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Recent geodetic measurements have detected recurrent slow slip events (SSEs) in many subduction zones. Numerical simulations suggest that the recurrence intervals and magnitudes of such SSEs decrease in the later stage of the interseismic period. Therefore, activities of SSEs and their temporal variations in recurrence intervals and magnitudes provide important clues for evaluating future large earthquakes. However, our knowledge of recurrent SSEs before the establishment of dense observation networks is limited. Here, we report a possible SSE in 1973 beneath the Kii peninsula along the Nankai subduction zone, southwest Japan, where magnitude 8 class earthquakes such as the 1944 Tonankai and 1946 Nankai earthquakes repeatedly occur. The possible SSE was detected by a horizontal pendulum tiltmeter installed in 1947 at the Kishu observatory. We recovered and converted the original tilt records from 1969 to 1974 drawn on bromide (light sensitive) papers to digital tilt records. In late-November 1973, there was a slow transient signal down to the northeast of about 1.4 μrad, lasting about 1–3 days. Assuming this crustal deformation was due to an SSE, we searched for a possible fault model by adopting the already-known SSE models occurring from 1996 to 2012 as template models. Among the 42 template fault models, five models were able to qualitatively explain the direction of the observed tilt change. The centroids of these fault models were located about 30–60 km to the west of the observatory. In the source region, two magnitude 5 class earthquakes occurred before the initiation of the transient signal, and thus may be related to the occurrence of the possible SSE. In contrast, the magnitude of the calculated tilt change was about 1–2 order smaller than that of the observed tilt change. This discrepancy could be partly explained by a slip amount larger than that in the template models, which suggests that the possible SSE in 1973 was larger than those detected by recent observations. Further investigation of historical data on crustal deformation will reveal more about the temporal change in the recurrence intervals and magnitudes of SSEs during earthquake cycles of large earthquakes. [ABSTRACT FROM AUTHOR]

Electrodynamic coupling between the ionospheric E and F regions is widely recognized as the underlying mechanism for generating medium-scale traveling ionospheric disturbances (MSTIDs) during nighttime at midlatitudes. Recently, the double-thin-shell approach has proven to be a useful tool for studying the E–F coupling. By using total electron content (TEC) measurements, this approach enables the simultaneous reconstruction of electron density perturbations in both the E and F regions with broad and continuous coverage. However, the current reconstruction performance is limited when using only GPS-TEC measurements from GEONET, a dense network of ground-based Global Navigation Satellite System (GNSS) receivers over Japan. The expansion of available data sources and the integration of multi-GNSS observation data are considered important to enhance the double-thin-shell model. Fortunately, SoftBank Corp., a Japanese telecommunications provider, has recently developed a dense independent GNSS observation network to improve positioning services. In this paper, we analyze the potential of the improved double-thin-shell approach and emphasize the importance of incorporating multi-GNSS observation data from both GEONET and SoftBank networks. The solvability analysis, simulation, and observation results collectively indicate a substantial improvement in the spatiotemporal resolution. Specifically, the longitudinal and latitudinal resolution is improved from 0.15° to 0.1° in the E region, and from 0.5° to 0.3° in the F region. The temporal resolution is also improved from 2 to 1 min. In addition, significant improvements have been achieved in the reconstruction performance, particularly for the E region under complex background conditions. Based on these assessments, we conclude that the incorporation of GEONET and SoftBank GNSS observation data holds significant potential for improving the double-thin-shell model and advancing our understanding of MSTIDs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Shikoku island, southwestern Japan lies in the western Nankai Trough and showcases along-strike segmentation of slow earthquake behavior. Whether the spatial variation of tremor behavior reflects the regional differences in structure/source properties and how much such differences can be recognized by the seismic signals themselves are two questions addressed in this paper. Taking advantage of advanced methods in recognizing and classifying signals using machine learning approaches, we attempt to answer them by conducting signal classification experiments in Shikoku. Based on the tremor catalog from 1 June 2014 to 31 March 2015, the tremors recorded in four different areas were treated as different classes and segmented into 60-s-long signals. The number of tremors in four different areas (A to D, from west to east) reached 15,000, 31,000, 10,000, and 16,000, respectively. To efficiently distinguish between tremors from different areas, we applied a k-nearest neighbor (k-NN) classifier with Fisher's class separability criteria to select the optimal feature subset. The resulting classification performance reached more than 90% at all 12 stations. We further designed a triangle test to select the features that can better represent the differences in source properties between areas. We found that the most efficient features were associated with (1) the number of peaks in the temporal evolution of discrete Fourier transforms and (2) the energy distribution in the autocorrelation function (ACF). To match the difference in behavior revealed by the ACF, the size of the tremor zone, which mainly controls how long the seismic energy lasts in a tremor episode, was determined to be largest in Area B and smallest in Area C. The heterogeneity of the asperities in a tremor zone, which may control how spiky the tremor signals developed over time, was determined to be strong in Areas B and C. Together with previously documented variations in slow earthquake behavior in the same area, we finally propose a conceptual model that provides a better understanding of the regional differences in the tremor source properties in Shikoku, Japan. [ABSTRACT FROM AUTHOR]

In the infrasonic observation of a fumarolic field, distinguishing multiple fumarolic sources is challenging. The array technique effectively estimates the source locations and identifies the target signal from other signals and noise. We conducted an experiment at Kirishima Iwo-Yama, Japan, where two active fumarolic areas were separated by ~ 450 m. A three-element array with an aperture of ~ 20 m was installed between the two fumarolic areas. In addition, a single microphone was installed near one of the fumaroles. The array combined with the waveform correlation analysis estimated the most prominent source but failed to estimate the other weak source. A joint analysis of the array and the single microphone effectively resolved the two sources. It was also confirmed that newly developed power-saving MEMS microphones were useful for observing the fumaroles. This paper presents the instrumentation and analytical method that would be beneficial for monitoring volcanoes that have multiple hydrothermally active vents. [ABSTRACT FROM AUTHOR]

Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced. [ABSTRACT FROM AUTHOR]

Crustal earthquakes are generally recognized as having recurrence periods of more than 1000 years. However, in northern Ibaraki Prefecture, Japan, two moderate-sized earthquakes occurred on almost the same fault plane in 2011 and 2016. Moreover, it has been reported that the slip distributions derived from crustal deformation data from these two events are almost identical. We performed a near-field waveform inversion analysis to estimate the details of the spatiotemporal distribution of the co-seismic rupture. Our results reveal large slip areas during each earthquake at different locations across the bend of the fault plane. These results indicate that the shear stress of the fault area where the main slip occurred during the 2011 event was not rebuilt in such a short period, a phenomenon that had not previously been discussed for this sequence. Our results also suggest that the fault geometry affected such complex fault ruptures. [ABSTRACT FROM AUTHOR]

Observations and theoretical analysis on the night-time mid-latitude ionospheric irregularities support the postulation of frequently coupled E and F regions. In this paper, we attempt at asserting this notion while using total electron content (TEC) measurements. The TECs are from a dense GNSS receiver network over Japan with more than 1200 stations and a mean distance of ~ 25 km between receivers; thus, ideal for analyzing small-scale perturbations in ionospheric electron density. We take an ansatz that mid-latitude night-time plasma instabilities concentrate at E and F layers. Then the integrated three-dimensional density perturbations are parameterized with a double-thin-shell model. At each shell, perturbation components are assumed identical at any point within a given grid block. Two days with events of night-time medium-scale traveling ionospheric disturbances (MSTIDs), but with different amplitudes, were investigated. Results show that the newly developed technique can infer several horizontal characteristics on E–F coupled instabilities; the coexistence of northwest–southeast (NW–SE) aligned irregular structures in E and F regions is evident. Both E- and F-region irregularities share similar propagation parameters, a shred of clear evidence of strong coupling. [ABSTRACT FROM AUTHOR]

