Kumamoto Earthquake Research Articles

Development of a New Method for Debris Flow Runout Assessment in 0-Order Catchments: A Case Study of the Otoishi River Basin

Debris flows are rapid, destructive landslides that pose significant risks in mountainous regions. This study presents a novel algorithm to simulate debris flow dynamics, focusing on sediment transport from 0-order basins to depositional zones. The algorithm integrates the D8 flow direction method with an adjustable friction coefficient to enhance the accuracy of debris flow trajectory and deposition modeling. Its performance was evaluated on three real-world cases in the Otoishi River basin, affected by rainfall-induced debris flows in July 2017, and the Aso Bridge landslide triggered by the 2016 Kumamoto Earthquake. By utilizing diverse friction coefficients, the study effectively captured variations in debris flow behavior, transitioning from fluid-like to more viscous states. Simulation results demonstrated a precision of 88.9% in predicting debris flow paths and deposition areas, emphasizing the pivotal role of the friction coefficient in regulating mass movement dynamics. Additionally, Monte Carlo (MC) simulations enhanced the identification of critical slip surfaces within 0-order basins, increasing the accuracy of debris flow source detection. This research offers valuable insights into debris flow hazards and risk mitigation strategies. The algorithm’s proven effectiveness in simulating real-world scenarios highlights its potential for integration into disaster risk assessment and prevention frameworks. By providing a reliable tool for hazard identification and prediction, this study supports proactive disaster management and aligns with the goals of sustainable development in regions prone to debris flow disasters.

Dynamically triggered seismicity in Japan following the 2024 Mw7.5 Noto earthquake

Abstract On January 1st, 2024, a moment magnitude (M w ) 7.5 earthquake occurred on an active reverse fault in the northern part of Noto Peninsula, being one of the largest intraplate events recorded in Japan. In previous studies, the dynamic triggering of seismicity in Japan following some large remote earthquakes has been well documented, such as in the case of the 2011 M w 9.0 Tohoku–Oki earthquake, the 2016 M w 7.1 Kumamoto earthquake, and other large teleseismic events. In this study, we investigate the remote triggering of microearthquakes by the 2024 Noto earthquake and their characteristics. We analyze waveform data recorded at high-sensitivity seismic stations in Japan, before and after the occurrence of the Noto mainshock. Local earthquakes are detected on high-pass filtered three-component seismograms. Low-pass filtered waveforms are used for visualizing the mainshock surface waves and estimating dynamic stresses. Our results show a relatively widespread activation of small earthquakes—none of them listed in the Japan Meteorological Agency (JMA) earthquake catalog—that were triggered by the passage of the mainshock surface waves in many regions of Japan. These include Hokkaido and Tohoku in northeastern Japan, Kanto in central Japan, and Kyushu in southern Japan. The triggering is mostly observed in volcanic regions, supporting the hypothesis that such places are relatively easy to be activated dynamically, likely due to the excitation of fluids by the passage of mainshock surface waves. The calculated dynamic stress changes estimated from peak ground velocities, which triggered the earthquakes after the Noto mainshock, are in the range 12.8–102.6 kPa. We also report potential, less well-constrained dynamic triggering by the Mw 5.3 Noto foreshock, which occurred ~ 4 min before the mainshock, at levels of stress about 100 times smaller. The analysis of a longer-term (1 month) seismicity pattern, based on the JMA catalog, revealed a statistically significant increase of seismicity in the remote Akita–Yakeyama (Tohoku region) volcanic area, following the Noto earthquake. Graphical Abstract

Insight into groundwater quality change before and after the 2016 Kumamoto earthquake

