We infer the temporal changes in the pressure sources that induced crustal deformation during the 2018 Shinmoe-dake eruption using strain and tilt observations and discern that the deep magmatic activity associated with the early stage of this eruption began approximately 19 h earlier than the previously defined onset of magmatic activity. Distinct tilt changes were observed from around 09:00 on 6 March to 12:00 on 8 March 2018 (JST), coincident with observed lava outflow into the crater and lava dome formation. Existing studies have attributed this tilt change to the onset of the deflation of a spherical pressure source located at ~ 7 km bsl (below sea level) to the northwest of Shinmoe-dake. Here we examine strain and tilt data that were acquired in the Kirishima volcanic group, and we find that the distinct changes in the measured strain at Isa-Yoshimatsu Observatory began at around 14:00 on 5 March. This change can be explained by the deflation of a spherical pressure source, thereby suggesting that the onset of magma ascent was earlier than previously thought. The time variation in the spherical pressure source is estimated using the time-dependent inversion of the Ensemble Kalman Filter; the deflation source ascended from ~ 11 to 7 km bsl during Phase 1 (14:00 on 5 March to 06:00 on 6 March) and descended from 7 to 8 km bsl during Phase 2 (06:00 on 6 March to 12:00 on 8 March). Interferometric synthetic aperture radar analysis suggests that a dike intrusion had occurred just below Shinmoe-dake crater until 5 March, and this inflatable crustal deformation is attributed to the emplacement of residual volcanic fluids from the 2011 eruption. It is also known that the surface eruptive activity increased during Phase 1, including an increase in ash venting from the night of 5 March. These strain and tilt observations, therefore, suggest that magma ascended from ~ 11 km bsl to the magma reservoir at 7 km bsl during Phase 1, followed by a deflation of the magma reservoir during Phase 2 due to the large magma supply to the surface.Graphical

A numerical investigation of surface wave propagation in the surface wave exploration of river embankments is implemented using 3D shallow soil models with different subsurface shapes and S-wave velocities. The focus is on the effects of surface and subsurface irregularities on the propagation and dispersion characteristics of Rayleigh waves generated by a surface source and observed at points along the crest of the embankment. Rayleigh wave dispersion curves observed along the embankment crest are found to be contaminated by reflected waves, with the amount of contamination affected more by the position of the velocity boundary at the bottom edge of the embankment than the surface topography. When the velocity boundary is close to the ground surface, especially at the toe of the slope, the effect of reflected waves is sufficiently large to generate biases in the phase velocity estimates.Graphical

Zodiacal light (ZL) is sunlight scattered by interplanetary dust particles (IDPs) at optical wavelengths. The spatial distribution of IDPs in the Solar System may hold an important key to understanding the evolution of the Solar System and material transportation within it. The number density of IDPs can be expressed as n(r) sim r^{-alpha }, and the exponent alpha sim 1.3 was obtained by previous observations from interplanetary space by Helios 1/2 and Pioneer 10/11 in the 1970s and 1980s. However, no direct measurements of alpha based on ZL observations from interplanetary space outside Earth’s orbit have been performed since then. Here, we introduce initial results for the radial profile of the ZL at optical wavelengths observed over the range 0.76-1.06 au by ONC-T aboard the Hayabusa2# mission in 2021-2022. The ZL brightness we obtained is well reproduced by a model brightness, although there is a small excess of the observed ZL brightness over the model brightness at around 0.9 au. The radial power-law index we obtained is alpha = 1.30 pm 0.08, which is consistent with previous results based on ZL observations. The dominant source of uncertainty arises from the uncertainty in estimating the diffuse Galactic light (DGL).Graphical

The source imaging for offshore earthquakes using terrestrial geodetic data has a limited estimation performance due to the low data resolution. One approach to overcome this limitation is the use of seafloor geodetic data. In this study, we focus on tsunami-source data, which is the spatial distribution of vertical crustal displacements above the source area and can be derived from tsunami waveform records. We evaluate how the use of this spatial seafloor geodetic data improves the estimation of a rectangular fault model. Here, the fault model of the 2016 off-Fukushima earthquake in Japan, which was a shallow intraplate earthquake (Mw 7.0), was estimated by three inversions: terrestrial Global Navigation Satellite System (GNSS) data only, tsunami-source data only, and a combination of the GNSS data and tsunami-source data. A Bayesian inversion approach was used to understand the distribution of the estimated fault parameters and their relationship. The results indicated that the terrestrial GNSS data have a low resolution for the analysis of the offshore earthquake, which resulted in a biased solution with large uncertainty. Conversely, the use of tsunami-source data significantly improved the resolution and reliability of source imaging and reduced the dependency among fault parameters. These results suggested that the high-spatial-resolution information of tsunami source is a powerful tool in source imaging of offshore shallow earthquakes. Moreover, the combined use of the two different geodetic data leads to a more robust estimation of fault parameters. We believe that the use of tsunami-source data is useful, not only for the post hoc source analysis, but also for estimating an earthquake rupture area just after a large earthquake, where GNSS data are currently used.Graphical