One of the characteristic features of the gravity recordings produced by the superconducting gravimeter CT #036 at Ishigakijima, Japan, is that it indicates gravity increase when a typhoon (hurricane) approaches the island. Since we are trying to detect small gravity signals associated with the long-term slow slip events in this region, it is very important in the interpretation of the observed data whether such gravity changes are of natural or instrumental origin. In this paper, we investigate whether or not nonlinearity in the sensor of the superconducting gravimeter is responsible for this phenomenon. Here we take the same theoretical approach as taken by our previous study which investigated the effect of coupling between horizontal and vertical components of the gravity sensor in order to understand the noise caused by the movements of a nearby VLBI antenna. From theoretical and experimental approaches, we prove that the gravity increase observed by CT #036 at the times of high background noise level cannot be explained by instrumental effects, such as the nonlinearity in the vertical component or the coupling between horizontal and vertical components of the gravity sensor. This implies that the observed gravity increases are real gravity signals of natural origin. [ABSTRACT FROM AUTHOR]

This paper demonstrates and assesses the capability of the advanced three-dimensional (3-D) ionosphere tomography technique, during severe conditions. The study area is northeast Asia and quasi-Japan-centred. Reconstructions are based on total electron content data from a dense ground-based global navigation satellite system receiver network and parameters from operational ionosondes. We used observations from ionosondes, Swarm satellites and radio occultation (RO) to assess the 3-D picture. Specifically, we focus on St. Patrick's day geomagnetic storm (17–19 March 2015), the most intense in solar cycle 24. During this event, the energy ingested into the ionosphere resulted in Dst and Kp and reaching values ~ − 223 nT and 8, respectively, and the region of interest, the East Asian sector, was characterized by a ~ 60% reduction in electron densities. Results show that the reconstructed densities follow the physical dynamics previously discussed in earlier publications about storm events. Moreover, even when ionosonde data were not available, the technique could still provide a consistent picture of the ionosphere vertical structure. Furthermore, analyses show that there is a profound agreement between the RO profiles/in-situ densities and the reconstructions. Therefore, the technique is a potential candidate for applications that are sensitive to ionospheric corrections. [ABSTRACT FROM AUTHOR]

We surveyed the relationship between the scale of space weather events and their occurrence rate in Japan, and we discussed the social impact of these phenomena during the Project for Solar–Terrestrial Environment Prediction (PSTEP) in 2015–2019. The information was compiled for domestic users of space weather forecasts for appropriate preparedness against space weather disasters. This paper gives a comprehensive summary of the survey, focusing on the fields of electricity, satellite operations, communication and broadcasting, satellite positioning usage, aviation, human space activity, and daily life on the Earth's surface, using the cutting-edge knowledge of space weather. Quantitative estimations of the economic impact of space weather events on electricity supply and aviation are also given. Some topics requiring future research, which were identified during the survey are also described. [ABSTRACT FROM AUTHOR]

The Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net) is presently the world's largest network of ocean bottom pressure sensors for real-time tsunami monitoring. This paper analyzes the efficacy of such a vast system in tsunami forecasting through exhaustive synthetic experiments. We consider 1500 hypothetical tsunami scenarios from megathrust earthquakes with magnitudes ranging from Mw 7.7–9.1. We employ a stochastic slip model to emulate heterogeneous slip patterns on specified 240 subfaults over the plate interface of the Japan Trench subduction zone and its vicinity. Subsequently, the associated tsunamis in terms of maximum coastal tsunami heights are evaluated along the 50-m isobath by means of a Green's function summation. To produce tsunami forecasts, we utilize a tsunami inversion from virtually observed waveforms at the S-net stations. Remarkably, forecasts accuracy of approximately 99% can be achieved using tsunami data within an interval of 3 to 5 min after the earthquake (2-min length), owing to the exceedingly dense observation points. Additionally, we apply an optimization technique to determine the optimal combination of stations with respect to earthquake magnitudes. The results show that the minimum requisite number of stations to maintain the accuracy attained by the existing network configuration decreases from 130 to 90 when the earthquake size increases from Mw 7.7 to 9.1. [ABSTRACT FROM AUTHOR]

This paper proposes a new technique, namely Phase bias-based Small Grid Model (PSGM), to derive absolute ionospheric vertical total electron content (VTEC) with observations of Global Navigation Satellite System Earth Observation Network of Japan (GEONET). The proposed technique deals with the phase observations alone without handling the pseudoranges, which reduces the noise in VTEC estimation. A new parameter, the arc bias (Barc), is introduced to combine the phase ambiguities and differential phase biases. To solve Barc, equations are constructed under the assumption that the VTEC is identical in the same 0.1° × 0.1° grid. The performance of PSGM is evaluated with the observations in solar maximum year 2014. The root mean square error (RMSE) of PSGM is 0.40 TECU in average, the maximum RMSE is 0.73 TECU and the minimum RMSE is 0.26 TECU. The fitting accuracy of the VTEC results is improved compared with most of the existing methods. [ABSTRACT FROM AUTHOR]

Correlation between rainfall and short period GPS-TEC (total electron content) variations are investigated by using the precipitation data obtained on the ground and estimated from satellite observations (JAXA/GSMaP) as a proxy of lower atmospheric wave activity. The GPS-TEC data obtained at a tropical station, PHIM, in Phimai, Thailand, for 2014–2020, and the data obtained at a mid-latitude station, NAKG, in Tokara Nakanoshima Island, Japan, for 2017–2019, are examined. A statistical analysis of MEM (maximum entropy method) power spectral density (PSD) in the period range from 50 to 1200 s over PHIM clearly shows an enhancement in the cases of rainfall from that in no-rainfall cases, in particular, on the dusk side. The enhancement is observed both acoustic wave periods less than 5–6 min and internal gravity wave periods more than 10 min. The enhancement after sunset could be an effect of strong rainfall more frequent on the dusk side than that in other local time, or it could suggest the importance of ionospheric electron density profile change for the TEC variation. On the other hand, the PSD does not show such clear enhancement over NAKG on the dusk side, although it shows a small enhancement on both dayside and night-side. A clear PSD bulge near the main vertical acoustic resonance periods, i.e., around 275 s, appears in the average PSD profile of the TEC at PHIM, which suggests that the resonance effect contribute to some extent the PSD enhancement under rainy condition. An event analysis also suggests the contribution of acoustic resonance to the enhancement of the short period TEC variation. A complicated spatial distribution of TEC oscillation over a rainfall area around PHIM, where the TEC oscillations with various periods co-exist, is presented. [ABSTRACT FROM AUTHOR]

Monitoring and predicting fault slip behaviors in subduction zones is essential for understanding earthquake cycles and assessing future earthquake potential. We developed a data assimilation method for fault slip monitoring and the short-term prediction of slow slip events, and applied to the 2010 Bungo Channel slow slip event in southwest Japan. The observed geodetic data were quantitatively explained using a physics-based model with data assimilation. We investigated short-term predictability by assimilating observation data within limited periods. Without prior constraints on fault slip style, observations solely during slip acceleration predicted the occurrence of a fast slip; however, the inclusion of slip deceleration data successfully predicted a slow transient slip. With prior constraints to exclude unstable slip, the assimilation of data after slow slip event occurrence also predicted a slow transient slip. This study provides a tool using data assimilation for fault slip monitoring and prediction based on real observation data. [ABSTRACT FROM AUTHOR]