The Kumamoto Earthquake is named after the Kumamoto Prefecture where the series of earthquakes occurred in the city located on the island of Kyushu, in southwest Japan.In this area, 100 % of the population depends on groundwater for drinking water; hence, groundwater is an extremely important lifeline for people. The effect of earthquakes on the water quality of groundwater has been suggested based on monitoring data measured by public agencies once or twice a year. Frequent water quality monitoring of the groundwater in the Kumamoto area has continued once a month for six years since 2014. In this study, we aimed to statistically evaluate the changes in groundwater quality caused by the 2016 earthquake and provide further insights into its mechanisms. The Stiff diagrams using major ionic components showed no apparent overall water quality change before and after the earthquake for 14 groundwater samples in the area. However, statistically significant changes were indicated for 11 samples by k-medoids clustering (non-hierarchical cluster analysis), divided into two clusters before and after the earthquake. In addition, the samples showing water quality changes did not vary even over the four years after the earthquake, indicating that the change was not temporal. The magnitude and trend of the change in individual water quality parameters depended on the sampling sites, while some parameters, such as nitrate, showed seasonal fluctuations due to precipitation conditions more than the effect of the earthquake. No particular relationship was found between the distance from the sampling sites to the epicenters or active faults and the degree of water quality changes. Finally, it was considered that the aquifer breaching/fluid mixing model (AB/FM model), through the change in the bleeding channel of water due to seismic shaking, most likely explains the main mechanism of water quality change in groundwater in the Kumamoto area.

Kumamoto Earthquake NST activity report: food problems in evacuation shelters in comparison with convalescent facilities

Kumamoto Earthquake NST activity report: food problems in evacuation shelters in comparison with convalescent facilities

Constraint on the background stress in the source region of the 2016 Kumamoto earthquake sequence based on temporal changes in elastic strain energies and coseismic stress rotation

SUMMARY We investigated the deviatoric stress magnitude of the background stress fields before the 2016 Kumamoto earthquake sequence in Japan based on temporal changes in elastic strain energies caused by the mainshock and coseismic stress rotation. We modelled the six components of background stress fields from stress orientations together with the parameter of effective friction coefficients (μ’ = 0.3, 0.15, 0.1, 0.05 and 0.03) using the 3-D Mohr diagram. We computed the absolute stress fields immediately following the primary events (the largest foreshock and mainshock) of the earthquake sequence by combining coseismic stress change fields with background stress fields. The total amount of elastic strain energy released by the mainshock increased with the effective friction coefficient. Considering the energy balance in which some part of the released energy must be consumed as radiated energy, we understood that the model with μ’ = 0.03 was unrealistic. We also examined the dependence of coseismic stress rotation on the effective friction coefficient. Furthermore, we applied a stress inversion method to moment tensor data of earthquakes to directly estimate coseismic stress rotation. Comparing the theoretical and estimated coseismic stress rotations, we concluded that the models with μ’ = 0.3 and 0.15 were more consistent with the observations than those with μ’ < 0.1. In the reasonable models, the deviatoric stress magnitudes were 37–65 and 39−70 MPa at a depth of 10 km on the northern and southern source faults, respectively.

Unwrap Intractable C‐Band Coseismic Interferograms: An Improved SNAPHU Method With Range Offset Gradients as Prior Information

AbstractC‐band Interferometric Synthetic Aperture Radar (InSAR) data are widely used to map coseismic deformation. However, phase unwrapping errors are commonly distributed near faults owing to decorrelation and steep phase gradients from short radar wavelengths. Here, we propose an improved SNAPHU phase‐unwrapping algorithm that considers the prior information of the range offset gradients (P‐SNAPHU) to overcome the constraints imposed by the phase continuity assumption. P‐SNAPHU exploits median filtering of homogeneous pixels for initial denoising of range offset and then refines range offset gradients divisionally through saliency extraction. The derived gradients are used to estimate the expected values of the cost functions for the low‐coherence fringes. The synthetic experiments show significant improvements in the phase‐unwrapping accuracy of the P‐SNAPHU method compared with classical unwrapping methods. We apply the P‐SNAPHU method to unwrap Sentinel‐1 coseismic interferograms of three large (Mw > 6.5) strike‐slip earthquake events that the existing classical methods could not successfully unwrap. In the comparison of the unwrapped interferograms with the external global navigation satellite system (GNSS) displacements and those from the classical methods, we find that P‐SNAPHU significantly reduces the phase unwrapping errors with the mean absolute error of 7.2, 2.3, and 1.8 cm for the 2023 Kahramanmaraş earthquake doublet, the 2016 Kumamoto earthquake, and the 2019 Ridgecrest earthquakes, respectively. Based on the unwrapped results derived from P‐SNAPHU, an estimate is made regarding the shallow slip deficit of the 2023 Kahramanmaraş earthquake doublet, which is approximately 7%. Therefore, P‐SNAPHU is useful for developing and applying C‐band InSAR data for large earthquakes and volcanic activity.

