermal fluid and conduit obstruction: inferences from numerical simulations

We investigate a volcanic hydrothermal system using numerical simulations, focusing on change in crater temperature. Both increases and decreases in crater temperature have been observed before phreatic eruptions. We follow the system’s response for up to a decade after hydrothermal fluid flux from the deep part of the system is increased and permeability is reduced at a certain depth in a conduit. Our numerical simulations demonstrate that: (1) changes in crater temperature are controlled by the magnitude of the increase in hydrothermal fluid flux and the degree of permeability reduction; (2) significant increases in hydrothermal flux with decreases in permeability induce substantial pressure changes in shallow depths in the edifice and decreases in crater temperature; (3) the location of maximum pressure change differs between the mechanisms. The results of this study imply that it is difficult to predict eruptions by crater temperature change alone. One should be as wary of large eruptions when crater temperature decreases as when crater temperature increases. It is possible to clarify the implications of changes in crater temperature with simultaneous observation of ground deformation.