The morphology of prompt penetration effects (PP) in the Equatorial Electrojet (EEJ) at Vencode (VEN, Geomagnetic latitude 0.29ºN) was studied using 5-min samples of EEJ and interplanetary Ey component data from 2011 to 2015. In contrast to previous studies that mainly focused on storm-time PP events, our study investigated the characteristics of PP effects on EEJ during both quiet and disturbed periods. Our findings reaffirm earlier reports of high PP efficiency during local noon for both quiet and disturbed days. Moreover, PP amplitudes were greater when IMF-Bz was oriented northward compared to southward, indicating the presence of overshielding effects even during quiet conditions. Additionally, we examined seasonal variations in PP amplitudes and observed weaker PP during the solstice compared to the equinox. Furthermore, our study analysed the equatorial counter electrojet (CEJ) caused by PP events associated with northward-turning IMF-Bz. We report 15% of CEJs at VEN associated with PP effects; a few PP events were observed while the equatorial electric field at VEN was already westward (i.e. PP within CEJ), indicating the combined effects of magnetospheric and ionospheric perturbations. Identifying CEJs caused by PP during quiet days helps to differentiate between magnetospheric and ionospheric mechanisms/processes. Additionally, we investigated the spatial variability in PP amplitudes at closely spaced sites, utilizing one year of concurrent data from Minicoy (MNC, Geomagnetic latitude 0.19ºN), VEN, and Campbell Bay (CBY, Geomagnetic latitude 0.29ºN) situated at geographic longitudes 72º, 77º and 93º, respectively. Our results showed significant differences in PP amplitudes between VEN-CBY and MNC-CBY, separated by 15º and 20º longitude, respectively. Observations from the three equatorial sites demonstrated a longitudinal trend, with PP amplitudes increasing westward towards MNC, inverse to the amplitudes of EEJ.Graphical

We describe a mode for two-dimensional UHF (445 MHz) radar observations of F-region irregularities using the 14-panel version of the advanced modular incoherent scatter radar (AMISR-14). We also present and discuss examples of observations made by this mode. AMISR-14 is installed at the Jicamarca Radio Observatory (JRO, 11.95°S, 76.87°W, ~ 0.5° dip latitude) in Peru and, therefore, allows studies of ionospheric irregularities at the magnetic equator. The new mode takes advantage of the electronic beam-steering capability of the system to scan the equatorial F-region in the east–west direction. Therefore, it produces two-dimensional views of the spatial distribution of sub-meter field-aligned density irregularities in the magnetic equatorial plane. The scans have a temporal resolution of 20 s and allow observations over a zonal distance of approximately 400 km at main F-region heights. While the system has a lower angular and range resolution than interferometric in-beam VHF radar imaging observations available at Jicamarca, it allows a wider field-of-view than that allowed with the VHF system. Here, we describe the mode, and present and discuss examples of observations made with the system. We also discuss implications of these observations for studies of ESF at the JRO.Graphical abstract

The predictability of the nighttime equatorial spread-F (ESF) occurrences is essential to the ionospheric disturbance warning system. In this work, we propose ESF forecasting models using two deep learning techniques: artificial neural network (ANN) and long short-term memory (LSTM). The ANN and LSTM models are trained with the ionogram data from equinoctial months in 2008 to 2018 at Chumphon station (CPN), Thailand near the magnetic equator, where the ESF onset typically occurs, and they are tested with the ionogram data from 2019. These models are trained especially with new local input parameters such as vertical drift velocity of the F-layer height (Vd) and atmospheric gravity waves (AGW) collected at CPN station together with global parameters of solar and geomagnetic activity. We analyze the ESF forecasting models in terms of monthly probability, daily probability and occurrence, and diurnal predictions. The proposed LSTM model can achieve the 85.4% accuracy when the local parameters: Vd and AGW are utilized. The LSTM model outperforms the ANN, particularly in February, March, April, and October. The results show that the AGW parameter plays a significant role in improvements of the LSTM model during post-midnight. When compared to the IRI-2016 model, the proposed LSTM model can provide lower discrepancies from observational data.Graphical

