Triggering intrusions of phreatic eruptions are often observed as seismic and ground deformation signals on a time scale of minutes. The current understanding of hydrothermal intrusions still needs ...improvement to obtain insight into the eruption scale from the observables. We examine local geophysical data from the precursory hydrothermal intrusion of the 2018 phreatic eruption of Kusatsu‐Shirane volcano. To achieve an integrated intrusion model, we divide analyzing time window into the onset, middle, and climax. Focusing on the transient response of tilt data for the sudden pressurization, we estimate a vertical tensile opening (1.7 × 103 m3/s in 40 s) at 1.1 km depth for the intrusion onset. Pressurization can represent the start of vapourization. Very long period (VLP, 0.033–0.1 Hz) seismic signals are adopted to constrain the middle and climax phases. We obtained two sequential semi‐horizontal tensile crack oscillation sources with peak volume changes of 3.6 × 104–1.9 × 105 m3 at 0.3–0.6 km depths. The second VLP source acted as a final trigger of the eruption to cause depressurization in the shallow portion of the intruded region, which is constrained as having reached 0.1 km depth by surface deformation. Simultaneously, we find another depressurization originated from depth in the climax due to a decrease in the hydrothermal intrusion rate. Through comparison with the 2014 Ontake phreatic eruption, the total inflation volume may correlate with eruption scales. Intruded hydrothermal fluid and local structure characteristics also may have to be considered to evaluate the eruptions scales from inferred signal source intensity.
Plain Language Summary
Phreatic eruptions, emitting non‐fresh magma fragments, are driven by underground water and gas mixtures intruding into the subsurface. Local geophysical observations often detect intrusions as seismic and ground deformation signals. Those observation data potentially provide insight into eruption and hazard scale evaluations. However, such an interpretation is still challenging for the current understanding. This paper examines seismic and ground deformation data associated with the precursory intrusion of a phreatic eruption at Kusatsu‐Shirane volcano in Japan, revealing a detailed hydrothermal intrusion process. Using ground deformation data, we have found that an aqueous intrusion started at 1.1 km depth. This starting process may represent the vapourization onset of underground water and gas mixtures. By analyzing seismic signals having a longer dominant period (>10 s), we revealed the intrusion path from the starting region to beneath the 2018 eruption vent and the final triggering for the eruption. Through comparisons with other phreatic eruptions, we find that the total deformation volume may correlate with the eruption intensity scales such as the mass emitted volcanic ash and the maximum eruption cloud height. However, there is still room for considering hydrothermal fluid characteristics and local structures to evaluate the eruption scale properly from observed data.
Key Points
A sudden hydrothermal intrusion onset is constrained by adopting transient tiltmeter responses
Very long period band seismic signals reflect the hydrothermal intrusion behaviors during the middle and climax phases
Tilt and seismic records show precursory depressurization in shallow depth as the final eruption trigger
Hayabusa 2 spacecraft revealed that a small carbonaceous asteroid 162173 Ryugu is a rubble pile and that its surface is covered with various sizes of boulders. A few percent of surface boulders have ...cracks. They are classified into straight cracks, sinuous cracks, arrested (incomplete) cracks, and complex (branched) cracks. We analyzed 538 boulders (777 cracks) and found 60% of their cracks have the meridional direction (±15deg from N-S) except complex cracks. Impacts or surface mass movement cannot have produced this preferred crack orientation. Thermal stress from solar heating would have grown cracks to meridional direction. Another feature of thermal stress – exfoliation – is also observed on the surface of small boulders of Ryugu as of Bennu.
Morphometrical studies on small volcanic edifices such as scoria cones have been extensively conducted. Nevertheless, some morphometric parameters have been difficult to obtain. For instance, it is ...often difficult to measure a slope angle of volcano edifice because DEM (Digital Elevation Model) with high spatial resolution are not always available. We used the K-GPS (Kinematic Global Positioning System) for high resolution measurement of the slope angle of Omuroyama, a typical young scoria cone in Eastern Izu, Japan, assuming no detailed DEM available in the region. We show the existence of constant slope of 32° from the base to the top. The result of the K-GPS measurement is consistent with the existing DEM of this region. We report the measuring system is simple and viable for summary survey of slope angle where no DEMs are available.
Shape analyses of tephra grains result in understanding eruption mechanism of volcanoes. However, we have to define and select parameter set such as convexity for the precise discrimination of tephra ...grains. Selection of the best parameter set for the recognition of tephra shapes is complicated. Actually, many shape parameters have been suggested. Recently, neural network has made a great success in the field of machine learning. Convolutional neural network can recognize the shape of images without human bias and shape parameters. We applied the simple convolutional neural network developed for the handwritten digits to the recognition of tephra shapes. The network was trained by Morphologi tephra images, and it can recognize the tephra shapes with approximately 90% of accuracy.
Seasonal cycle of Martian relative humidity shows that, in the southern and the northern RSL sites, humidity decreases when RSL increment while humidity is high on the fading season. We suggest that ...the recurrence of RSL is caused by the different angle of response between wet and dry grains. 2D DEM simulation indicates that, when the ratio of a liquid bridge volume to a particle volume is the order of 10^{-7}, the repose angle of Martian sand can increase to ~40 degrees due to cohesion, which is near the slope around RSL starting area. Thus, if this amount of liquid bridge is generated by deliquescence or capillary condensation of vapor and if the water evaporates following the humidity and temperature cycles, Martian sand can induce avalanche on RSL slope. When grains are wet again, sand and dust deposit on RSL, which may be the mechanism of RSL disappearance and recharge of grains for the next season. In our model, because liquid induces RSL indirectly (RSL itself is the granular flow), the amount of liquid can be drastically reduced compared with the wet mechanism. In this case, as well as temperature, the seasonal cycle of relative humidity plays an important role in the recurrence of RSL.
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