Japanese

SUMMARY OF MASTER'S THESIS AND GRADUATION THESIS

Graduated in March 2022


2022.3 Master's thesis

Ayaka IKEDA
Reproduce Past Earthquakes,1987 Chibaken-Toho-Oki Earthquake(M6.7)

In Japan, the damaging earthquake is frequently occurred, and many people and things be victims. The earthquake damage is repeated. If an earthquake occurs with similar topography, ground, earthquake scale in different places, the same kind of earthquake damage will occur. Therefore, to be able to respond quickly in the event of a similar earthquake, it is important to analysis the past earthquakes.
Recently, the methods of collecting and analyzing information about earthquakes have become more diverse. The development of wide-area seismic networks and GPS-based observations is recent. On the other hand, past earthquakes, even if surveyed, have little information, are stored analogically, and there is information dissipation. It is important to analyze past earthquakes but in fact there are few damaging earthquakes for which there are records and documents that can be used for analysis. 1987 Chibaken-Toho-Oki Earthquake's maximum seismic intensity was 5, causing casualties and destroying many homes, in the Tokyo metropolitan area. Even though the seismic records have been scattered, we were able to find three seismic records which is very valuable and including values that indicate seismic motion, but it is not has be able to have centralize organize and manage them in a centralized manner. So, before the records start to dissipate, we collect as much seismic information, and archive the valuable data to recreate the situation at that time as a distribution map of actual observation records. In addition, we use" Attenuation Relationship formula" by Mr. Si and Mr. Midorikawa, which can calculate the peak ground acceleration at a certain point using magnitude, epicenter depth, and earthquake type as variables, so we calculate the seismic motion in the Tokyo metropolitan area for detailed analysis. Furthermore, we will compare the results of the calculations with the actual damage and investigate the relationship between them.
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Taichi KUMAGAI
Study on the Long-distance Ground-flow Mechanism in Palu City during the 2018 Sulawesi Earthquake, Indonesia

An earthquake of Mw 7.5 occurred in central Sulawesi, Indonesia, on 28 September 2018. In the Palu, occurred strong seismic and suffered damage due to liquefaction, tsunamis up to 3 m high, and long-distance ground-flow. This long-distance ground-flow occurred in several areas and caused extensive damage in the Palu. The damage is rarely damage that the maximum slope was gentle, at about 3.9%, and that the maximum flow area was about1.5 km, which is unprecedented in the world.
The mechanism of the ground-flow is not simple and has been considered by various researchers. One consideration is that the ground may have been covered by groundwater due to the results of post-disaster borehole tests, damage and topographical conditions, and that an impermeable layer may have trapped the covered groundwater. It is thought that during the earthquake the impermeable layer may have ruptured, causing the pressurised groundwater continue to supply the groundwater. In this study, the ground in which the impermeable layer is destroyed were assessed using numerical analysis. Specifically, using the results of borehole tests in the flow area, considering four cases: total de-liquefaction, liquefaction above the impermeable layer, liquefaction below the layer and liquefaction of both the above and below layers.
The results of the analysis showed that the shear wave velocity in the layer above the impermeable layer was small, which caused large strains; liquefaction in the layer above the impermeable layer caused the largest strains; and liquefaction in the layer below the impermeable layer did not cause large strains in the whole ground. Therefore, it is considered that the liquefaction of the upper layers above the impermeable layer caused large shear strain, which may have led to the inflow of groundwater that had been under pressure due to damage to the impermeable layer, resulting in the damage described above. In addition, a simplified liquefaction assessment was carried out in non-flow areas, which showed that liquefaction was less likely to occur in non-flow areas, suggesting that the occurrence of liquefaction may be related to the damage caused by the earthquake. In the future, effective stress analysis considering the other case. And, it is also considered necessary to conduct experiments using field samples and the analytical parameters into the field.
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Hikaru TAKADA
Effects of surface ground characteristics on seismic motion characteristics