Seismic design parameters of double-layer diamatic domes

The effect of past earthquakes (e.g., the 1995 Kobe and 2016 Kumamoto earthquakes) on the space structures revealed the fact that the assumption of the invulnerability of these structures is incorrect despite their light self-weight and high degree of redundancy, and space structures may suffer severe damage during strong ground motions. Therefore, special attention should be paid to the design of this type of structure in high seismic-risk regions. Nevertheless, the existing seismic design codes have not provided the seismic design parameters (i.e., the seismic response modification factors), including the behavior factor, R, and the displacement amplification factor, Cd for space structures. This research aims to extract the seismic design parameters of double-layer diamatic domes (DLDDs) to answer the challenges that structural engineers face in the design of this type of structure. For this purpose, 16 DLDD models with different rise-to-span ratios (RSRs) and span lengths were considered. After designing the models and performing the nonlinear time history analysis using the simultaneous effect of the three transitional components of ground motions, the target displacement at the control node was determined. Then, the pushover analysis was carried out to derive the capacity curve of the structures and calculate the seismic response modification factors in both horizontal and vertical directions. The results indicate that the domes with RSRs of 1/7 and 1/5 should be designed such that they remain in the elastic state in the horizontal direction. Then, with the growth of RSR, the value of R increases and reaches 1.792. Finally, equations were presented in this study for the period, behavior factor, and displacement amplification factor of DLDD structures in both horizontal and vertical directions using the nonlinear regression analysis.

Long-Term Impact of the Kumamoto Earthquake on Out-of-Hospital Cardiac Arrest With Cardiac and Non-Cardiac Origins - An Interrupted Time Series Analysis.

Possible etiologies of out-of-hospital cardiac arrest (OHCA), including aortic dissection, ruptured aortic aneurysms, and pulmonary embolism, may be classified as non-cardiac causes. We investigated whether cardiac and non-cardiac OHCAs increased following the Kumamoto earthquake and whether the impact on OHCAs extended to regions far from the epicenter. We prospectively analyzed a nationwide registry of patients who experienced OHCAs between January 2013 and December 2019. Data from cases registered in 7 prefectures, including Kumamoto (Kyushu region; n=82,060), in the All-Japan Utstein Registry were analyzed for OHCAs of cardiac and non-cardiac origin. The numbers of OHCAs before and after the Kumamoto earthquake were compared using an interrupted time series analysis. The incidence of both cardiac (rate ratio [RR] 1.22) and non-cardiac (RR 1.27) OHCAs in Kumamoto Prefecture increased after the earthquake. The difference disappeared when the analysis was limited to patients with non-cardiac OHCAs with a clear cause of cardiac arrest. The number of cardiac and non-cardiac OHCAs did not increase in other prefectures within the Kyushu region. The Kumamoto earthquake led to an increase in the incidence of cardiac and non-cardiac OHCAs. However, this was attenuated by increasing distance from the epicenter. Except for cardiac causes, cases complicated by earthquake-related events may include non-cardiac OHCAs due to vascular diseases that might be overlooked.

