The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan: eruption and intrusion of hydrothermal fluid observed by a borehole tiltmeter network

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.

The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan: Eruption and intrusion of hydrothermal fluid observed by a borehole tiltmeter network

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 crack azimuth 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 * 10 5 and 3.6 * 10 5 m 3 , respectively, meaning that 1.5 * 10 5 m 3 of expanded volume formed underground. The total heat associated with the expanded volume is estimated to have been ≥10 14 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 yr. Although the ejecta mass of the 2018 phreatic eruption was small, the 2018 MPCG eruption was not negligible in terms of the energy budget of Kusatsu–Shirane volcano. A water mass of 0.1–2.0 * 10 7 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.

Characterizing volcanic tremor sources associated with collapses in Halema‘uma‘u Crater at the beginning of the 2018 Kilauea eruption

<p>We analyze data from one tiltmeter and twelve broadband seismic stations recorded at the beginning of the 2018 Kilauea eruption, to provide an integrated view of distinct tremor sources that preceded and accompanied this eruption. Studying the beginning of the eruption is challenging because of the diversity and complexity of signals that were recorded during this phase. But such undertaking represents a key aspect for understanding the dynamics of the different processes that took place at the start of the lava lake withdrawal on May 2 and during the twelve major collapses that occurred in Halema‘uma‘u Crater through May 26. The application of a network-based method to automatically detect and locate seismic tremor, combined with physical modeling of the underlying source processes, enables a characterization of these tremor sources in unprecedented detail.</p><p>Our analyses document one tremor source active during the period preceding the eruption, which is attributed to the quasi-steady radiation from a shallow hydrothermal system located at the south-southwest edge of Halema‘uma‘u Crater. These analyses further document two newly described sequences of gliding tremor. The first sequence is attributed to progressive jerky intrusions of a rock piston into a shallow hydrothermal reservoir between May 7 and May 17. The second sequence is attributed to the gradual degassing of a bubbly magma within an east striking dike below Halema‘uma‘u Crater, impacted by repeated roof collapses, and resulting in a quasi to totally degassed magma by May 26.</p>

Low frequency seismic source investigation in volcanic environment: the Mt. Vesuvius atypical case

We present a detailed analysis of the low frequency seismicity occurred at Mt. Vesuvius in the time range 2003–2018. This kind of seismicity is atypical for the volcano and poorly studied, therefore we characterized it in terms of spectral analysis, waveform cross-correlation, location and polarization properties. The different decay patterns of the spectra, the existence of both earthquake families as well as single events, the relatively wide seismogenic volume inferred from the locations and polarization features, indicate that the events are caused by distinct source mechanisms: slow brittle failure in dry rocks and resonance of fluid-filled cracks. On these basis, we classified the earthquakes as Low Frequency (LF) and Long Period (LP). Despite the differences between the two classes, both the event types are ascribable to the dynamics of the deep hydrothermal reservoir which induces variations of the fluid pore pressure in the medium. The fluid amount involved in the generation process, as well as the physical-chemical properties of the surrounding rocks are the essential factors that control the occurrence of a mechanism rather than the other.

A Major Hydrothermal Reservoir Underneath the Tatun Volcano Group of Taiwan: Clues from a Dense Linear Geophone Array

A dense linear geophone array is deployed across the Tatun volcano group (TVG) at the northern tip of Taiwan, where more than 7 million residents live in the Taipei metropolis. The array is composed of 50 geophones with a station spacing of ~ 200 m in average, and it is designed for striking in the NW–SE direction to record the many earthquakes in eastern Taiwan, where the Philippine Sea plate subducted beneath the Eurasia plate. The detailed examination of felt earthquakes shows consistent P-wave delays are recorded at particular stations of the array. The further forward modeling indicates there is a low-velocity zone (LVZ) at depths between ~ 0.5 and ~ 2.5 km beneath the major fumarole sites. Combining this preliminary result with previous studies including clustering seismicity, volcanic earthquakes, low-resistivity zone, strong degassing processes and shallow velocity structures, we suggest that the LVZ might be associated with the major hydrothermal reservoir at the TVG. The identification of the hydrothermal reservoir by the LVZ not only implies a potential volcanic threat, such as phreatic eruptions, in the future, but also provides the possibility of sustainable geothermal resources for replacing traditional nuclear and fossil fuel power plants. Detailed images of the LVZ and other volcanic structures will be obtained soon when dense geophone arrays with more than 600 geophones are deployed from 2020 to 2022.

Geothermal Linkage between a Hydrothermal Pond and a Deep Lake: Kuttara Volcano, Japan

Kuttara Volcano, Hokkaido, Japan, consists of temperate Lake Kuttara and the western Noboribetsu geothermal area. In order to explore geothermal relations between Lake Kuttara and the geothermal area, the heat budget of a hydrothermal pond, Okunoyu, was evaluated, and the heat storage change in the lower layer of Lake Kuttara was calculated by monitoring the water temperature at the deepest point. The lake water temperature consistently increased during the thermal stratification in June–November of 2013–2016. The heat flux QB at lake bottom was then calculated at a range of 4.1–10.9 W/m2, which is probably due to the leakage from a hydrothermal reservoir below the lake bottom. Meanwhile, the heat flux HGin by geothermal groundwater input in Okunoyu was evaluated at 3.5–8.5 kW/m2, which is rapidly supplied through faults from underlying hydrothermal reservoirs. With a time lag of 5 months to monthly mean QB values in Lake Kuttara, the correlation with monthly mean HGin in Okunoyu was significant (R2 = 0.586; p < 0.01). Applying Darcy’s law to the leakage from the hydrothermal reservoir at 260–310 m below the lake bottom, the time needed for groundwater’s passage through the media 260–310 m thick was evaluated at 148–149 days (ca. 5 months). These findings suggest that the hydrothermal reservoir below lake bottom and the underlying hydrothermal reservoirs in the western geothermal area are both connected to a unique geothermal source in the deeper zone as a geothermal flow system of Kuttara Volcano.

Secondary minerals in the geyserites of the Geysers Valley (Kamchatka)

Secondary minerals assemblages that are deposited from thermal solutions at the top of geysers (Velikan, Bolshoy) were investigated. It is established that assemblages are represented mainly by opal and high-silica zeolites (mordenite and heulandite). As conditions of feeding hydrothermal reservoir change, minerals of the kaolinite group and smectites may appear.