In the 2004 Niigata-ken Chuetsu Earthquake, strong seismic motions were generated near the epicenter and caused a lot of damage over a wide area. Many houses collapsed in the Budokubo area of the former Kawaguchi Town near the epicenter. In the Budokubo district of the former Kawaguchi town near the epicenter, many houses collapsed. In previous studies, the tendency of earthquake damage to wooden houses did not differ significantly depending on the construction age and structure of the houses, suggesting that the earthquake ground motion generated by different ground characteristics was different and caused the damage. In addition, microtremors were measured at several points in the area and the dominant frequency of the ground was estimated. The estimated frequencies were classified into categories, and a correlation was found between the frequencies and the damage. However, since the results of the vibration frequency by the constant microtremor are discrete, the boundary of each category is unclear, although it was possible to roughly categorize the ground properties.
In this study, we estimated the ground structure (two-dimensional S-wave velocity structure) using the surface wave survey method, which enables us to understand the continuous ground properties for the purpose of continuous evaluation of ground properties. In addition, we compared the distribution of earthquake ground motions with the distribution of damage, and discussed the causes of damage to wooden houses.
As a result, it was found that soft surface layers with low S-wave velocity were thickly distributed in the area with large and medium damage, while soft layers were not thickly distributed in the area with small damage. This result reflects the difference of topography and geology in the area, and the seismic damage, 2-D S-wave structure, topography and geology show similar trends. Therefore, the results suggest that there is a possibility of severe damage to wooden houses in the area where the soft layer in the surface soil is thickly distributed. The results of the comparison between the earthquake damage and the characteristics of the surface soil in the area suggest that the thick, loosely deposited clay layer had a significant influence on the earthquake damage to wooden houses, and that the difference in S-wave velocity and layer thickness affected the characteristics of the earthquake motion and the degree of damage to wooden houses in a narrow range.
2022.3 Graduation thesis

Aoto KUBOTA
Analysis of frequency characteristics of seismic waves observed in inland crustal earthquakes around Niigata Prefecture

Given the large number of damaging earthquakes that occur in Japan every year, reducing the damage from these earthquakes is considered to be an important issue. In particular, since seismic waves generated by earthquakes have a great impact on buildings, understanding the frequency characteristics contained in observation records is necessary for examining the possibility of damage caused by major earthquakes that are expected to occur in the future, and understanding the frequency characteristics will lead to countermeasures for buildings not only after the occurrence of major earthquakes but also before the occurrence of earthquakes.
In this research, we analyzed the frequency characteristics of the records observed in recent large earthquakes of magnitude 6.
The running spectrum was used as a method to analyze the frequency characteristics of the observation records.
For the calculation of running spectra, we used parameters downloaded from the strong-motion observation network (K-NET, KiK-net) of records observed at the KNET Nagaoka site (NIG017), the target site for analysis, for five earthquakes: the Niigata-ken Chuetsu earthquake, the Niigata-ken Chuetsu-oki earthquake, the earthquake in northern Nagano prefecture, the earthquake in northern Tochigi prefecture, and the earthquake off Yamagata prefecture. The results of the analysis are divided into three groups: Group 1 for earthquakes in northern Nagano Prefecture, Niigata Prefecture, and northern Tochigi Prefecture, which are located to the south of the observation point (NIG017); Group 2 for earthquakes off Yamagata Prefecture and Niigata Prefecture, which are located to the north.
From the results of the analysis, it is considered that the components of the high frequency band were dominant in Group 1 (the Niigata-Chuetsu earthquake, the earthquake in northern Nagano Prefecture, and the earthquake in northern Tochigi Prefecture), and that the NIG017 was relatively susceptible to strong shaking. The way the seismic wave propagated suggests that it passed through a hard ground.
In Group 2 (earthquakes off the Niigata-Chuetsu-Oki coast and off the Yamagata coast), the high frequency component is not as dominant as in Group 1, and strong shaking is not felt as easily. In addition, the way the seismic waves were transmitted suggests that they passed through soft ground. In this research, we analyzed the frequency characteristics of the records observed in recent large earthquakes of magnitude 6. In this study, we focused on Nagaoka point (NIG017) in Niigata Prefecture, and analyzed five earthquakes recorded there. In order to analyze the frequency characteristics in more detail in the future, it is important to increase the number of earthquakes with different arrival directions and sizes.
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Shogo TSUNO
Study on the applicability of equivalent linear analysis method to liquefied ground