Temporal variations of the ‘in-situ’ nonlinear behaviour of shallow sediments during the 2016 Kumamoto Earthquake sequence

SUMMARY Strong ground shaking has the potential to generate significant dynamic strains in shallow materials such as soils and sediments, thereby inducing nonlinear site response resulting in changes in near-surface materials. The nonlinear behaviour of these materials can be characterized by an increase in wave attenuation and a decrease in the resonant frequency of the soil; these effects are attributed to increased material damping and decreased seismic wave propagation velocity, respectively. This study investigates the ‘in-situ’ seismic velocity changes and the predominant ground motion frequency evolution during the 2016 Kumamoto earthquake sequence. This sequence includes two foreshocks (Mw 6 and Mw 6.2) followed by a mainshock (Mw 7.2) that occurred 24 hr after the last foreshock. We present the results of the seismic velocity evolution during these earthquakes for seismological records collected by the KiK-net (32 stations) and K-NET (88 stations) networks between 2002 and 2020. We analyse the impulse response and autocorrelation functions to investigate the nonlinear response in near-surface materials. By comparing the results of the impulse response and autocorrelation functions, we observe that a nonlinear response occurs in near-surface materials. We then quantify the velocity reductions that occur before, during, and after the mainshock using both approaches. This allows us to estimate the ‘in-situ’ shear modulus reduction for different site classes based on V$_{S30}$ values (V$_{S30}\lt 360$ m s−1, $360\lt $V$_{S30}\lt 760$ m s−1 and V$_{S30}\gt 760$ m s−1). We also establish the relationships between velocity changes, shear modulus reduction, variations in predominant ground motion frequencies and site characteristics (V$_{S30}$). The results of this analysis can be applied to site-specific ground motion modelling, site response analysis and the incorporation of nonlinear site terms into ground motion models.

Methodology for estimating regional production capacity loss rate in industrial sectors caused by disasters: A case study of the 2016 Kumamoto earthquakes

The estimation of regional Production Capacity Loss Rate (PCLR) in industrial sectors after disasters is critical for disaster mitigation, resource allocation and restoration. Recent studies to estimate the PCLR in industrial sectors were based on hypothetical disaster survey data for resistance analysis and the linear regression model for recovery analysis, which may cause non-negligible bias. Moreover, these models did not incorporate the impact of lifeline disruption and restoration, thus further amplifying the estimation bias. To fill this gap, this study proposes a comprehensive regional PCLR estimation framework that incorporates the impacts of lifeline disruptions into resistance analysis and lifeline restoration into recovery analysis to improve the PCLR estimation accuracy. Specifically, this study utilizes lifeline resilience factors (LRFs) to quantify the impact of lifeline disruption on the production capacity rate immediately after the earthquake, and apply a semi-Markov recovery function to model the recovery rates considering the initial damage and lifeline availability influences. Then, the proposed methodology is applied to a case study of 2016 Kumamoto earthquakes to verify its effectiveness and accuracy. In this case study. Results shows that the non-manufacturing sector is more resistant than the manufacturing sector, yet lower recovery ability, ultimately resulting in greater production losses. The Mean Square Errors between the estimated PCLR and the official statistical of Kumamoto prefecture's Index of Industrial Production in each industry are less than 0.05, proves the validity of the proposed model and indicates the improved accuracy of loss estimation compared to previous studies. In addition, comparison results indicate improved accuracy of loss estimation compared to previous studies.

Post-earthquake building services downtime distribution: a case study of the 2016 Kumamoto, Japan, earthquake

Seismic damage to building services systems, that is, mechanical, electrical, and plumbing systems in buildings related to energy and indoor environments, affects the functionality of buildings. Assessing post-earthquake functionality is useful for enhancing the seismic resilience of buildings via improved design. Such assessments require a model for predicting the time required to restore building services. This study analyzes the downtime data for 250 instances of damage to building services components caused by the 2016 Kumamoto earthquake in Japan, presumably obtained from buildings with minor or no structural damage. The objectives of this study are (1) to determine the empirical downtime distribution of building services components and (2) to assess the dependence of the downtime on explanatory variables. A survival analysis, which is a statistical technique for analyzing time-to-event data, reveals that (1) the median downtime of building services components was 90 days and, 7 months after the earthquake, the empirical non-restoration probability was approximately 32%, (2) the services type and the building use are explanatory variables having a statistically significant effect on the downtime of building services components, (3) the log-logistic regression model reasonably captures the trend of the restoration of building services components, (4) medical and welfare facilities and hotels restored building services components relatively quickly, and (5) the 7-month restoration probability was observed to be highest for electrical systems, followed by sanitary systems, then heating, ventilation, and air conditioning systems, and finally life safety systems. These results provide useful information to support the resilience-based seismic design of buildings.