The physical properties of seafloor massive sulfides are crucial for interpreting sub-seafloor images from geophysical surveys, shedding light on the evolution of seafloor mineral deposits. While some studies have explored the relationship between electrical properties and the volume of conductive minerals in rocks from seafloor massive sulfide deposits, they primarily focused on artificial samples, leaving the characteristics of natural samples less understood. Moreover, there has been no comprehensive study detailing the general characteristics of electrical properties, particularly chargeability and relaxation time, in relation to the volumetric fraction of sulfides in rocks from massive sulfide mounds in typical hydrothermal areas. In this study, we employed complex conductivity measurements, elemental concentration analysis, and mineral content identification on to rock samples from the active hydrothermal zones of the Okinawa Trough in Japan. The complex conductivity observed was remarkably high, with a pronounced imaginary component and a broad frequency range. This is attributed to induced polarization extending beyond our measurement range. The rock samples were rich in conductive sulfide minerals such as pyrite, chalcopyrite, and galena. Using the Cole–Cole rock physics model, we established a correlation between rock chargeability and relaxation time coefficient with the volume fraction of conductive sulfide minerals, which deviated from previous findings. The intensity of induced polarization was notably higher than anticipated in earlier studies using artificial samples. Furthermore, we observed a distinct positive correlation between the coefficient of relaxation time and the increase in sulfide volume, likely due to the geometric characteristics of the sulfide minerals. Our findings suggest that rocks in massive sulfide mounds may generally construct sulfide clusters that lengthen the conductive path of the electrical carrier. [ABSTRACT FROM AUTHOR]

The Noto region in Japan has been experiencing earthquake swarm activity since mid-2018, with repeated rises and falls in activity. Crustal deformation (expansion and uplift) observed there since the end of 2020 has been connected to crustal fluids. Studies in other regions have suggested that tides are related to earthquake swarm activities associated with crustal fluids. Therefore, we investigated whether tides are also involved in the Noto earthquake swarm activity. Our results suggest a tidal correlation only at greater depths in the southern part of the analyzed area ('region Sd'). There, we inferred that an increase in pore fluid pressure caused by the inflow of deep fluids may have led to a decrease in fault fracture strength, making the local seismicity relatively susceptible to the effects of tidal forces. The significantly high value in region Sd of the scaling parameter b of the Gutenberg–Richter law (describing the earthquake magnitude–frequency distribution) and observed crustal deformation are consistent with this interpretation. [ABSTRACT FROM AUTHOR]

Seismic activity in the Noto region of Ishikawa Prefecture, central Japan, has increased since August 2020 and has continued as of August 2023. Stress changes due to subsurface sources and increases in fluid pressure have been discussed as the causes of the seismic activity increase. In this study, S-wave polarization anisotropy was investigated by S-wave splitting analysis using temporary and permanent stations located in the epicenter area. We also investigated the seismic wave velocity structure in the source region by analyzing seismic wave velocity tomography. The fast orientations of anisotropy (fast shear wave oscillation direction, FSOD) were generally NW–SE in the southern part of the focal area and east–west in the northern part. The NW–SE anisotropy generally coincides with the direction of the maximum horizontal compression axis, both near the surface and at earthquake depths. Therefore, stress-induced anisotropy can be the cause of the observed NW–SE anisotropy. On the other hand, faults with strike directions generally east–west have been identified, and structural anisotropy may be the cause of the observed east–west anisotropy. We examined the time variation of anisotropy at N.SUZH, one of the permanent stations. No significant time variation was observed in the FSOD. Larger anisotropy was observed, particularly for the activity in the western part of the focal area, from about June–September 2021 compared to the previous period. A high Vp/Vs region was identified beneath the focal area, at a depth of 18 km. This high Vp/Vs region has slightly larger P-wave velocities than the surrounding area. Since Tertiary igneous rocks are distributed in the target area, the high Vp/Vs region may represent a Tertiary magma reservoir, suggesting that fluids released through the old magma reservoir are involved in this seismic swarm. This seismic activity started in the southern part of the area, where relatively immature fault structure exists, where stress-induced anisotropy is distributed, and where high Vp/Vs regions suggestive of fluid at depth are identified. Subsequently, seismicity became more active in the northern part, where structural anisotropy with well-developed fault structures is distributed. [ABSTRACT FROM AUTHOR]

On October 21, 2016, an earthquake with Japan Meteorological Agency (JMA) magnitude 6.6 hit the central part of Tottori Prefecture, Japan. This paper demonstrates two notable effects of the surface geology on strong ground motions due to the earthquake. One is a predominant period issue observed over a large area. A seismic intensity of 6 lower on the JMA scale was registered at three sites in the disaster area. However, the peak ground acceleration ranged from 0.3 to 1.4 G at the three sites because of the varying peak periods of observed strong ground motions. The spectral properties of the observations also reflect the damage around the sites. Three-component microtremors were observed in the area; the predominant ground period distributions based on horizontal to vertical spectral ratios were provided by the authors. The peak periods of the strong motion records agree well with predominant periods estimated from microtremor observations at a rather hard site; however, the predominant periods of the microtremors are slightly shorter than those of the main shock at the other two soft sites. We checked the nonlinear effect at the sites by comparing the site responses to small events and the main shock. The peak periods of the main shock were longer than those of the weak motions at the sites. This phenomenon indicates a nonlinear site effect due to large ground motions caused by the main shock. A horizontal component of the accelerogram showed rather pulsating swings that indicate cyclic mobility behavior, especially at a site close to a pond shore; ground subsidence of ~20 cm was observed around the site. The peak periods of weak motions agree well with those of the microtremor observations. This implies an important issue that the predominant periods estimated by microtremors are not sufficient to estimate the effect of surface geology for disaster mitigation. We have to estimate the predominant periods under large ground motions considering the nonlinear site response of soft sediment sites. [ABSTRACT FROM AUTHOR]

Construction of regional geomagnetic secular variation curves for the last several tens of thousands of years is important for understanding the behavior of non-dipole fields and applications to geochronology. Around Japan, secular variation records of older than 10 ka was scarce, in particular for relative paleointensity (RPI). Here, we conducted a paleomagnetic study of a sediment core covering the last ~ 40 kyr taken from a small basin in the Nankai Trough. The core consists of homogenous hemipelagic sediments except for turbidites and volcanic ashes. The age model was constructed based on seven 14C datings and two volcanic ashes. Turbidites and volcanic ashes were excluded from the construction of secular variation curves because of geologically instantaneous deposition. It was revealed that the magnetization of this core is carried largely by detrital magnetic minerals, although magnetofossils are also contained. Bulk magnetic properties show some temporal changes in magnetic concentration and grain size, but still homogeneous enough for reliable RPI estimations except for turbidites and volcanic ashes. The resultant RPI shows no correlation with the normalizer, anhysteretic remanent magnetization, of the RPI estimations or with a proxy for a magnetic grain size and/or the proportion of magnetofossils to detrital magnetic minerals. The obtained RPI record shows a long-term increasing trend since ~ 40 ka, which coincides with global stack curves. On the other hand, there are some differences in shorter timescale variations, which may reflect non-dipole fields. This study demonstrated that hemipelagic sediments in the Nankai Trough have potential for recovering high-quality RPI records when turbidites and volcanic ashes were excluded and are useful for accumulating records to construct a regional master curve. [ABSTRACT FROM AUTHOR]

Volcanic earthquakes provide essential information for evaluating volcanic activity. Because volcanic earthquakes are often characterized by swarm-like features, conventional methods using manual picking require considerable time to construct seismic catalogs. In this study, using a machine learning framework and a trained model from a volcanic earthquake catalog, we obtained a detailed picture of volcanic earthquakes during the past 12 years at the Kirishima volcano, southwestern Japan. We detected ~ 6.2 times as many earthquakes as a conventional seismic catalog and obtained a high-resolution hypocenter distribution through waveform correlation analysis. Earthquake clusters were estimated below the craters, where magmatic or phreatic eruptions occurred in recent years. Increases in seismic activities, b values, and the number low-frequency earthquakes were detected before the eruptions. The process can be conducted in real time, and monitoring volcanic earthquakes through machine learning methods contributes to understanding the changes in volcanic activity and improving eruption predictions. [ABSTRACT FROM AUTHOR]