Ocean bottom pressure-gauge (OBP) records play an essential role in seafloor geodesy. Oceanographic fluctuations in OBP data, however, pose as a significant noise source in seafloor transient crustal deformation observations, including slow slip events (SSEs), making it crucial to evaluate them quantitatively. To extract the significant fluctuation phenomena common to multiple observation networks, including oceanographic fluctuations and tectonic signals, we applied principal component analysis (PCA) to the 3-year Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) OBP time series for 40 stations during 2016–2019. PCA could separate several oceanographic signals based on the characteristics of their spatial distributions, although evident transient tectonic signals could not be confirmed from the observed pressure records during this observed period. The spatial distribution of the first four principal components (PCs) reflected the common component, inclined component along sea depth, longitude component, and parabola-like pattern, respectively. By subtracting each PC (in particular, PC-2 and PC-4) from the time series, we could significantly reduce the sea depth dependence of OBP records, which has been highlighted in several previous studies and is also evident in this region. We interpreted PCs 2–4 as the reflection of the strength and meandering of ocean geostrophic currents based on a comparison with the PC spatial distribution of the numerical oceanographic models. In addition, to evaluate the ability of PCA to separate transient tectonic signal from OBP time series, including oceanographic fluctuations, we conducted a synthetic ramp assuming an SSE by rectangular fault and then applied PCA. The assumed synthetic tectonic signal could be separated from the oceanographic signals and included in the principal component independently depending on its amplitude, suggesting that the spatial distribution of each PC would change if the amplitude of the synthetic signal were sufficiently large. We propose a transient event-detection method based on the spatial distribution difference of a specific PC with or without a tectonic signal. We used the normalized inner product (NIP) between these PCs as the indicator of their similarities. This method can detect transient tectonic signals more significantly than the moment-magnitude scale of 5.9 from OBP records.Graphical

Tephra rings that surround maar craters are typically inferred from field observations to have been emplaced rapidly over a time period of days to years and thus monogenetic, which is, however, rarely assessed quantitatively. This paper describes a polycyclic origin of the Mamiyadake tephra ring (Japan), comparing the paleomagnetic directions obtained from throughout the stratigraphy. The new data show that the paleomagnetic directions change systematically with stratigraphic height through the sections, which is interpreted to record paleosecular variation (PSV) of the geomagnetic field during formation of the tephra ring. The paleomagnetic results, together with using an average rate of PSV in Japan, indicate that the Mamiyadake tephra ring was constructed episodically with five major eruptive episodes, separated by centuries or longer, over a period of at least 1000 years. The findings demonstrate that detailed paleomagnetic characterization can uncover the temporal evolution of tephra rings, providing a useful criterion for identifying time breaks, even where field evidence is lacking, and a minimum estimate of the time interval for their emplacement. The approach used here may be applicable to volcanoes of any type.Graphical

We present horizontal ground motion predictions at a soft site in the Kumamoto alluvial plain for the Mj 5.9 and Mj 6.5 Kumamoto earthquakes of April 2016, in the framework of an international blind prediction exercise. Such predictions were obtained by leveraging all available information which included: (i) analysis of earthquake ground motions; (ii) processing of ambient vibration data (AMV); and (iii) 1D ground response analysis. Spectral analysis of earthquake ground-motion data were used to obtain empirical estimates of the prediction site amplification function, with evidence of an amplification peak at about 1.2 Hz. Horizontal-to-vertical spectral ratio analysis of AMV confirmed this resonance frequency and pointed out also a low-frequency resonance around 0.3 Hz at the prediction site. AMV were then processed by cross-correlation, modified spatial autocorrelation and high-resolution beamforming methods to retrieve the 1D shear-wave velocity (Vs) structure at the prediction site by joint inversion of surface-wave dispersion and ellipticity curves. The use of low frequency dispersion curve and ellipticity data allowed to retrieve a reference Vs profile down to few thousand meters depth which was then used to perform 1D equivalent-linear simulations of the M 5.9 event, and both equivalent-linear and nonlinear simulations of the M 6.5 event at the target site. Adopting quantitative goodness-of-fit metrics based on time–frequency representation of the signals, we obtained fair-to-good agreement between 1D predictions and observations for the Mj 6.5 earthquake and a poor agreement for the Mj 5.9 earthquake. In terms of acceleration response spectra, while ground-motion overpredictions were obtained for the Mj 5.9 event, simulated ground motions for the Mj 6.5 earthquake severely underestimate the observations, especially those obtained by the nonlinear approach.Graphical