Analyzing building damage from the Kumamoto earthquake using Sentinel-1 data: impact of different acquisition conditions

Analyzing building damage from the Kumamoto earthquake using Sentinel-1 data: impact of different acquisition conditions

Stress change in southwest Japan due to the 1944–1946 Nankai megathrust rupture sequence based on a 3-D heterogeneous rheological model

The Nankai trough has repeatedly experienced large megathrust earthquakes at intervals of 100–200 years. The inland region of southwest (SW) Japan has a seismically active period from 50 years before to 10 years after megathrust earthquakes. To assess the activities of inland earthquakes after megathrust earthquakes, we need to quantitatively evaluate the postseismic stress accumulation on the inland source faults considering plausible viscoelastic relaxation. Recent studies have shown the importance of low-viscosity layers along the lithosphere-asthenosphere boundary (LAB layer) in postseismic deformation. In the present study, we focus on the most recent ruptures, the 1944 M7.9 Tonankai and the 1946 M8.0 Nankai earthquakes, estimating the 4-year stress change on the source faults in SW Japan in a forward modeling approach. The 1944–1946 megathrust rupture sequence was followed by severe ~ M7 inland earthquakes, such as the 1945 M6.8 Mikawa and 1948 M7.1 Fukui earthquakes. For stress calculation, we used a highly detailed finite element model incorporating the actual topography and the plausible viscoelastic underground structure from past studies. The calculated inland stress field shows the dominance of the coseismic change during the 1944 and 1946 earthquakes and little contribution from viscoelastic relaxation. In contrast, viscoelastic relaxation has a significant effect on stress in the slab, indicating the importance of quantifying the viscosity of the LAB layer. Based on the calculated stress, we evaluated the change in the Coulomb failure stress (ΔCFS) on each source fault. The ΔCFS is generally positive on the strike-slip faults east of 135°E due to the 1944 rupture. In contrast, the ΔCFS on the faults west of 135°E, including the Median Tectonic Line segments, became positive due to the 1946 rupture. The occurrence of the damaging earthquakes in 1945 and 1948 can be explained by the calculated ΔCFS. The ΔCFS on the recent earthquake faults of the recent damaging earthquakes such as the 2016 M7.2 Kumamoto earthquake is generally negative, suggesting the delay in stress accumulation. The ΔCFS on the source faults of the intra-slab earthquakes differ significantly as large as tens of kilopascals depending on the viscosity of the LAB layer.Graphical

A digital twin prototype to visualize heterogeneous seismic damage simulation results on web-GIS

ABSTRACT Access to a wide range of information tailored to specific purposes, including extensive damage projections and statistics for target areas and the potential for individual building and dwelling damage predicted through simulations, enhances cities’ resilience against large-scale earthquakes. Recognizing the significance of this, we have developed a prototype of a digital twin capable of comprehensively visualizing various simulations on a web-based geographic information system (GIS) platform. This prototype employs three seismic damage simulators: the Integrated Earthquake Simulation for area damage, the wallstat for wooden houses, and the E-Simulator for a reinforced concrete building. A trial of earthquake damage simulations was conducted to assess this prototype’s efficacy, focusing on the damage incurred in Mashiki Town during the 2016 Kumamoto earthquake. The trial successfully verified the capability of the system to visualize and integrate various types of simulation data alongside records obtained from actual earthquake disasters. This paper provides an overview of the digital twin prototype. It reports on the trial’s results, focusing on the visualization and integration achieved through this trial.