Vulcanian activity is one of the most common eruption styles of arc andesitic volcanism on Earth. It ejects and deposits volcanic bombs around the source crater. Although paleomagnetic studies of volcanic bombs are limited, such studies can potentially provide more opportunities for high-resolution paleomagnetic dating of volcanic activity. In this study, paleomagnetic dating was applied to large (> 1 m) volcanic bombs around active craters in the Azuma volcano group, NE Japan. Oriented samples were collected from the interior parts of five large volcanic bombs situated on gentle slopes, a few hundred meters from the source crater. More than six core samples were collected from each bomb and all samples were subjected to a range of rock magnetic experiments, including anisotropy of magnetic susceptibility (AMS) and thermal/alternating field demagnetization (THD/AFD) analyses. The Characteristic Remanent Magnetization (ChRM) directions for specimens from all bombs were well-defined, have small α95 (< 2.5º), and are in close agreement with each other. Comparing our measured overall mean direction (Dm = 355.5º, Im = 49.8º, α95 = 1.6º) with modeled geomagnetic field estimates and a reference secular variation curve for this area (using MATLAB-based archaeomagnetic dating tool), we suggest that the volcanic bombs were produced in the historical Meiji period (1893–1895 CE) eruption. In addition, a combination of the data of ChRM, AMS, thermomagnetic analyses, hysteresis measurement, and XRF analysis indicates that the volcanic bombs were derived from a plug of lava in the conduit under the solidification point (ca. 800 °C), but above the Curie point of the titanomagnetite remanence carrier (around 300 °C). We show that volcanic bombs can be powerful for paleomagnetic dating if certain sampling conditions, such as quantity, situation, size and portion are satisfied. [ABSTRACT FROM AUTHOR]

Fluid-inclusion and thermochronometric analyses have been applied to hydrothermal alteration zones and their host rocks outcropping in the Hongu area of the Kii Peninsula, southwestern Japan in an attempt to detect thermal anomalies related to hydrothermal events and quantify the thermal effects on the host rocks. Hydrothermal events at ~ 150 °C and ~ 200 °C were identified by fluid-inclusion microthermometry of quartz veins in the alteration zones. For the host rocks and alteration zones, in the youngest population zircon yielded U–Pb dates ranging between ~ 74.7–59.2 Ma, fission-track dates of ~ 27.2–16.6 Ma, and (U–Th)/He single-grain dates of ~ 23.6–8.7 Ma. Apatite yielded pooled fission-track ages of ~ 14.9–9.0 Ma. The zircon U–Pb dates constrain the maximum depositional ages of the sedimentary samples. However, the fission-track and (U–Th)/He dates show no clear trend as a function of distance from the alteration zones. Hence, no thermal anomaly was detected in the surrounding host rocks based on the thermochronometric data patterns. The fission-track and (U–Th)/He dates are rather thought to record regional thermal and exhumation histories rather than a direct thermal imprint of fluid flow, probably because the duration of such activity was too short or because fluid flow occurred before regional cooling events and were later thermally overprinted. Apatite fission-track ages of ~ 10 Ma may reflect regional mountain uplift and exhumation related to the obduction of the SW Japan lithospheric sliver onto the Shikoku Basin, or the rapid subduction of the Philippine Sea slab associated with the clockwise rotation of the Southwest Japan Arc. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via the original article. [ABSTRACT FROM AUTHOR]

We present the 1D subsoil structure and local site effects at KUMA strong ground motion station in Kumamoto City, Japan. We analyze data from a field campaign conducted in the framework of the Blind Prediction BP1 test of the 6th IASPEI/IAEE International Symposium: Effects of Surface Geology on Seismic Motion. In parallel with other participants of the BP1 test, we process data from passive and active source measurements aiming to determine the shear-wave velocity, Vs, structure and the site response at KUMA station. Passive measurements are associated to five microtremor arrays. In each array, seven seismometers have been deployed in a common-center triangle shape, recording microtremors simultaneously for 1 to 2 h. The vertical component of microtremors was analyzed using the spatial autocorrelation (SPAC) method. Cross-correlation coefficients were computed for all station pairs available for each array. By fitting the average SPAC's coefficients to the first-kind zero-order Bessel function, J0, and assuming that microtremors primarily comprise fundamental mode Rayleigh waves, phase velocity dispersion curves were determined. Phase velocity values for frequencies > 15 Hz were obtained from data of a close-by active source geophone profile. We integrated the data with those of the passive measurements and obtained an experimental phase velocity dispersion curve. The resulting curve shows low velocity variation, from 150 to 200 m/s, in the surface layers, whereas significant dispersion appears in frequencies below 2.5 Hz. By inverting this curve, we achieved to determine the 1D shear-wave velocity structure at KUMA station. Site response characteristics were determined by applying the Horizontal-to-Vertical-Spectral-Ratios method. Significantly amplified peaks in the frequency range between 0.3 to 1.5 Hz dominate HVSR spectral ratios. These peaks correspond to resonant frequencies of soils and originate from different impedance contrasts within the substratum of the site. [ABSTRACT FROM AUTHOR]

The explosive eruption of the Hunga Tonga-Hunga Ha'apai volcano on 15 January 2022 generated atmospheric waves traveling around the Earth, which caused ionospheric disturbances on various spatio-temporal scales. A HF Doppler sounding system in Japan detected characteristic ionospheric disturbances showing periodic oscillations in the Doppler frequency with a period of ~ 4 min. In this study, such periodic oscillations were examined by comparing Doppler frequency data with Total Electron Content data obtained by Global Navigation Satellite System. The observed periodic oscillations in the Doppler frequency were characterized by a sawtooth or S-letter shaped variation, implying the passage of the traveling ionospheric disturbances through the reflection points of the HF Doppler sounding system. It was also found that the periodic oscillations occurred prior to the arrival of the tropospheric Lamb wave excited by the Tonga eruption. From the total electron content data, the traveling ionospheric disturbances causing the periodic oscillations were excited by the tropospheric Lamb waves at the conjugate point in the southern hemisphere, namely, the electric field perturbations due to the Lamb waves in the southern hemisphere mapped onto the sensing area of the HF Doppler sounding system in the northern hemisphere along the magnetic field lines. The periodic oscillations were observed only in the path between Chofu transmitter and Sarobetsu receiver, whose the radio propagation path is almost aligned in the north–south direction. This suggests that the traveling ionospheric disturbance has a structure elongating in the meridional direction. The variation in the Doppler frequency was reproduced by using a simple model of the propagation of the traveling ionospheric disturbances and the resultant motion of the reflection point. As a result, the vertical motion of the reflection point associated with the periodic oscillations was estimated to be about 1 km. It is known that 4-min period variations are sometimes observed in association with earthquakes, which is due to resonances of acoustic mode waves propagating between the ground and the lower ionosphere. Therefore, a similar resonance structure in the southern hemisphere is a plausible source of the traveling ionospheric disturbances detected in the northern hemisphere. [ABSTRACT FROM AUTHOR]

We present a novel method for coastal tsunami prediction utilizing a denoising autoencoder (DAE) model, one of the deep learning algorithms. Our study focuses on the Tohoku coast, Japan, where dense offshore bottom pressure gauges (OBPGs), called S-net, are installed. To train the model, we generated 800 hypothetical tsunami scenarios by employing stochastic earthquake models (M7.0–8.8). We used synthetic tsunami waveforms at 44 OBPGs as input and the waveforms at four coastal tide gauges as output. Subsequently, we evaluated the model's performance using 200 additional hypothetical and two real tsunami events: the 2016 Fukushima earthquake and 2022 Tonga volcanic tsunamis. Our DAE model demonstrated high accuracy in predicting coastal tsunami waveforms for hypothetical events, achieving an impressive quality index of approximately 90%. Furthermore, it accurately forecasted the maximum amplitude of the 2016 Fukushima tsunami, achieving a quality index of 91.4% at 15 min after the earthquake. However, the prediction of coastal waveforms for the 2022 Tonga volcanic tsunami was not satisfactory. We also assessed the impact of the forecast time window and found that it had limited effects on forecast accuracy. This suggests that our method is suitable for providing rapid forecasts soon after an earthquake occurs. Our research is the first application of an artificial neural network to tsunami prediction using real observations. In the future, we will use more tsunami scenarios for model training to enhance its robustness for different types of tsunamis. [ABSTRACT FROM AUTHOR]