In recent earthquakes, many cases of liquefaction caused by seismic motion have been reported. To prevent liquefaction damage from occurring, it is important to understand the behavior of liquefied soils. However, it is difficult to understand the behavior of liquefied ground in detail, and seismic response analysis is required. Currently, effective stress nonlinear analysis is used for seismic response analysis of liquefied ground, which takes into account the nonlinearity of the ground and the generation of pore water pressure. However, it is a very difficult and labor-intensive analysis method due to the difficulty of setting the necessary parameters and the large number of parameters. By the way, Equivalent linear analysis is a simpler method of seismic response analysis that is commonly used in practice. This analysis method is very effective if it can be applied to liquefied ground, but it is difficult to apply to large-strain earthquakes that cause liquefaction. Therefore, in order to apply equivalent linear analysis to liquefied ground, the solution is limited to the maximum acceleration and maximum velocity, which are important indices for design and other purposes, but do not change over time. This study focuses on the effective strain coefficient, a parameter that plays an important role in equivalent linear analysis but for which the method of determination is unclear, and aims to determine values that can be used to determine the maximum acceleration and velocity during liquefaction. In conducting this study, Port Island during the 1995 Hyogo-ken Nanbu Earthquake was used as the target site, provided that the site actually liquefied and that the necessary data for the analysis were available. A ground model was created from observation records, and after determining the input earthquake ground motion, the analysis was conducted while varying the effective strain coefficient from 0.3 to 1.0 in increments of 0.05.
The results of the analysis showed that the effective strain coefficient could not be determined for the maximum velocity because the error was too large. The error was 0.7 in the N-S direction and 0.55 in the E-W direction for the maximum acceleration. 0.5, which had the smallest error, was determined to be the optimum effective strain coefficient, provided that the relative error was positive in order to be on the safe side when using the results in design and other applications. The agreement between the analysis results and the observed acceleration waveforms when the effective strain coefficient was set to 0.5 was also checked, and the agreement was good in sections where the degree of liquefaction was considered to be small, such as the section where the maximum value was recorded. This value is only valid for determining the maximum acceleration.
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Yuki NISHIJIMA
Analysis of distance attenuation characteristics of inland crustal earthquakes occurring around Niigata Prefecture

In general, the further away from the epicenter of an earthquake, the smaller the shaking becomes. This phenomenon is called distance attenuation, and one of the methods for evaluating seismic motion is the distance attenuation formula, which focuses on this characteristic. Although the distance attenuation formula is difficult to evaluate precisely, it is a very simple method to estimate the maximum seismic motion if the magnitude of the earthquake and the distance from the epicenter are obtained.
In this study, we focus on the distance attenuation equation, and use the distance attenuation equation of Shi and Midorikawa (1999), which is used as a standard equation in Japan, as the basic equation. Based on this basic equation, the purpose of this study is to analyze the distance attenuation characteristics around Niigata Prefecture by comparing the maximum acceleration and velocity estimated from the distance attenuation equation with the actual observed data. We also analyzed the influence of the direction of arrival of the earthquake motion and the ground structure of Niigata on the distance attenuation.
After determining the observation points and target earthquakes, we collected the necessary data for inputting them into the distance attenuation equation by using the strong-motion observation networkand the broadband seismic observation network. In this study, the target earthquakes are the 2004 Niigata-ken Chuetsu earthquake, the 2007 Niigata-ken Chuetsu-oki earthquake, the 2011 Nagano-ken/Niigata-ken border earthquake, the 2013 northern Tochigi-ken earthquake, the 2014 northern Nagano-ken earthquake, and the 2019 Yamagata-ken offshore earthquake. From the earthquake data collected in this way, we determined the fault plane, transformed the latitude and longitude into rectangular coordinates, and calculated the shortest distance between the fault and the site, which is the shortest geometric distance on the fault plane. Using the shortest distance of the fault plane, the distance decay curves of the maximum acceleration and the maximum velocity were made and compared with the observation points.
The results of the analysis of maximum acceleration and maximum velocity show that the observed records of maximum acceleration and maximum velocity tend to be overestimated for the 2004 Niigata-Chuetsu earthquake and the 2019 Yamagata-Oki earthquake. On the other hand, for the 2007 Niigataken Chuetsu-oki earthquake, the 2011 Nagano-Niigata prefectural border earthquake, and the 2013 northern Tochigi prefecture earthquake, both the maximum acceleration (PGA) and the maximum velocity (PGV) tended to underestimate the observed records. These trends were found to be strongly influenced by the ground through which the seismic waves pass.
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