The damping characteristic is an important parameter in the design calculation of the structure. In this study, the natural period and damping characteristics of the entire bridge system were calculated from the continuous behavior observation records of the entire bridge system using the observation records of weak earthquakes and the records of constant vibration. System identification based on the subspace method was used to calculate the natural period and damping characteristics. The purpose of this study was to develop a method for calculating the vibration characteristics of the entire bridge system, and verified the applicability of the subspace method for multipoint observation records of bridges. As a result, the average value of the mode attenuation of the entire bridge system of the target bridge in this study was evaluated as h = about 1.7% by system identification based on the subspace method.

 In recent years, reinforcements of railway embankments have been conducted for increasing the seismic resistance. Reinforcements of embankment body using soil nails are often adopted. For the prevention of seepage and erosion to rainfall, slope protection works are also constructed. The works can also contribute to the increase of seismic performance by keeping the embankment in unsaturated condition and connection with soil nails. However, these effects have not been studied clearly. In this paper, series of shaking table tests of model embankments with slope protection works and soil nails were conducted, with rainfall infiltration. From the series of shaking table tests, it was revealed that the seismic resistance of the embankment was increased by the slope protection works due to decreasing the degree of saturation in the embankment by covering the slope surface, and increasing the extension force of soil nails by connecting with the works.

 In this study, in order to accurately evaluate the Arias intensity used for damage prediction of infrastructures during an earthquake, we investigated the relationship between site amplification factors for Arias intensity and site effects. First, we derived the relational expression between Arias intensity, acceleration Fourier spectrum, and site effects. Next, we evaluated the site amplification factors and site effects of seismic observation sites in the Chugoku region, and the characteristics of their distribution was investigated. As a result of investigating the correlation between the amplification factor and the integrated value of the site amplification effect squared value, the integration range with a high correlation coefficient was clarified. Finally, the ground amplification factor prediction formula was obtained using the average S wave velocity of the surface layer 30 m and the information deeper than the engineering bedrock. As a result, it was clarified that the prediction accuracy is higher when the information deeper than the engineering bedrock is used.

 The interaction between the structure and surrounding ground should be appropriately evaluated in the seismic design of cut and cover tunnels. In particular, the shear stiffness ratio (G_g / G_s) between the structure and the surrounding ground is a crucial parameter to evaluate the shear strain ratio (shear strain of the structure γ_s / shear strain of the ground γ_g). However, the effect of the nonlinearity of ground on these relationships has not been clarified. In this study, a model experiment using a small shear tank was conducted using an aluminum rod laminated model ground, which can simulate the nonlinearity of the ground. There are three experimental cases with different structural stiffness. The results are compared with elastic FEM analysis. It is confirmed that the effect of nonlinearity of the ground on the shear strain ratio during an earthquake can be evaluated by modeling the ground with equivalent linear stiffness under the condition of a large shear stiffness ratio.

 Due to decreased socio-economic activities in response to the Novel Coronavirus Disease (COVID-19) pandemic in 2020, amplitude reductions of high-frequency (>1 Hz) microtremors (seismic ambient noise) had been reported at some seismic stations in areas around Tokyo by some researchers. This study investigated the existence of amplitude decreases of the microtremor records obtained at each of 169 MeSO-net (Metropolitan Seismic Observation network) stations in and around Tokyo using continuous seismic data from January 2019 to December 2020. We compared the variations of microtremor spectral characteristics with the number of populations, the traffic volume around each station, and the number of students in schools near the location of the seismic stations before and during the pandemic. We found that (1) amplitudes of microtremor between 20 and 90 Hz decreased from early March 2020 at many MeSO-net stations regardless of the number of population, (2) amplitude reductions below 10 Hz were also observed during the first state of emergency (April 7–May 25, 2020), (3) amplitude reductions during the first state of emergency were more significant (60–80 % of 2019 levels) at stations installed in schools with large numbers of students, and (4) there were no significant amplitude reductions at seismic stations in deep boreholes (>1900 m). These observations indicate that the microtremor amplitude reductions in 2020 were mainly affected by local phenomena where seismic sensors were installed.

 An intra-slab earthquake with a magnitude of 7.3 that struck in off the coast of Fukushima on February 13, 2021 caused damage to slopes and houses. This article investigates the generation mechanism of this earthquake and the damage to the slopes based on the strong motion data recorded by the National Research Institute for Earth Science and Disaster Resilience (NIED) and the dataset from previous studies. Analyses of the data indicated that the peak ground acceleration of the earthquake showed larger value than the interplate earthquakes. In addition, microscopic observation and energy dispersive X-ray analysis of samples from the collapsed road slope near Matsukawaura bridge revealed that microorganism such as diatoms and green algae might induce the rock collapse.

 The 2018 Hokkaido Eastern Iburi earthquake caused a large number of slope failures and casualties in Atsuma town and its surrounding areas. In previous studies, several prediction methods for slope failures have been proposed, and the prediction accuracy has been discussed. However, the prediction models suitable for extensive areas are not solidly developed. In view of the current situation, this study uses machine learning to predict the slope failures during the 2018 Hokkaido Eastern Iburi earthquake. In addition, the influence of NDVI of satellite image on the prediction results of machine learning model is discussed.

 Gunma Prefecture has many mountainous regions and there is an active fault called Fukaya Fault Zone in the south. The slope failure caused by heavy rainfall led to road closure and isolation in Kanna Town during Typhoon No. 19 in 2019. Regional isolation is likely to recur due to road closure due to slope failure caused by strong earthquake as well. In addition, road closure due to bridge damage caused by earthquake is also considered as a cause of isolation. This research will evaluate the risk of isolation caused by scenario earthquake in Tano Country, including Kanna Town, by considering road closure due to slope failure and bridge damage. As result, it was confirmed that the risk of isolation in Tano Country is very high. In addition, the probability of slope failure and road closure due to slope failure caused by earthquake showed the same trend as the damage caused by Typhoon No. 19 in 2019.