Many active volcanoes and various types of seismic activities exist in the southern part of the Northeast Japan subduction zone. One of the geologically most interesting features in this area is the sequential explosive eruptions of a group of volcanoes. The group consists of Mt. Azuma and Mt. Adatara on the volcanic front line, Mt. Bandai west of the volcanic front, and Mt. Numazawa on the back-arc side. A previous petrological study on the eruption products regarded Mt. Numazawa as an anomalous back-arc volcano because its lavas are similar to those of volcanoes on the volcanic front. The reason behind this unique connection was unclear, and hence, this study was intended to understand the deep fluids distribution beneath the area. For this purpose, a 3-D regional electrical resistivity structure was estimated from a series of wide-band magnetotelluric surveys, with 45 observation points deployed from the fore-arc to the back-arc sides. The most important feature of the resistivity structure is a large conductive zone in the central part of the area, spanning from the upper mantle to the lower crust. Interestingly, the lateral elongation of the conductor is oblique to the volcanic front line and consistent with the spatial distribution of the group of volcanoes and the low-frequency earthquake clusters around them. Therefore, the conductor most likely represents a large, elongated magmatic fluid reservoir beneath the volcanoes. Hydrous partial melting might be the cause of the enhanced conductivity. [ABSTRACT FROM AUTHOR]

Estimates of heat flow can contribute to our understanding of geological structures in plate convergent zones that produce great earthquakes. We applied automated velocity analysis to obtain the accurate seismic profiles needed for precise heat flow estimates using six new seismic profiles acquired during R/V Kaimei KM18-10 voyage in 2018. We calculated heat flow values in the accretionary wedge of the Nankai Trough off the Kii Peninsula, Japan, from the positions of widespread bottom-simulating reflectors (BSRs) in seismic reflection profiles. Calculated conductive heat flow values from the depth of the BSR agree with previous studies where a regional trend is observed from ~ 50 mW/m2 to < 40 mW/m2 60 km landward from the deformation front. This trend is caused by thickening of accretionary sediments and the subduction of the Philippines Sea plate. Segments of profiles are marked by anomalous high heat flow values. Such anomalies represent alterations of the shallow crustal thermal structure caused either by a combination of topographic affects, surface erosion of the seafloor, or by fluid flow that transports heat by advection. We interpret heat flow anomalies (~ 100 mW/m2) as indicators of active faulting, which correspond to low seismic velocity zones along faults. Our results also showed relatively high heat flow at the landward end of several survey lines close to the Kii Peninsula, which we interpret to the possible presence of plutonic rocks that underlie the Kii Peninsula and extend offshore and may be the cause of geothermal springs, steep geothermal gradients, and high heat flow. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Mt. Ontake is an active volcano in central Japan where phreatic eruption activity is prominent. A shallow subsurface structure in the summit region of this volcano has been scarcely studied despite its importance. Our study examines the structure from three seismic analyses: a typical P-wave velocity from the semblance of the vertical propagation of intermediate-depth earthquakes (IDEs), a layered velocity model from P- and S-wave arrival times of shallow volcanic earthquakes, and a pseudoreflection profile from the autocorrelation functions of the IDEs. Our results consistently indicate the presence of three layers, which are interpreted as younger (> 0.1 Ma) and older (0.39–0.78 Ma) eruptive deposits and the basement. A comparison of the structure with hypocentres and deformation sources suggests fluid migration controlled by the structure. [ABSTRACT FROM AUTHOR]

To investigate the dominant deformational patterns and stress conditions in the upper crustal structure of the Kinki region, southern-central Japan, we constructed a high-resolution 3D azimuthal anisotropy model to a depth of ~ 11 km. We used 6-month-long ambient noise data recorded by the densely distributed permanent and temporary stations. From this dataset, cross-correlations were retrieved. We then obtained a 3D isotropic velocity model by inverting Rayleigh wave dispersion data, followed by a direct joint inversion for both 3D azimuthal anisotropy and additional isotropic velocity perturbation. The resolved 3D azimuthal anisotropy reveals significant contrasts of anisotropy across the Kinki region. The predominant fast axes observed in the northwestern Kinki region align with the direction of the maximum horizontal compressional stress and the principal strain rate axes, suggesting that the observed anisotropy is mainly stress-induced. In the southern part of the study area, furthermore, the predominant fast axes trend NE–SW and near E–W, also indicating the presence of stress-induced anisotropy. On the depth profile of the anisotropy, we found depth-dependent variation of azimuthal anisotropy. There exists a significant consistency between the anisotropy observed beneath 3 km depth and the dense distribution of earthquake hypocenters. This interrelationship between anisotropy and seismicity demonstrates that the observed anisotropy could be linked to local crustal stress or fractures relevant to earthquake ruptures. Our 3D anisotropy model therefore contributes towards understanding the locations and features of the seismicity region. [ABSTRACT FROM AUTHOR]

In this study, we detected the horizontal ground strains induced by the atmospheric Lamb wave emitted from the 2022 eruption of the Tonga–Hunga Ha'apai undersea volcano, at an underground observatory in Kamioka, Japan. The observed strains were in the range of 10 - 11 – 10 - 10 and were measured precisely using a 1500-m laser strainmeter with a high resolution in the order of 10 - 12 . This was one of the first observations of a Lamb wave using a laser strainmeter. The strainmeter was constructed in a tunnel of the KAGRA gravitational-wave telescope. Our observations demonstrate that strain and atmospheric pressure were clearly correlated, resulting in a regression coefficient of - 2.3 - 3.7 × 10 - 10 strain/hPa. This finding was compared with the responses under regular pressure conditions and the estimations obtained using the local deformation and traveling wave models. The observed coefficients for the Lamb wave exhibited smaller magnitudes than those observed under regular conditions and take values between that of the two theoretical models. These results reflect the intermediate scale of the pressure distribution of the Lamb wave between the assumptions of the simple models. The strain variations were also found to have started earlier than the corresponding pressure changes at the observation site with characteristic time lags ranging from 25 to 155 s. In addition, several aspects related to the mechanism that created the time lags are discussed. [ABSTRACT FROM AUTHOR]

Precisely evaluating the source, path, and site terms in a broadband frequency range is indispensable for quantitatively predicting strong motions. We conducted a generalized spectral inversion of strong motion in Japan to delineate both the spectral amplitude and phase characteristics for statistical Green's functions. To predict ground motion from future megathrust earthquakes, we also need to model a kinematic source with a stochastic representation of the slip and rupture velocities on the fault surface. In this fundamental study, we first reported the basic features of statistical Green's functions used for summation. We then demonstrated the construction of a kinematic source with distinctive strong-motion-generation areas with spatially random slip and rupture velocity variations. After the summation of statistical Green's functions following the constructed kinematic source, we found that the peak ground accelerations and peak ground velocities of synthetics for the Mw8.2 1944 Tonankai earthquake are in good agreement with those of the well-established empirical formula in Japan because the medians of the synthetic values are well within the average plus/minus one standard deviation of the formula. Based on the simulation results of conducted parametric studies, we observed relatively small but meaningful effects of random slip distribution, as well as relatively small effects of random rupture velocity perturbation. In conclusion, our implementation of the complex source model and the empirically obtained statistical Green's function used as an element source can be a viable combination for broadband (0.1 to 20 Hz) strong-motion simulations, realistic in terms of the amplitude and duration, without any hybrid scheme. [ABSTRACT FROM AUTHOR]

