Takashi Hirai

Development of coastal areas in Japan for various land uses since the 1960s has contributed to industrial upgrades and improved the efficiency of transportation networks. However, there are concerns about the vulnerability of developments on alluvial plains and reclaimed lands to geological events, like ground subsidence due to liquefaction during large earthquakes. Realistic assessment of earthquake and tsunami hazards and evaluation of possible countermeasures require accurate estimation of the amount of subsidence that can be expected from liquefaction at coastal and riverside sites supporting various structures. In this study, to evaluate the amount a river embankment structure might be expected to settle as a result of strong motion from an assumed Nankai Trough great earthquake, we conducted a numerical simulation using the soil–water coupled finite deformation analysis code GEOASIA. We then investigated the effect of the estimated embankment subsidence on tsunami inundation, which was simulated by using nonlinear shallow-water equations and a grid spacing as fine as 3.3 m. The influence of urban structures on the inundated area was taken into account by using a structure-embedded elevation model (SEM). The results showed that subsidence of river embankments and the collapse of parapet walls on top of them would increase both the depth and area of inundation caused by a tsunami triggered by a Nankai Trough scenario earthquake. Our findings underscore the importance of evaluating not only earthquake resistance but also vulnerability of coastal and riverside structures to strong motion in tsunami hazard analyses. Furthermore, the importance of tsunami inundation analysis using a SEM for predicting the behavior of tsunami flotsam in urban areas was demonstrated.

A new method of permanent displacement estimation and simplification of seismic ground motion is developed. In the method, the displacement is extracted from the acceleration record or the velocity record using wavelets. The basic wavelet used in this study is expressed as a product of the Gaussian function and the complex exponential function. To extract the characteristic element of original wave including the pulse-like ground motion causing the permanent displacement, a wavelet set defined as the linear combination of basic wavelets is used. Each wavelet in the linear combination reflects a permanent or transient displacement component. Applying the method to the ground motion records during the 2016 Kumamoto Earthquake, the validity of the method is demonstrated. The method is particularly useful in the case of the ground motion record which has slight change of the acceleration baseline. In addition, it is shown that the method is available to simplification of the seismic ground motion by applying to the strong motion record during the 1995 Southern Hyogo Prefecture Earthquake. The acceleration record is rapidly decomposed to element waves by the method. Especially, a complicated wave element can be extracted by the method, since the wavelet set used seems to be a Fourier series in the limited time interval.

Long-period seismic waves are affected by the propagation path, and have a different nature depending on the source location. In addition to the accretionary wedge and sedimentary basins could affect the long-period seismic waves. The characteristics of ground motions on the irregular sedimentary basin structure vary depending on the incident direction of the seismic wave and the type of incident seismic wave. Therefore it is important to focus on the source location, and to consider the path effect together with the effect of the sedimentary basin beneath the site. In this study, the 3D finite difference analysis based on the reciprocity theorem is employed to clarify the path effect and the effect of the sedimentary basin beneath the site. In regard to the path effect, the accretionary wedge could decrease the specific periodic band of seismic waves, in contrast to the Niigata sedimentary basin which could amplify the specific periodic band of seismic waves. In addition, the Kanto plain could so much not amplify the waves on the site in Chukyo area. In regard to the effect of the sedimentary basin beneath the site, the analysis confirm that the characteristic of ground motion fluctuates depending on the incident direction of seismic waves. Especially, the results of some observation points show that when the source located in a particular direction, the seismic wave is strongly amplified by the sedimentary basin. Specific amplification of seismic waves is examined by the numerical analysis of wave propagation using the simple soil structure models. As a result, it is revealed that the seismic waves are strongly amplified by the step of the layer near the site. Therefore, the ground motion at the site is amplified and the duration time is extended. At last, amplifications of seismic wave radiated from sources of various directions are compared at the site on Chukyo sedimentary basin using the seismic ground motion records. The study revealed that the amplification of the seismic wave radiated from eastern sources is larger than other direction at the specific periodic band. The behavior similar to the result of numerical analysis is shown by the study using the seismic ground motion records. According to the result that the effect of the wave propagation path and the sedimentary basin beneath the site varies with respect to the source location, the damage of specific long-period building could be large in specific cases of earthquakes. The examination of the ground motion using 3D finite difference analysis based on the reciprocity theorem is beneficial since the above mentioned effects are complicated and different around the earthquakes.

