Haruhisa Nakamichi

Understanding the spatio‐temporal variations in the physical properties of the Earth's crust is crucial for comprehending the dynamic processes linked to earthquakes. Conventional seismometer‐based analyses have limitations in spatio‐temporal resolution due to the sparse station distribution. Using seismic interferometry with distributed acoustic sensing (DAS), we mapped coseismic velocity changes associated with a M4.1 earthquake 40 km away from Sakurajima volcano, Japan, with high spatial resolution. We observed velocity decreases of up to 1.0% in the 2–3 Hz band in sedimentary or old lava regions with low S‐wave velocity, demonstrating enhanced sensitivity of unconsolidated materials to seismic perturbation. By stacking seismic ambient noise cross‐correlation functions (CCFs) from many channel pairs, we improved the signal‐to‐noise ratio sufficiently to estimate relative velocity changes with a temporal resolution of 30 min. Although peak ground velocities observed at seismic stations are one‐tenth (0.16 cm/s) of previous studies, we observed comparable seismic velocity reductions and recovery rates until approximately 12 hr after the earthquake. Since DAS can provide dense measurements along fiber‐optic cables, it offers new opportunities to investigate seismic velocity changes with high spatial detail and, through stacking seismic ambient noise CCFs from many channel pairs, improved temporal resolution.

Eruption models developed using the waveform inversion method for explosion earthquakes differ across studies, and a unified model has not been established. Here, Vulcanian eruption processes of the Sakurajima volcano, Japan, were clarified using seismic waveform inversion analysis in the long- and very-long period bands for two specific events: one with a clear preceding infrasound phase (July 17, 2022) and the other with almost no preceding infrasound phase (July 24, 2022). The waveform inversion analysis used the layered velocity structure obtained from artificial seismic experiments and the detailed topography of the Sakurajima edifice. In both events, the single-force component, in addition to the moment tensor, was a necessary parameter for explaining the eruption process. The source location of the explosion earthquake on July 17 was 480 m below sea level (1140 m below the crater), while that of the explosion earthquake on July 24 was 440 m above sea level (220 m below the crater). In the former event, the magma reached supersaturation at depth by the time of the explosion. Magma ascent and bubble growth appeared as the expansion and contraction of the crack in the volcanic conduit beneath the crater. In addition, as the magma rose, vertical drag and reaction forces were exerted on the conduit. In the latter event, magma supersaturation remained shallow within the conduit. Following rapid magma ejection from the shallow part of the conduit and the expulsion of volcanic bombs, the vesiculation level of magma in the conduit decreased, resulting in gradual ash ejection. The preceding phase of infrasound was seen in the July 17 event but not in the July 24 event; the presence or absence of the preceding phase did not seem to determine the strength of the lava plug since there were six events between the two events, and the rate of SO₂ emission during that period showed an increasing trend. Overall, these findings highlight the necessity of considering the single-force component and the moment tensor to explain the eruption process. This study also provides insights into the source locations of explosion earthquakes, as well as the dynamics of magma ascent and bubble growth. Graphical Abstract

Lidar observations of volcanic ash in Sakurajima, Japan, were conducted during 2016–2024 to determine the volcanic impact on the atmosphere. The range from the lidar observatory to the vent was divided into four zones, and the characteristics of the extinction coefficient were then analyzed. The discrimination threshold between cloud and volcanic ash was suggested, and volcanic activity reports by the Japan Meteorological Agency (JMA) were employed to explain the long-term variation of the extinction coefficient coupled with the high depolarization ratio measured by lidar. The correlation between the number of eruptions mentioned in the JMA report and the monthly mean extinction coefficient was low. This result suggests other smaller-scale eruptions affect atmospheric conditions around the volcano.

Backend=biber, style=alphabetic, sorting=ynt ]biblatex Understanding the spatio-temporal variations in the Earth’s crustal structure is crucial for comprehending the dynamic processes linked to earthquakes. Conventional seismometer-based analyses have limitations in spatio-temporal resolution due to the sparse station distribution. Using seismic interferometry with distributed acoustic sensing (DAS), we mapped coseismic velocity changes associated with a M4 earthquake 40 km away from Sakurajima volcano, Japan, with a high spatial resolution. We observed velocity decreases of up to 1.0% at the 2-3 Hz band in sedimentary or old lava regions with low S-wave velocity, demonstrating enhanced sensitivity of unconsolidated materials to seismic perturbation. By stacking seismic ambient noise cross-correlation functions or relative velocity change measurements from many channel pairs, we achieved tracking recovery processes of seismic velocities with a time resolution of 30 minutes. Although peak ground velocities observed at seismic stations are one-tenth (0.16 cm/s) of previous studies, we observed comparable seismic velocity reductions and recovery rates until the second half of the day after the earthquake. Since DAS can provide measurements every few meters along fiber-optic cables extending tens of kilometers, DAS observation offers new opportunities for investigating the mechanisms of seismic velocity changes related to earthquakes with both spatially continuous and high temporal resolution.