 A maximum instrumental seismic intensity of 6+ was observed in near source area during the 2019 off the Yamagata Prefecture earthquake, Mj 6.7. The Koiwagawa area of Yamagata Prefecture located in near source area, was one of the areas most affected by this earthquake. In the Koiwagawa area, about one third of the wooden houses suffered damage to their roofs. In this study, we investigated the surface layer S-wave velocity structures of this area by surface wave survey. The results show that shallow subsurface layers with low shear wave velocity are distributed in a thickness of 3 m to 10 m. The thicker these layers are, the higher the damage rate of the roof tiles.

 In recent years, some attempts have been made to conduct three dimensional (3D) site response analyses considering relatively wide scale of area such as block scale and city scale for the purpose of estimating earthquake damage. In this study, we conducted city scale 3D soil liquefaction analysis for a city located in an alluvial plain in Kanto area Japan with the aim of confirming feasibility of practical procedure of a city scale liquefaction analysis. As a result, it is shown that i) the tendency of the calculated natural frequencies of subsurface in the area seems to be consistent with the measured, ii) the 3D soil liquefaction analysis can obtain similar acceleration response during liquefaction with the 2D soil liquefaction analysis at the place where the 2D plane strain condition can be assumed. A series of results in this study is thought to almost demonstrate the validity of 3D soil liquefaction analysis in terms of acceleration responses and build-up of excess pore water pressures.

 In accordance with the Law Concerning Special Measures against Large-Scale Earthquakes, Shizuoka Prefecture had been implementing disaster prevention measures based on the premise of predicting the occurrence of the assumed Tokai earthquake in advance. Since the 2011 off the Great East Japan Earthquake, the assumed earthquake of the largest class has become the Nankai Trough Earthquake, and the policy has been changed from the premise of prior prediction to the sudden response. And, the new information which was named Nankai Trough Earthquake Extra Information was introduced. The last 10 year of disaster measures have changed significantly from the previous 30 years. Therefore, we conducted a questionnaire survey for the purpose of grasping the residents awareness of the assumed tsunami situation and the tsunami evacuation. The results of the survey showed that the local residents were aware of the early start time of tsunami evacuation and the evacuation action by walking. It was also shown that there was not enough understanding of the assumed tsunami situation for evacuation and the specific measures for those who need assistance to evacuation.

 This study extended the existing earthquake–tsunami multihazard interaction diagram by considering foundation uplift as a limit state. Taking up a three-story building supported through a mat foundation as a model, 30 ground motions recorded in the coastal area of three prefectures in Tohoku, Japan during the 2011 Tohoku earthquake and the tsunami hydrodynamic force were applied by varying their magnitude to put together the diagram. The obtained earthquake–tsunami interaction diagram illustrates that after the earthquake, the remaining resistance differs depending on the different levels of foundation uplift during the earthquake.

 This note makes an overall summary of the major points of modeling and analysis performance of several seismic analysis models for underground vertical shaft structures that the author has published over the past few years. In the previous reports, single analysis model was taken up each time, and discussions were conducted by demonstrating practical calculation examples. As a result, usable numerical data for five types of analysis models have been accumulated.

 In this note, using them, a simultaneous analysis dealing with the same example is performed, and summarized the characteristics of each model by comparing the calculation results. The five types are three types of 3D-FEM technique that integrate the ground and underground structure together: solid element model, shell element model, and beam element model, and two types of seismic deformation method in which the structure is separated from the ground: conventional beam-and-spring model, and the improved model that is an improvement of it.

 During the Great East Japan Earthquake of 2011, approximately 27,000 detached houses were damaged by settlement and tilting due to liquefaction of the ground. To reduce similar liquefaction disasters in the future, it is urgent to develop and deployment of new liquefaction countermeasure for detached houses. In this study, we propose a new liquefaction countermeasure to suppress the settlement and tilting of detached houses that can satisfy the conditions of “narrow construction space” and “construction cost.” The concept of this liquefaction countermeasure is to compensate for the reduced bearing capacity of a spread foundation when the bearing layer liquefies using the axial force of the pile and tension of the wire. In this thesis, the mechanism of the countermeasure effect of this method is discussed using 1/25 scale 1G shaking table tests under the experimental conditions of pile arrangement and wire arrangement between piles. From a series of shaking table tests, it was observed that the piles were placed in pairs near the corners of the building model, and the horizontal displacement of the pile heads and pile tips was constrained, which significantly reduced the amount of settlement.

 The importance of a structural system with high robustness against seismic forces with high uncertainty has been shown, and such a structural system is required. Efforts are being made to realize a highly robust bridge structure by combining seismic isolation bearings and seismic dampers. In this study, we investigated the influence of characteristics of seismic isolation bearings and seismic dampers on the robustness of bridge systems using seismic isolation bearings and seismic dampers. It was found that characteristics of seismic isolation bearings and seismic dampers have a great influence on the coefficient of variation of the maximum response displacement of a pier and maximum plasticity rate.

 A supplementary formulation was added to the author’s previous study regarding the differentiability of ground motion phase. An intuitive explanation was also added. As a result, it was clarified that ground motion phase is differentiable with respect to angular frequency and group delay time can be defined as long as the Fourier amplitude is nonzero. Also, to cope with numerical difficulty in calculating group delay time, the author devised a new, numerically stable method which can calculate both the mean and standard deviation of the group delay time.