The eruption activity of Shinmoedake in the Kirishima volcanic group of Japan resumed in 2017–18, following a quiet period during 2011–17. Subplinian eruptions preceded lava effusion in 2011; however, no subplinian eruption occurred during 2017–18. Petrological studies and melt inclusion analyses were conducted to investigate the ascent and degassing of the magma to understand the cause of the different eruption styles. Chemical analysis of the melt inclusions from the 2011 eruption indicates that mafic magma with high volatile content (6.2 wt% H2O, 0.25–1.4 wt% CO2) ascended into the shallow felsic magma (1.9–3.7 wt% H2O, 0.025–0.048 wt% CO2) at depths of 5–6 km. Calculations indicate that the mafic magmas were of lower density (1717–1835 kg m−3) than the felsic magma (2264–2496 kg m−3) at 125 MPa and that the two magmas were mixed. The 2011 mixed magma with high volatile content (4.0 wt% H2O, 0.14–0.70 wt% CO2) had a bubble volume of approximately 50 vol% at 50 MPa, which is likely to have caused the subplinian eruption. The whole-rock and chemical compositions of the plagioclase, clinopyroxene, and orthopyroxene phenocryst cores from 2018 and 2011 were similar, suggesting that the 2018 magma was a remnant of the 2011 magma. Chemical analyses of the groundmass from 2018 and the MELTS calculation indicate that the magma approached chemical equilibrium during 2011–18. Melt inclusion analyses and volcanic gas observation noted a lower bulk volatile content in the 2018 magma (2.1–3.0 wt% H2O, 0.087–0.10 wt% CO2) than that in the 2011 magma. Comparison of the degassed-magma volumes estimated from the S and Cl contents of the melt inclusions, SO2 flux and volcanic gas composition, and erupted-magma volume indicates that excess degassing has been occurring in the magma due to convection since February 2011, which may have decreased the volatile content of the magma. The relatively low volatile content meant that the 2018 magma could not erupt explosively and lava was instead erupted via effusion. [ABSTRACT FROM AUTHOR]

Site amplification is an important component of strong ground motion prediction as it differs among sites, reflecting its specific local subsurface geology. Here, we confirm that site amplifications are similar in a neighborhood area over a long period. However, few studies have investigated the spatial properties in a wide region (i.e., the whole of Japan). In this study, we explored the spatial properties of site amplifications based on the generalized inversion technique (GIT) using Fourier amplitude spectra (FAS) as well as pseudo-velocity response spectra (pSv) as the latter is an important index for engineering purposes and the most similar type of response spectra to FAS. The spatial distributions of S-wave site amplifications (SA-S), especially within large sediment basins (e.g., the Kanto and Osaka Basins in Japan), were found to be relatively similar in proximate areas for a long period ranging from 2 to 8 s. This suggests that we could easily predict the site amplifications using an empirical approach through spatial interpolation based on the properties obtained by the GIT. Furthermore, we propose a prediction procedure for site amplification for the whole duration from the SA-S at an arbitrary site. We used the correction function, which converts the SA-S to the site amplification for the whole duration (SA-W), including an S-wave portion and a subsequent portion. This function is called the whole-duration to S-wave spectral ratio (WSR) and is stable in terms of spatial properties. As we could estimate the SA-S either by theoretical transfer functions or observed microtremors, we can easily predict the SA-W based on the proposed WSR concept. We found that SA-S in pSv is more or less similar to SA-S in FAS, however, SA-W in pSv fails to capture the effects of the long duration of ground motions inside a large basin so that we cannot recommend to use pSv for the prediction of whole duration of ground motion. [ABSTRACT FROM AUTHOR]

Infrasonic stations are sparse at many volcanoes, especially those on remote islands and those with less frequent eruptions. When only a single infrasound station is available, the seismic–infrasonic cross-correlation method has been used to extract infrasound from wind noise. However, it does not work with intense seismicity and sometimes mistakes ground-to-atmosphere signals as infrasound. This paper proposes a complementary method to identify the seismic component and the infrasonic component using a single microphone and a seismometer. We applied the method to estimate the surface activity on Ioto Island. We focused on volcanic tremors during the phreatic eruption on April 11, 2013, and during an unconfirmed event on September 12, 2018. We used the spectral amplitude ratios of the vertical ground motion to the pressure oscillation and compared those for the tremors with those for known signals generated by volcano-tectonic earthquakes and airplanes flying over the station. We were able to identify the infrasound component in the part of the seismic tremor with the 2013 eruption. On the other hand, the tremor with the unconfirmed 2018 event was accompanied by no apparent infrasound. We interpreted the results that the infrasound with the 2013 event was excited by the vent opening or the ejection of ballistic rocks, and the 2018 event was not an explosive eruption either on the ground or in the shallow water. If there was any gas (and ash) emission, it might have occurred gently undersea. As the method uses the relative values of on-site records instead of the absolute values, it is available even if the instrument sensitivity and the station site effects are poorly calibrated. [ABSTRACT FROM AUTHOR]

The COVID-19 pandemic that started at the end of 2019 forced populations around the world to reduce social and economic activities; it is believed that this can prevent the spread of the disease. In this paper, we report an analysis of the seismic noise during such an induced social activity reduction in the Tokyo metropolitan area, Japan. Using seismic data obtained from 18 stations in the Metropolitan Seismic Observation Network (MeSO-net), a two-step seismic noise reduction was observed during the timeline of COVID-19 in Tokyo. The first noise reduction occurred at the beginning of March 2020 in the frequency band of 20–40 Hz. This corresponded with the request by the Prime Minister of Japan for a nationwide shutdown of schools. Although social activity was not reduced significantly at this juncture, local reduction of seismic wave excitation in the high-frequency band, 20–40 Hz, was recorded at some MeSO-net stations located in school properties. The second reduction of seismic noise occurred at the end of March to the beginning of April 2020 in a wider frequency band including lower frequency bands of 1–20 Hz. This timing corresponds to when the Governors of the Tokyo metropolitan area requested citizens to stay home and when the state of emergency was declared for the Tokyo metropolitan area by the government, respectively. Since then, the estimated population at train stations abruptly dropped, which suggests that social activity was severely reduced. Such large-scale changes in social activity affect the seismic noise level in low-frequency bands. The seismic noise level started to increase from the middle of May correlating with increase in population at the train stations. This suggests that social activity restarted even before the state of emergency was lifted at the end of May. The two-step seismic noise reduction observed in this study has not been reported in other cities around the world. Unexpected reduction of social activity due to COVID-19 provided a rare opportunity to investigate the characteristics of seismic noise caused by human activities. [ABSTRACT FROM AUTHOR]

At active volcanoes, the surface temperature and its spatial distribution can indicate changes in the underlying magmatic and hydrothermal system. Surface temperature monitoring has been widely performed using thermal remote sensing with thermal infrared (TIR) cameras. One of the drawbacks of this method is that the unknown viewing orientation of TIR images inhibits quantitative evaluations of the spatial extent and distributions of thermal anomalies. Therefore, many studies have performed 3D temperature-field reconstructions by processing TIR images photogrammetrically. However, these studies have not included the correction of TIR wave atmospheric attenuation and viewing-angle variation in emissivity in the reconstruction procedure, which can result in significant temperature misestimation. We propose a simple method that incorporates the correction into the reconstruction process, which can improve the estimation of surface temperature, especially in rugged terrains. We demonstrate our method for the active crater of Aso volcano in Japan. We create digital elevation models by applying the Structure from Motion–Multi-view Stereo algorithm to aerial visible images taken at the same time as the TIR images and then project the TIR images onto the DEM with the direct georeferencing method. The correction is carried out using the geometric parameters acquired in the process. We create 1-m resolution orthorectified thermal images of the crater on two separate dates (18 August 2020 and 16 March 2022) and calculate the heat discharge rates from steaming grounds and a volcanic lake. We find that the heat discharge rate from the fumarolic field on the south crater wall showed a sevenfold increase after the phreatic explosions that occurred on 14 and 20 October 2021. [ABSTRACT FROM AUTHOR]

