SUMMARY OF MASTER'S THESIS AND GRADUATION THESIS
Graduated in March 2021
- 2021.3 Master's thesis
- Haruki SUZUKI
- Study on strong ground motion and earthquake damage in Koiwagawa area during the 2019 off Yamagata prefecture earthquake
On June 18, 2019, at 22:22 (JST), an Mw6.4 earthquake occurred off the coast of Yamagata Prefecture. A maximum acceleration of 1191.3 gal was also observed at a nearby seismic station. Strong ground motions were generated at near source area, but there was no major damage. Main damage is roof tiles damage of the wooden house. A damage rate of roof tile at Koiwagawa area was the highest in damage area. On the other hand, Oiwagawa area and Wasada area which are adjustment in Koiwagawa had minor damage. Therefore, the purpose is to clarify the relationship between the ground characteristics of the Koiwagawa area and housing damage. Geophysical exploration was conducted in the Koiwagawa area of Tsuruoka City, Yamagata Prefecture, and the ground characteristics were clarified. We clarified the ground characteristics from geophysical exploration and acquired the parameters for seismic response analysis. Seismic response analysis was carried out from the obtained analysis parameters, and the relationship between seismic motion and housing damage was clarified by calculating the seismic motion in the Koiwagawa area. From the geophysical survey results, it was found that the surface layer was deeper in the area where the damage was large in the Koiwagawa area. In addition, seismic response analysis revealed that the deeper the surface layer, the greater the seismic motion. Therefore, it was found that there is a difference in housing damage depending on the depth of the surface layer. If you look at the acceleration response spectrum, you can see that it shows a large response value as a whole. Therefore, it is presumed that Koiwagawa observed shaking that could have caused enormous damage. However, it is not clear at present why the great damage did not occur. It was found that the depth of the surface layer influences the magnitude relationship of the small number of damages.
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- Atsuya MINAGAWA
- Study on amplification factor of maximum acceleration for seismic hazard assessment
Background and objective
While efforts are being made in Japan regarding earthquake disaster prevention measures, there are many cases in which ground survey data and other data are not sufficient overseas and earthquake risk is not properly evaluated. This study focuses on earthquake risk countermeasures in Yangon City. Since the earthquake damage is affected by the shaking of the surface ground, we focus on the surface ground amplification characteristics that evaluate the regional distribution. In previous studies, the maximum velocity amplification factor (ARV) was evaluated for the amplification characteristics of the ground in Yangon City using terrain classification, boring, microtremor measurement, etc. Since it is important to evaluate acceleration as an index, such as vibration analysis of structures, we attempted to calculate the maximum acceleration amplification factor (ARA).
Method and result
Since the amplification factor is calculated using the ratio of the maximum amplitude of the base and the maximum amplitude of the surface ground, vertical array observation with strong motion seismographs installed on the basement and the ground surface is required. However, Yangon City has no observation network facilities, so similar points were selected in Japan. Since the observation record is an acceleration time history waveform, the velocity time history waveform is calculated by integration. Since the observed waveform is a composite wave that includes various frequencies, filter correction was performed to limit the frequency range in consideration of the effect on the building. ARV and ARA were plotted on each of the xy axes, and the relational expression of the linear linearity was calculated by the least squares method. The results differed depending on the correction width of the filter. As an adaptive formula for Yangon City, we decided to use the calculation result with a filter 5.0-10Hz with a surface layer thickness of 15m or more. It was found that ARA tends to be about 1.1 times higher than ARV, and the amplification factor of acceleration is overestimated rather than velocity.
Summary
This survey method is considered to be very effective for area evaluation when targeting areas such as Yangon City where information on earthquake disaster countermeasures is not sufficiently accumulated. Since local information is required to consider reliability and accuracy, it will be necessary to disseminate seismic observations for earthquake disaster prevention measures in the future.
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- Yoshito YOKOE
- Evaluation of seismic risk in Yangon City using empirical seismic motion evaluation method
The distance decay equation is a simple method for evaluating earthquake risk. However, although the distance attenuation method has the advantage of easy risk assessment, it does not take into account detailed fault parameters such as directional effects of fault rupture and fault heterogeneity. However, it cannot take into account detailed fault parameters such as fault rupture directional effects and fault heterogeneity. Therefore, this study aims to conduct risk assessment using the waveform synthesis method in Myanmar.
As a background to the study of Myanmar, a project called "Strengthening the Disaster Response System in Myanmar and Building a Platform for Industry-Academia-Government Collaboration" is underway as part of SATREPS, a joint research program between researchers in developing countries and JICA. In this program, research is being conducted to determine the ground amplification factor in Yangon and to calculate the damage to buildings. However, the earthquake ground motions used in these studies are considered to be 30kine unified. Therefore, in this study, we use waveform synthesis in Yangon to obtain detailed engineering basis.In this study, the earthquake ground motions from six faults are assumed based on the past earthquake history and literature. For reference, we also considered an inland earthquake of M7 class directly under Yangon City. We used EMPR, an empirical seismic prediction method that can take into account detailed fault parameters such as fault heterogeneity and directional effects of fault rupture. The fault parameters required for EMPR are the fault length, width, seismic moment, fault strike, fault dip, moment ratio of each sub-fault, and the relative position of the fault to the site (Xs, Ys). The fault parameters were basically set according to the strong-motion prediction recipe.