Radon concentration in seawater as a geochemical indicator of submarine fault activity in the Yatsushiro Sea, Japan

This study examined the relationship between radon (222Rn) concentrations in seawater and crustal activity in the Yatsushiro Sea by investigating the submarine fault zone situated at the southern end of the Futagawa–Hinagu fault zone, activated by the 2016 Kumamoto earthquake (M7.3). We conducted an analysis of 222Rn concentration in samples of bottom water just above the seafloor and pore water in sediments, utilizing multiple and piston cores from the Hakuho Maru Expedition KH18-3. The findings revealed significantly elevated 222Rn concentrations in the central sites of the Yatsushiro Sea, coinciding with a high-stress field exhibiting dense active faults. Seismicity analysis revealed heightened moment release and a low b-value post the 2016 Kumamoto earthquake, indicative of increased seismic activity and the potential for substantial earthquakes in the Yatsushiro Sea vicinity. Our results indicate that heightened concentrations of 222Rn in seawater can serve as an effective tracer for identifying and estimating submarine fault activities. Moreover, our research highlights the utility of 222Rn concentrations in detecting active submarine faults and assessing their activity. It contributes to a comprehensive understanding of the potential for significant earthquakes in the Yatsushiro Sea in the future.

Integrated Investigation on Heterogeneous Lower Crust Rheology in Kyushu and Afterslip Behavior Following the 2016 Mw7.1 Kumamoto Earthquake

AbstractThe viscoelastic lower crust beneath Kyushu Island, influenced by the volcanic arc, interplays with active crustal faults in this region and helps to shape local tectonics. In this study, we employed a three‐dimensional viscoelastic finite element model to gain insights into the lithospheric rheology and crustal faulting kinematics, through modeling the postseismic deformation processes of the 2016 Mw 7.1 Kumamoto earthquake. Our model reveals a viscosity of 2 × 1020 Pa s for the lower crust and 2 × 1019 Pa s for the upper mantle. A reduced lower crust viscosity of 2 × 1019 Pa s in the volcanic arc area is required for better reproducing the Global Positioning System data. The stress‐driven afterslip decays rapidly over time and is up to 0.3 m within 5 years after the earthquake. We propose additional normal‐component afterslip to better explain the complex postseismic deformation in the near field, which may be due to the interaction between the fault and volcano Aso.

Dynamic Triggering of Earthquakes in Northeast Japan before and after the 2011 M 9.0 Tohoku-Oki Earthquake

ABSTRACT Previous studies documented a relative scarcity of remote dynamic triggering of earthquakes in Japan and suggested that it could be related to Japan’s predominantly compressive tectonic regime or the more frequent occurrence of large earthquakes in Japan. For example, remote triggering in California, characterized by extensional tectonics, occurs at levels of stress change as small as 0.1 kPa, whereas in Japan, transient stresses ≥30 kPa are required. However, the dynamic triggering threshold in Japan, following the 2016 Mw 7.0 Kumamoto earthquake, has been found to be of just a few kilopascals, significantly smaller than reported previously. It was proposed that a decrease in the triggering threshold may have taken place in Japan, in particular at volcanic and geothermal areas, after the 2011 Mw 9.0 Tohoku-Oki earthquake. In this study, we analyze the possible change in dynamic triggering conditions in five areas in northeast Japan, where swarm earthquakes have occurred immediately after the Tohoku-Oki earthquake. The triggering thresholds in these five areas have been estimated based on the analysis of waveform recordings of 49 teleseismic earthquakes that occurred between 2004 and 2020. A decrease of the triggering threshold (or triggering ability) is apparent in all but one region. However, a statistical significance Kolmogorov–Smirnov test could not reject, at a 5% level, the null hypothesis stating that “the distribution of dynamic stress changes for triggering earthquakes that occurred before and after the Tohoku-Oki event is the same.” We interpret the changes in the triggering threshold to be related to the pore pressure increase (and thus a fault strength decrease) in the crust following the Tohoku-Oki earthquake. Our results also indicate that dynamic triggering in Japan is more common than reported previously.

