Phreatomagmatic eruptions are one of the most common and hazardous eruption styles on Earth. To understand the explosive phreatomagmatic eruptions, it is essential to constrain the mechanisms of magma–water interaction and the environments in which they occur. We investigated the quenching depths of ascending magma during the large-scale and explosive phreatoplinian eruption that occurred at Kutcharo volcano, Japan, based on the water content measurements for volcanic glass shards using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and Fourier transform infrared spectroscopy (FT-IR) analysis. Results for the two analytical methods show approximately similar total water content ranges despite using different glass shards. This indicates that the water content ranges can be regarded as representative of those of this phreatoplinian eruption. However, for FT-IR results, OH and H2Om species contents against H2Ot contents for individual glass shards do not plot within the equilibrium curves calculated using the H2O speciation model of rhyolite with temperatures of 500–800 °C. This suggests that glass shards have experienced secondary hydration after deposition, resulting in disequilibrium H2O speciation by adding only H2Om. Using measured OH contents and applying appropriate models for H2O solubility, speciation, and a glass transition temperature of 800 °C, we estimated quenching pressures of up to ~24.3 MPa. These correspond to quenching depths ranging from near-surface levels to approximately 1,200 m, depending on the assumed pressure gradient. When the presence of a paleo-caldera lake is considered, quenching depths concentrate at 100–300 m below ground level, suggesting that magma-water interaction occurred primarily in the upper conduit directly beneath the lake bottom. Bubble-wall/micro-pumice glass shards and dense/blocky glass shards show overlapping OH contents and quenching depths, implying that volatile-driven fragmentation and magma-water interaction occurred concurrently within the conduit. Together with the high mass discharge rate, our results indicate that this phreatoplinian eruption was fundamentally highly explosive, with external water mainly modifying fragmentation, quenching, and depositional characteristics rather than driving the overall eruption intensity. Phreatoplinian eruptions may therefore develop during highly explosive silicic eruptions when abundant external water interacts with ascending magma, conditions that may be particularly favorable in mature caldera systems.
Coseismic rupture length is a key parameter for predicting earthquake magnitude and strong ground motion. In contrast to the conventional assumption of fault segmentation, multi-segment earthquakes and partially ruptured earthquakes occur. Therefore, a probabilistic approach to evaluating rupture propagation plays a crucial role in hazard assessment. Some empirical studies have proposed rupture propagation probability at the fault complex and an associated method for calculating the final rupture length probability. However, in this context, case studies on actual surface rupture and objective validation are relatively limited. Here, we developed a semi-automatic method for measuring fault geometry and extracting possible rupture paths and applied it to the 2023 Kahramanmaraş earthquake sequence. The results showed that the area with relatively high rupture probabilities calculated for the Mw 7.8 Pazarcık earthquake was more extensive than the corresponding area obtained assuming rupture initiation from other segments. The actual fault rupture tends to follow the path with the least geometrical penalty. However, this cannot always be confirmed to be the case, potentially because of the uncertainty around the subsurface structure or the inherent limitation of the trace dataset used in the geometrical approach. During the Mw 7.5 Elbistan earthquake, the rupture propagated to the Doğanşehir fault, bypassing the geometrically advantageous Sürgü fault. This could be explained by the insufficient stress accumulated at the Sürgü fault, which may have forced the rupture to shift direction. These findings highlight the importance of incorporating geological parameters related to stress accumulation into a geometrical method. To explore the effect of these geological factors, we incorporated the normalized elapsed time since the last earthquake, which serves as a proxy for stress conditions and fault activity history. As a result, we find a significant probability decrease at the Pütürge segment for eastward propagation when recent stress release is considered. For westward rupture propagation, results for different assumed event times suggest that stress accumulation since before 1822 supports the long-distance rupture observed in 2023. This approach has a potential to enhance our understanding of rupture propagation paths in seismic hazard assessment.
We conducted a seawater imaging study using multichannel seismic reflection (MCS) data acquired off the eastern coast of the Noto Peninsula in the Japan Sea following the 2024 Noto Peninsula earthquake (MJ 7.6). A shallow prominent reflector (SPR) was identified within the water column of the Toyama Trough. The SPR exhibited significant spatial variations, appearing as a continuous, high-amplitude reflection in select areas while appearing weak and discontinuous in others. To investigate the mechanisms underlying the formation of this reflector, we compared MCS images with concurrently collected conductivity, temperature, and depth (CTD) data. The results suggest that the SPR was generated by a notable change in the acoustic impedance associated with a high vertical density gradient within the pycnocline, which functions as a transition layer between the surface mixed layer and the underlying Japan Sea Proper Water. We conclude that the spatial variations in the characteristics of the SPR could be controlled primarily by changes in the thickness of the surface mixed layer driven by wind-induced mixing, eventually affecting the magnitude of the vertical density gradient. These findings demonstrate the potential of MCS data for visualizing upper-ocean stratification and mixed-layer variability in the Japan Sea.
Previous studies of storm-time hemispheric asymmetry in thermospheric density have focused on direct drivers such as Joule heating, but overlooked the influence of Earth’s magnetic field topology. This study first links the Earth’s dipole tilt angle, which influences magnetic field geometry and its solar wind interaction, to the hemispheric density response asymmetry. Using in situ neutral density data from China’s TianMu-1 constellation, CHAMP, Grace FO and Swarm C, we analyzed 62 intense geomagnetic storm events (Kp ≥ 7, 2000–2025), yielding 171 satellite samples, to quantify the correlation between dipole tilt angle and storm-time hemispheric density response asymmetry. Statistical results show that asymmetry increases linearly with the dipole tilt angle at storm onset (R = 0.66; slope ≈ 2.29
We present here synthetic BepiColombo observations of Mercury’s magnetosphere obtained from two global fully kinetic particle-in-cell simulations performed with the iPIC3D code under two interplanetary magnetic field (IMF) configurations: a southward IMF and one along the average Parker-spiral direction at Mercury. Electromagnetic fields and particle bulk measurements and energy distributions are sampled self-consistently along four Mio case study orbital trajectories using a virtual spacecraft approach, providing a synthetic characterization of the plasma environment expected during the orbital phase of the mission. The simulated observations reproduce the sequence of solar wind, magnetosheath, and magnetospheric regions encountered along each orbit and reveal signatures of magnetotail reconnection near closest approach, including enhanced field-aligned electron flows and associated suprathermal particle populations. Comparison of the dawn and dusk flank trajectories underlines asymmetries between the two regions, including evidence of enhanced plasma density on the dusk side and differences in magnetosheath extent that depend on the IMF orientation. We further investigate temperature-anisotropy-driven instabilities along the trajectories. Ion mirror mode and Alfvén/ion-cyclotron instabilities are found to occur systematically within the magnetosheath, whereas electron whistler and electron firehose conditions are preferentially reached near closest approach and in localized magnetosheath intervals. We also examine the role of IMF orientation in shaping the plasma stability properties along the trajectories. The reconstructed particle energy spectra and electromagnetic fluctuations provide additional diagnostics of trapped populations and kinetic processes associated with magnetotail activity. Overall, these results establish a physically consistent reference framework for the interpretation of BepiColombo observations during the forthcoming orbital phase at Mercury.