&nbsp;Recently, it is said that the characteristics of ground motions on the irregular sedimentary basin structure vary depending on the incident direction of the seismic wave. The incident seismic waves are affected by the path, and have a different nature depending on the source location. Therefore, it is important to focus on the source location and to consider the path effect.<br>&nbsp;In this study, the long-period ground motion by the earthquakes in 3 zones (Off-Sanriku, Off-Kii Peninsula, and the Niigata sedimentary basin) are compared in the path effect and the site effect. Especially, the ground motion at the site near the seismic source and outside the Chukyo sedimentary basin have different nature. The result shows that long duration and long-period seismic wave propagates to Chukyo area in the case of an earthquake in the Niigata sedimentary basin zone and Off-Kii Peninsula zone, compared with Off-Sanriku zone. It is known that strong long-period seismic wave propagates to Chukyo area due to the sedimentary wedge, if an earthquake occurs in Off-Kii Peninsula zone. On the other hand, strong long-period seismic wave from the Niigata sedimentary basin zone is considered to be affected by the Niigata sedimentary basin.<br>&nbsp;By using the 3D finite difference method, the same phenomena were simulated for 5 earthquakes in the Niigata sedimentary basin zone. If the seismic wave do not pass the Niigata sedimentary basin, the long-period ground motion is not caused in Chukyo area. Therefore, the earthquake beneath the Niigata sedimentary basin could radiate long-period seismic wave to Chukyo area from the northern part of Nagano Prefecture.<br>&nbsp;In addition, 3D finite difference analysis based on the reciprocity theorem of Green's function is applied to the bedrock site between the Niigata sedimentary basin and Chukyo area. The sedimentary basins on the seismic wave path including the Niigata sedimentary basin affect the seismic wave. The sedimentary basins cause an increase of the amplitude and extension of the duration time of the seismic wave, and some of them cause an increase of the amplitude at the specific period range. It is suggested that these effects may be different depending on the shape and the size of the sedimentary basin and the seismic wave paths.<br>&nbsp;According to the above mentioned result, the damage of specific long-period building could be larger depending on the source location. It is important to evaluate the path effect including the sensitive period range and the site effect together.

Anew method to predict a long-period ground motion due to arbitrary seismic source was developed. In the method, the ground motion due to an earthquake is generated from the ground motion record due to other earthquake by using the transfer function defined as the spectral ratio between the theoretical ground motions due to each earthquake. The theoretical ground motions are simulated based on a soil structure model by 3D finite difference method. In addition, a technique to compute the Green's functions giving the ground motions at a station by arbitrary seismic sources based on the reciprocity theorem is used in order to reduce calculation load. Applying the new method to three earthquakes, it was suggested that the ground motion record of a small earthquake is accurately scaled up to that of a large earthquake.

On the sedimentary basin, the characteristics of ground motion varies with incident angle and azimuth angle of the seismic wave due to the complicated soil structure. Especially, the predominant period of the seismic ground motion, which is very important in the design of high-rise buildings, often varies with the direction of the hypocenter. In this study, we focus the effect of sedimentary basin structure on dynamic characteristics of seismic ground motion, using the three dimensional finite difference method and the reciprocity theorem of Green's function. We study especially focusing effect and trapping effect of seismic waves. We also discuss the fluctuation of predominant period for the seismic design of high-rise buildings.

Listening to a sound representing ground motion is valuable to intuitively understand an earthquake, compared with conventional methods such as observing waveforms, Fourier spectra, and response spectra. In this study, we propose a new method for generating sound representing an earthquake, which has several differences in comparison with existing methods. In our new method, an earthquake-like sound is generated by modifying the frequency information of a generating function based on the symmetric Fourier analysis theory. Applying the method to existing seismograms illustrates three features of the sound generated by this method: the sound has the same duration as the seismogram, the sound pitch corresponds to the instantaneous frequency of the seismogram, and the sound volume corresponds to the envelope amplitude of the seismogram. Furthermore, we constructed a web-based earthquake-like sound generating system called Naion. Other applications will be implemented in the future.

Many metropolises, such as Tokyo, Osaka, and Nagoya, are on the sedimentary basin in Japan. On the sedimentary basin, the characteristics of seismic ground motion varies with the incidence directions of seismic waves due to the complicated soil structure. Especially, the predominant period of the seismic ground motion, which is very important in the design of high-rise buildings, often varies with the direction of the hypocenter. In this study, we develop a new method to evaluate the dynamic characteristics of the sedimentary basin using the three dimensional finite difference method and the reciprocity theorem of Green's function. The finite difference method is advantageous to the complicated soil structure. In addition, the calculation cost is dramatically reduced using the reciprocity theorem of Green's function. By using our new method, the fluctuation range of the predominant period of the seismic ground motion can be revealed for the observation point on the sedimentary basin.

The asperity model was developed to explain plate boundary behavior such as interplate earthquakes. Asperity is defined as a strongly-coupled region on the plate interface. Since interplate earthquakes are considered to occur on asperities, it is important to know the asperity distribution, which can be inferred from interseismic crustal deformation through estimation of the slip deficit distribution. Slip deficit is the difference between the long-term plate convergence rate and the actual relative displacement rate of the plate interface. It is a kinematic description of plate interaction. The relation between the estimated slip deficit and the asperity is still not clearly understood. We have conducted a quantitative comparison between them by combining a forward simulation of crustal deformation, as a result of plate subduction, and a geodetic data inversion. We found that the seismic moment accumulation rate is likely to be overestimated in most cases. The degree of overestimation increases in the case of small asperity areas. Conversely, if no slip deficit is detected by geodetic data inversion, it is highly probable that no asperity exists. Such a misinference may lead to an incorrect estimation of strong ground motion in future earthquakes, and appropriate measures should be taken to allow for this.