Strengthening damaged columns on the soft first story of reinforced concrete building using ultra-high-strength fiber-reinforced concrete panels

In this study, ultra-high-strength fiber-reinforced concrete (UFC) panels were used as a quick and effective measure for seismic strengthening of damaged reinforced concrete (RC) columns. Four 1/3-scale specimens, which replicated RC columns of the soft-first-story of a 10-story condominium building that was heavily damaged in the 2016 Kumamoto Earthquake, were constructed and tested. The specimens were strengthened using UFC panels after being preloaded to the drift ratio, at which the maximum load capacity of the target column was measured, and then subjected to the main loading until the ultimate state was reached. The UFC panels were installed on the two faces of the column in a direction parallel to the assumed loading direction. Two of the specimens had a UFC or RC wing wall attached to one side of the column, which was also aligned with the assumed loading direction. During the preloading and main loading, the specimens were subjected to a cyclic lateral load and varying axial load that simulated an earthquake load. The proposed method improved the maximum strength and ultimate drift ratio, and helped restore the initial stiffness of specimens with UFC panels and a wing wall to that of a column specimen during preloading. The test results of a previous study, wherein the target column, loading method, and strengthening method were the same but the specimens were not damaged before strengthening, were compared to study the impact of the damage on the RC column before strengthening.

Application of the on-site P-wave earthquake early warning method based on site-specific ratios of S-waves to P-waves to the 2016 Kumamoto earthquake sequence, Japan

The on-site P-wave earthquake early warning (EEW) based on the site-specific ratios of S-waves to P-waves has been applied to large-sized offshore earthquakes, and the efficiency of the method has been validated. However, the method requires the P-waves including earthquake ground motions radiated from a large slip area while avoiding the inclusion of S-waves. In this study, we investigated the applicability of the on-site P-wave EEW method for ground motions near an earthquake source fault region, using strong-motion data observed during the 2016 Kumamoto earthquake sequence in Japan. At first, we examined the appropriate time-window length following the arrival of the P-waves. As a result, P-waves with a time-window length of 2.56 s after the arrival at most strong-motion stations were required at least to predict appropriately S-waves for the 2016 Kumamoto earthquake sequence, including the large-sized earthquakes. On the other hand, in the case of the large-sized earthquake as the mainshock (Mj 7.3), the method can predict within a brief time of 0.5 to 2 s in the operational use that strong ground motions exceeding a certain threshold (e.g., acceleration of 150 cm/s2) will come. Moreover, we found that the method was not strongly affected by the non-linearity of soil deposits due to strong ground motions during the 2016 Kumamoto earthquake sequence. The variability of the relationship between P- and S-waves at the seismic bedrock influenced by the source and path effects is larger than the variability of the relationships between P-/S-waves at the seismic bedrock and at the ground surface by the site effects, and therefore, it hides the effect of the non-linearity of soil deposits.Graphical

Deep-Seated Fluids in Thermal Waters Before and After the 2016 Kumamoto Earthquakes.

Noble gases, oxygen-hydrogen isotope ratios, and ion compositions were measured at three sampling points (KUM, OTN, and ASO) from December 2013 to July 2021. The 3He/4He values at the three sampling points remained stable in the range of 3-4 Ra throughout the observation period, suggesting that the supply of deep-seated gases to the aquifer was stable. The 4He/20Ne values of KUM and OTN indicate that the supply of surface-source fluids to the aquifer decreased relative to that of deep-seated fluids at KUM and OTN. In contrast, in the ASO site, both the surface- and deep-seated fluids supplied to the aquifer were stable. The δD-δ18O relationship indicated the supply of deep-seated water to the KUM and OTN aquifers but not to the ASO aquifer. Nevertheless, the δD-δ18O relationship remained stable throughout the observation period, suggesting that the supply of deep-seated water to the three stations was stable. The Li/Cl and 1/Cl relationships for the three sampling points were plotted within a narrow range throughout the observation period, suggesting that the groundwater recharge was stable. Neither spikes nor step changes owing to the 2016 Kumamoto earthquake were observed in any of the data. These results indicate that the KUM and OTN aquifers are constantly supplied with deep fluids from the fluid-rich zone beneath the Kumamoto region, and that only deep-seated gas was supplied to the ASO aquifer. We also confirmed that these supply conditions were unaffected by the 2016 Kumamoto earthquake or the subsequent aftershock activity.