The correct absolute time of a seismogram is an important prerequisite for many seismological processing methods. Recently, seismic interferometry has been applied to continuously estimate instrumental clock errors in certain regions. However, previous studies have focused on estimating clock errors in only one target region for each study, and data processing methods that provide more stable results in different regions have not been explored. This study presents a new method to estimate clock errors that are more stable in various regions based on seismic interferometry with windowed cross-correlation and least absolute deviation regression. The applicability of this method was validated by its application to 50 active volcanoes in Japan. We found clock errors at 23 stations at 13 volcanoes between January 2017 and December 2021. The maximum amplitude of the clock errors ranged between 0.24 and 18.27 s. This value range is significantly larger than the amplitude of the background fluctuation in the estimated values of clock errors, which is less than 0.05 s for most volcanoes. A comparison of different approaches for the estimation of clock error shows that our proposed method that uses windowed cross-correlation and least absolute deviation regression can reduce the effect of waveform changes in seismic ambient noise cross-correlation functions, probably because of noise source fluctuations. Managing the time stamps of data using the method proposed in this study will contribute to data quality assurance and ensure the reliability of analysis using time information in various study regions. [ABSTRACT FROM AUTHOR]

The station coordinates derived from GNSS (Global Navigation Satellite System) with a conventional static method underpin the study of Earth and planetary science and surveying and mapping. For the continuous provision of high-quality coordinates, it is mandatory to cope with the increasing deviation from the global standard reference frame and the launch of modern GPS (Global Positioning System) satellites. To provide coordinates agreed with ITRF2014 (International Terrestrial Reference Frame 2014) at several millimeters for GEONET (GNSS Earth Observation Network System) in Japan, we developed a new analysis strategy named F5 and assessed the reprocessed station coordinates from 1996. The major updates in F5 can be summarized as incorporating global network processing and enhancements in troposphere modeling. As for the troposphere enhancements, a modern mapping function VMF1 (Vienna Mapping Functions 1) was employed and time intervals for troposphere estimates were shortened. Station coordinates in the global network showed a great agreement with ITRF2014 at several millimeters in the recent 20 years and comparable or slightly better performance with IGS (International GNSS Services) Analysis Centers. The RMS (root mean square) averaged over all GEONET stations indicated very high accuracy of 3.2 mm (horizontal) and 7.3 mm (vertical); the latter accounts for an improvement of roughly 10% from the previous strategy. Sensitivity tests about troposphere estimates revealed that the reduced RMS was completely due to the short time intervals, not the use of VMF1, which contributed to partly suppressing the spurious vertical annual deformation. These results confirm that F5 is sufficiently accurate for the requirements of individual applications and infer the capability of detecting smaller signals the previous strategy could not resolve. [ABSTRACT FROM AUTHOR]

The study used data acquired by the ESG6 Blind Prediction Step BP1 Working Group for purposes of facilitating a comparison of interpretation methods for obtaining shear-wave velocity profiles (Vs) from array observations of microtremor (passive seismic) noise. This work uses the direct-fitting MMSPAC method and the krSPAC method on passive seismic data supplied from four seven-station nested triangular arrays with apertures ranging from 1 to 962 m, located within Kumamoto City, Japan. The data allow a useful frequency range of 38 Hz down to 0.3 Hz, giving depth sensitivities from 2 m to > 1000 m. Results are presented as a seven-layer model which has time-averaged shear wave velocities for top 30 m and 300 m of Vs30 = 189 m/s and Vs300 = 584 m/s, respectively. HVSR spectra show two significant peaks at 1.2 and 0.35 Hz which are indicative of major Vs contrasts at depths 26 m and 750 m. The MMSPAC method (and its krSPAC variant) also proved viable on one asymmetric array where four of the seven stations were corrupted by incoherent low-frequency noise. Indications of a lateral variation in Vs could be detected due to the non-concentric geometry of the four arrays, and also from variations in HVSR spectra at stations of the largest array. Further analysis in step 4 of the blind trials, making use of geological data and a Preferred model supplied to participants, showed apparent discrepancies between the Preferred and our BP1 model for the upper 40 m where a supplied PS log appears to be inconsistent with geological data and the blind BP1 model. At low frequencies 0.5–2.5 Hz dispersion data and the BP1 model suggest that use of the Rayleigh effective mode is superior to use of the fundamental mode in deducing the Vs model at depths below 100 m. The method of direct fitting of model and observed SPAC spectra used in MMSPAC also enabled the use of a bandwidth 0.5–38 Hz for interpretation, which is a wider bandwidth than that achieved by other participants for use of passive seismic data alone. [ABSTRACT FROM AUTHOR]

A major source of error in interferometric synthetic aperture radar (InSAR), used for mapping ground deformation, is the delay caused by changes in the propagation velocity of radar microwaves in the troposphere. Correcting this tropospheric delay noise using numerical weather models is common because of their global availability. Various correction methods and tools exist; selecting the most appropriate one by considering weather models, delay models, and delay calculation algorithms is essential for specific applications. We compared the performance of two tropospheric delay correction methods applied to Advanced Land Observing Satellite-2 (ALOS-2) data acquired over Japan, where the atmospheric field is complex with significant seasonal variation. We tested: (1) a method of delay integration along the slant radar line-of-sight (LOS) path using the mesoscale model (MSM) provided by the Japan Meteorological Agency and (2) the Generic Atmospheric Correction Online Service (GACOS) for InSAR, which estimates delay using the high-resolution forecast (HRES)-European Centre for Medium-Range Weather Forecasts (ECMWF) products along with an iterative decomposition approach. The results showed that the tropospheric delay correction using the slant-delay integration approach with MSM, which has a finer temporal and spatial resolution, performed slightly better than GACOS. We further found that the differences in the refractivity models would have limited significance, suggesting that the difference in performance mainly originates from differences in the numerical weather models being used. This study highlights the importance of using the best-available numerical weather model data for tropospheric delay calculations. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via the original article. [ABSTRACT FROM AUTHOR]

In subduction zones, slip deficit monitoring along the plate interface is important for understanding the seismogenesis of megathrust earthquakes. In the last two decades, aseismic slip transients, such as slow slip events (SSEs), which are usually synchronized with tectonic tremors, have been detected in subduction zones worldwide. Frequent SSEs are particularly important for releasing slip deficits during the inter-seismic periods of megathrust earthquakes. In southwest Japan, deep short-term SSEs have been primarily monitored with strain and tilt records because the SSEs in this region are small. However, strain and tilt records are so sensitive that they record not only SSEs, but also rainfall and local groundwater movements, which temporally affect the quality of data making it difficult to apply an automated detection algorithm. Therefore, previously reported short-term SSE catalogs, based on strain and tilt records, were created by visual inspections, although they are not suitable for generating a long-term catalog. In this study, a quantitative detection algorithm was developed to detect short-term SSEs using strain and tilt records. The problem of temporally varying data quality was solved by introducing the prior probability of log-normal distributions in the fitting variance. This method was applied to an 8-year (2013–2020) dataset of strains and tilts from southwest Japan. A total of 96 events were detected, among which, 78 corresponded with SSEs previously reported by the Geological Survey of Japan (GSJ). Although the GSJ catalog contained more events with smaller magnitudes, such events were difficult to distinguish from noise using the developed method. Three of the remaining 18 events were considered SSEs that were not reported in the GSJ catalog. Others could be artifacts because there were no obvious signals in the global navigation satellite system records (with events of magnitude > 6.0). Previous studies have suggested the existence of aseismic transients deeper or shallower than regular short-term SSEs in southwest Japan. However, detection results from this study did not confirm such events. [ABSTRACT FROM AUTHOR]