The prediction of volcanic eruptions and eruption hazards is important to mitigate volcanic hazards. The purpose of the Volcano Program Promotion Panel in the Research Program on Earthquake and Volcanic Observations is to elucidate the conditions and logic of the event branch of volcanic activities, construct a transition model of volcanic activity, and promote research for the prediction of volcanic eruptions. During 2019–2023, submarine and remote-island volcanic eruptions were prominent. Additionally, terrestrial volcanic eruptions highlighted issues in volcanic disaster prevention. Significant progress was realized in material science research. The effects of water content, conduit size, and magma ascent rate on the magma supply systems and eruption explosiveness were evaluated. The relationship between the increase in phenocryst and decrease in eruption size was also elucidated, and the time evolution of the silicic magma system leading up to the caldera-forming eruption was elucidated using a new analytical method based on isotopes. The isotopic ratios of volcanic gases revealed the effects of magma foaming on shallow hydrothermal systems. The importance of rapid analysis of eruptions was highlighted, and magma supply systems and volcanic activity transitions were modeled. Remarkable progress was also made in geophysical observation research. An increase in the amplitude of volcanic tremors and a change in the epicenter location were detected, which were presumed to be caused by the rising of magma before the eruption. New observation techniques were actively introduced, and detailed magnetization structures and temporal changes were detected from aeromagnetic observations using uncrewed aerial vehicles. Distributed acoustic sensing observations were used to determine the epicenter of volcanic earthquakes and estimate the ground structure. Furthermore, seismic survey and ejecta analyses indicated the volume of deposits and magma production process in caldera-forming eruptions. Standardization of the volcanic activity index was promoted through applications of the index for multiple volcanoes as an objective method of volcanic activity evaluation. The addition of highly accurate information on the subsurface structure of volcanoes has advanced our understanding of the eruptive activity and processes. Through these results, the trial and verification of predictions based on the transition model of volcanic activity will be conducted in the following research plan.

The interaction of faults, fractures, and hydromagmatic systems of volcanoes can lead to complicated stress patterns that vary over short spatial and temporal scales. Here we study stress-induced anisotropy using observations of shear-wave splitting at 12 stations across Ontake volcano, Japan. The results reveal a complicated pattern of anisotropy indicating that the volcano perturbs the local stress field. In 2007, a minor phreatic eruption (VEI 0) occurred at Ontake, but there is little evidence of changes in splitting parameters during this eruption. In contrast, the much large eruption of 2014 (VEI 3) shows clear temporal changes in splitting parameters following the eruption. The average background magnitude of anisotropy, as described by the delay time between the fast and slow shear wave, doubles to nearly 0.2 second at the onset of the 2014 eruption, but the percent anisotropy increases dramatically from 3% to 20%. Contemporaneously, the polarisation of the fast shear wave rotates towards sHmax. We interpret these observations in terms of basal heating of the hydrothermal system. We suggest that a lack of temporal variation in anisotropy parameters during the 2007 eruption indicates that a critical stress or crack density threshold must be overcome to exhibit a change in anisotropy, which may be indicative of a more significant eruption.

Sakurajima Volcano Observatory, with a history exceeding 60 years, is recognized by researchers in Japan and overseas for conducting frontier research centering on Sakurajima, the most active volcano in Japan. It is owned by Kyoto University and represents the university in the surrounding communities. This paper reviews the open-door events, which have been conducted yearly for the last ten years as a part of a university-wide effort by Kyoto University. They make use of the facilities at Sakurajima Volcano Observatory, which aims to enhance the understanding of volcanoes among the general public in collaboration with geo-guides as “boundary workers.”

The sudden eruption and tragedy of Mt. Ontake in 2014, a volcano located in central Japan, showed the fact that the volcanic eruption is the event with uncertainty, and it is important to let citizen to be aware of such uncertainty. To find measures to raise citizen’s disaster awareness, this study focuses on the risk communication between citizens and volcano observatories, which are attached to universities. It examines the role of observatories, focusing on the activities of the Sakurajima Volcano Research Center, which monitors Mt. Sakurajima, one of the most active volcanoes in Japan, and suggests the necessity of human resource development that is able to connect citizen and science.

This study applied the numerical simulations of lava flows to evaluate the damage caused by volcanic hazards. The accuracy of the numerical simulation was governed by the reliability of the input parameters in the numerical models. One of the most important parameters is the information pertaining to terrain data, which is generally provided as a digital elevation model (DEM). The recently-developed 8K technology, mainly applied in the broadcasting field, provides a precise photographic survey of the terrain and can produce a detailed digital topographic map. The Japan Broadcasting Cooperation (NHK) deployed an 8K drone to generate a digital elevation model of Sakurajima Volcano, Japan. These data were used to formulate the lava flow under certain prescribed conditions. This study analyzes the influence of DEM mesh size on lava flow simulations, showing that high-precision DEM outlines the distribution of inundated areas in detail. Moreover, a smaller mesh resulted in a faster arrival time of the lava flow. The study’s results are useful for both risk and crisis management.

The performance of a multi-channel seismometer system with phase-shifted optical interferometry was improved by newly introduced sensors and a processing unit. The current version of the system consists of three optical wired seismometers and the unit. We deployed the system at Sakurajima Volcano and successfully operated it from June to December 2019. As the Sakurajima Volcano frequently erupts, a number of eruption events were observed during the observation period, as were a number of lightning strikes. In this study, we evaluated the observation performance of the volcanic earthquake and the noise caused by the lightning, using the spectrum and amplitude of the waveform. The results show that this sensor can observe earthquakes caused by eruptions as well as ordinary seismometers do. When the lightning struck, pulsed noise with power in a wide frequency band was observed in the existing seismometer, but not in the new sensor. Therefore, the observation was not affected by lightning. In addition, this system was found to be effective in the array analysis of volcanic earthquakes.