The strong motion prediction recipe is a standard methodology to predict the strong motion of an earthquake with high accuracy by specifying the source fault. The evaluation points are divided into a mesh of Yangon city at intervals of 1 km, 30 km long by 25 km wide. The ground amplification factor obtained from the study of Matsumoto (2020) was used as the ground amplification factor and interpolated into the mesh. The results showed that the seismic waves amplified in the direction of rupture from the north to the south, and there were no cases where the amplification exceeded 30 kine. The reason for the amplification of seismic waves is the fault directionality effect. In addition, we found that the seismic motions on the engineering basis, assuming past faults, did not produce results exceeding 30 kine.
In Yangon, the seismic resistance of buildings tends to be weaker than in Japan, so I believe that the earthquake ground motions obtained in this study can be effectively used to calculate the damage.
- 2021.3 Graduation thesis
- Tsubasa SATO
- Study on dynamic seismic characteristics of wooden houses based on site measurement
In June 2019，an earthquake with a seismic intensity of over 6 occurred off the coast of Yamagata Prefecture. By this earthquake，A seismic intensity of over 6 was observed at Fuya， Murakami city，but the wooden structure didn’t collapse．Therefore，in this study，I calculated the natural frequency of the wooden structure in Fuya and investigated the possibility that the response of the wooden structure was amplified．The natural frequency was calculated using the transfer function from the waveform obtained by microtremor observation．In addition，We measured the microtremor of two wooden houses in Nakahama． As a result of calculating the natural frequency，For wooden house A，ridge direction is 3.8Hz and span direction is 4.0Hz，For wooden house B，ridge direction is 5.7Hz and span direction is 5.0Hz． From this result，the natural frequency of ridge direction was smaller than the natural frequency of span direction in the house A．The reason why the natural frequency was reversed was considered to be the influence of Niigata earthquake．Niigata earthquake caused cracks in the ridge direction．It was considered that the natural frequency was reversed due to this crack in the ridge direction．Finally，the obtained natural frequency was evaluated．Therefore，I made the acceleration response spectrum of the earthquake off Yamagata Prefecture．Then，both the wooden house A and the wooden house B deviated from the frequency at which the acceleration response peaked．However，when compared with the acceleration response spectrum of 1995 South Hyogo Prefecture Earthquake，the maximum acceleration response of the earthquake off the coast of Yamagata Prefecture was larger．Therefore，it is necessary to investigate from the point of view of seismic strength of wooden structures in the future．
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- Yuta MATSUYA
- Attenuation relation of peak ground motion considering earthquake types
Earthquakes that occur around Japan are roughly divided into inland earthquakes and subductionzone earthquakes, and it is important to consider the earthquake type in the evaluation of ground motion. The shaking of an earthquake has a characteristic called distance attenuation, which decreases as the distance from the epicenter increases. One of the seismic motion evaluation methods is a distance attenuation formula that focuses on this characteristic. Although it is difficult to make a precise evaluation using the distance attenuation formula, the maximum seismic motion can be estimated if the magnitude of the earthquake and the distance from the epicenter can be obtained, which is a very simple method. In this study, we focused on this distance attenuation formula, and used the Si-Midorikawa formula as the basic formula, which is famous and can consider the earthquake type. In addition, since the evaluation accuracy and the application range are in a trade-off relationship, we focused on the evaluation accuracy and limited the application range to Tochigi Prefecture.
Furthermore, considering the three types of earthquakes that damage Tochigi Prefecture, the range of earthquake occurrence is limited to these three ranges. Based on the above, The purpose of this study is to construct a distance attenuation equation with a limited range of application by modifying the coefficients of the equation so that the distance attenuation characteristics could be expressed. We also focused on predicting strong ground motion.
Seismic data were collected using the strong motion observation network (K-NET, KiK-net) and the broadband seismic observation network (F-net). When collecting seismic data, we set the standard for the maximum value of ground motion, the standard for the location of observation points, and the standard for seismic scale (M). From the seismic data collected in this way, the fault plane is determined, the latitude and longitude are converted to orthogonal coordinates (UTM coordinates), etc., and the geometrical shortest distance between the fault and the site (observation point of the strong motion observation network). The shortest fault plane distance was calculated.
As a result of collecting seismic data, the observed value was larger than the estimated value of the Si-Midorikawa formula in Tochigi prefecture. From this result, it was necessary to increase the estimated value of the Si-Midorikawa formula, but it was expected that the correction amount would be different for the three earthquake types, and in the Si-Midorikawa formula, the effect of the earthquake type was expressed by a coefficient d. Therefore, we decided to correct the coefficient d used in the equation because different values are used for each earthquake type. As a result of the correction, the coefficient d for each earthquake type was proposed, and as a result of using each coefficient d, the accuracy equal to or higher than that of the Si-Midorikawa equation was obtained. In fact, it was confirmed that it is possible to improve the accuracy of the distance attenuation formula by considering the earthquake type and application range.
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