S-net is a large-scale ocean bottom (OB) network in the Japan Trench area, consisting of inline-type 150 observatories equipped with seismometers and pressure gauges. Among them, 41 observatories have been buried about one meter beneath the seafloors in the shallow water regions (water depth <1500 m). We analyzed the strong-motion data recorded at the S-net sites from earthquakes with magnitudes 3.5 < Mw ≤ 7 and focal depths < 70 km to understand the site amplification effect on the recorded motions. We used the spectral inversion technique and obtained some fundamental properties of the earthquake source spectra, path attenuation, and site factors from the horizontal-component S-wave portions of the recordings. We obtained that the source spectra followed the ω - 2 source model generally well, and the estimated magnitudes were mostly within ± 0.3 magnitude units of the catalog magnitudes. The stress drops increased systematically with the focal depths, and the values for the shallow earthquakes in the Pacific Plate were higher than those for the interplate earthquakes with comparable focal depths. The path-averaged quality factors were generally frequency-dependent and were somewhat larger than those in the past studies. The peak site frequencies ranged between about 0.2 and 10 Hz, while the peak amplification factors ranged between 10 and 50. Even though the peak frequencies and amplification factors differed from site to site, the peak frequencies were mostly higher than 2 Hz at the outer trench stations, while they were lower than 2 Hz at many inner trench stations. The amplification factors at a few OB sites in the shallow water regions were comparable with the theoretical ones computed from the 1-D subsea model. The amplification factors at intermediate frequencies (~ 0.3 to 2 Hz) generally increased with P-wave travel time in the sediments estimated from the multi-channel seismic survey. At about 20% of the sites (mainly at the unburied stations), spurious site spectra were recognized at frequencies higher than about 4 Hz. If the site spectra between 4 and 10 Hz are required, using only the X-component records is recommended. [ABSTRACT FROM AUTHOR]

The Izu–Bonin–Mariana Subduction System (IBM) is one of the longest subduction zones in the world with no instrumental history of shallow focus, great earthquakes (Mw > 8). Over the last 50 years, researchers have speculated on the reason for the absence of large magnitude, shallow seismicity on this plate interface, exploring factors from plate age to convergence rate. We approach the question from a different point of view: what if the IBM has hosted great earthquakes and no documentable evidence was left? To address the question of observability, we model expected tsunami wave heights from nine great earthquake scenarios on the IBM at selected locations around the Pacific Basin with an emphasis on locations having the possibility for a long, written record. Many circum-Pacific locations have extensive written records of tsunami run-up with some locations in Japan noting tsunami back to 684 CE. We find that most IBM source models should theoretically be observable at historically inhabited locations in the Pacific Basin. Surprisingly, however, some IBM source models for earthquakes with magnitudes as high as Mw 8.7 produce tsunami wave heights that would be essentially unobservable at most historically populated Pacific Basin locations. These scenarios aim to provide a constraint on the upper bound for earthquake magnitudes in the IBM over at least the past 400 years. [ABSTRACT FROM AUTHOR]

We propose a statistical analysis method to identify common features of seismic activity that are indistinguishable from most other seismicity, and to find anomalous activity that differs from these common features. Using the hypocenter catalog of earthquakes which occurs in and around Japan during the past 20 years, we apply this method for the parameters of the frequency-magnitude distribution and a parameter that expresses the correlation of seismicity with tides as indices, with a focus on objectively understanding the regional characteristics of seismicity. As a result, we extracted a "typical" probability density distribution of each index value common to the most analysis regions and "anomalous" regions with index-value distributions that differ significantly from the typical distributions. In terms of the frequency-magnitude distribution, most estimated values of indices in the anomalous activity areas can be explained as variations corresponding to the effects of fluids, interplate coupling, and stress fields that control faulting styles that have been pointed out in previous studies. By extracting typical index values for the frequency-magnitude distribution, common features of the frequency-magnitude distribution that depend on the earthquake occurrence interval were identified. Although seismicity showed no clear correlation with tides, the index value for tidal correlation changes to reflect the proportion of earthquakes occurring in a series of periods shorter than the tidal period; it is therefore useful as an index to capture the characteristics of such earthquake occurrence intervals. We also show that the typical probability density distribution of these index values can be represented by existing models or their extensions. By using the proposed models as a reference, it is possible to quantify the degree of anomaly using the same concept as that of the method presented here; hence, such a method should be applicable to monitor seismic activity. [ABSTRACT FROM AUTHOR]

Density perturbations accompanying seismic waves are expected to generate prompt gravity perturbations preceding the arrival of P-waves. Vallée et al. (Science 358:1164–1168, 2017, https://doi.org/10.1126/science.aao0746) reported the detection of such pre-P-wave signals in broadband seismograms during the 2011 Tohoku-oki earthquake. Kimura et al. (Earth Planets Space 71:27, 2019, https://doi.org/10.1186/s40623-019-1006-x) considered that their detection involved some uncertain points, including a concern regarding their signal processing procedure. Specifically, to remove the instrumental response, Vallée et al. (2017) applied acausal deconvolution to the seismograms truncated at the P-wave arrivals. Generally, acausal deconvolution produces artifacts at the edge of the time window. However, they did not present quantitative assessment whether the detected signals were artifacts due to the signal processing. To avoid this concern, Kimura et al. (2019) employed another procedure that eliminated acausal processes, resulting in the detection of a pre-P-wave signal with a statistical significance of 7σ in stacked broadband seismograms. Subsequently, Vallée et al. (Earth Planets Space 71:51, 2019, https://doi.org/10.1186/s40623-019-1030-x) commented that the procedure employed by Kimura et al. (2019) for the signal detection was inappropriate because it dismissed the low-frequency components of data. Although we admit the loss of low-frequency components in the data in Kimura et al. (2019), Vallée et al. (2019) have not yet provided a full account of the validity of their own procedure. Here, we assessed the validity of the procedure employed by Vallée et al. (2017) by quantitatively evaluating the magnitude of the acausal artifacts. First, we investigated how the input acceleration waveform, having an ideal signal-like shape, was distorted by their procedure. Their acausal deconvolution indeed generated a large-amplitude terminal artifact; however, it was removed by the causal band-pass filtering performed after the deconvolution and consequently became negligible. Next, we constrained the maximum amplitude of the artifact due to the noise in a seismogram and showed that it was sufficiently small compared to the reported signal amplitudes. These results suggest that the signal waveforms seen after their procedure were not artifacts but were representing the input acceleration with sufficient accuracy. Namely, their procedure well functions as a detection method for pre-P-wave signals. In the context of this validation, we replied to the comments of Vallée et al. (2019). [ABSTRACT FROM AUTHOR]

We carried out seismic tomography study to reveal three-dimensional (3D) seismic velocity structures in the Noto peninsula, Japan, where swarm-like seismic activity started in December 2020. The obtained results reveal a highly heterogeneous structure in the crust. The most striking feature is the existence of a low-velocity anomaly in the lower crust beneath the Noto earthquake swarm. Although the data set used in this study cannot resolve the upper mantle structure, previous regional tomographic studies suggest that a low-velocity anomaly exists at depths of 50–150 km around the Noto peninsula that is probably interpreted as a fluid-rich region. We infer that fluids have been supplied from the uppermost mantle to the lower crust over a geological time scale and a large volume of fluids have accumulated below the seismogenic zone beneath the Noto peninsula. A further upward migration of fluids to the upper crust, which may have suddenly started in December 2020, probably triggers numerous earthquakes at depths of 10–15 km. Since major active faults exist at shallower extensions of the hypocenters of the Noto earthquake swarm, we consider that the earthquake swarm occurs along pre-existing and weak fault planes. Dense temporary seismic observations will highlight a smaller-scale (5–10 km) 3D seismic velocity model and finer hypocenter distribution, which provide additional information for better understanding of the generation mechanisms of the Noto earthquake swarm. [ABSTRACT FROM AUTHOR]