5:15 PM - 7:15 PM
[SSS11-P01] Ground Amplification Characteristics in the Northern Noto Peninsula Area
Keywords:Hazard risk assessment, Subsurface structure, Microtrmor array observation
Introduction
In order to compare damage trends and ground amplification characteristics in cities and towns in the northern Noto Peninsula that suffered severe damage to residential areas from the 2024 Noto Peninsula earthquake, microtremor array surveys (250 m mesh intervals) were conducted at about 423 points mainly in lowland areas in the northern Noto Peninsula, including Wajima City, Suzu City, Shiga Town, Anamizu Town, Noto Town, etc.The objectives of this study are confirm the amplification characteristics of the Noto Peninsula earthquake of 2024 in detail, mainly in the shallow areas below the engineering foundation, because the degree of damage differs in each of the above-mentioned areas, especially the correlation with the surface soil, and construct a detailed integrated shallow and deep ground structure model to improve earthquake ground motion prediction.
Observation and analysis methods
The microtremor array survey consists of 6 to 8 units per site, each of which is basically a 60 cm array, set up in irregular shapes from 10 m to 25 m, with a minimum of 15 minutes of simultaneous observation.When the quality of the microtremor data was not sufficient with the accelerometer, the velocity meter was also utilized to obtain records that sufficiently confirmed AVS30 and engineering base depth (top surface around Vs 400). We have confirmed that the dispersion curve is generally around (Vs700) and the period can be observed from 0.5 to 1.0 second. Since these observations were made with three components for both acceleration and velocity, the H/V spectral ratio was also calculated. The ground amplification factor is obtained from AVS30 by Fujimoto and Midorikawa (2006).
Analysis Results and Discussion
In Wajima City (central area), AVS30 ranged from 108 to 410 (surface ground amplification factor was 0.98 - 3.05), and the average amplification factor in low-lying areas was about 1.85 times. In particular, in the Kawai to Fugeshi area of Wajima City, where the damage to houses was concentrated, soft ground with an amplification factor of 2.5 or higher and a period of ground excitability of 0.75 seconds or higher was observed in some places.
In the central part of the Anamizu district, areas with an amplification factor of about 2.5 were locally observed, mainly in K-NET Anamizu. In addition, areas showing large amplification ratios of 2.0 to 2.5 times were observed in the hinterland wetlands and valley floor lowlands of Machino District, Wajima City, and in the inland delta and coastal lowlands of Shoin District and Iida District, Suzu City. On the other hand, in the Monzen area of Wajima City and the Tomirai and Takahama areas of Shiga Town, the amplification was found to be relatively small, generally less than 2 times (average of about 1.35 times in the lowland areas).
It is thought that the amplification is not so large in this area due to the development of dunes and sandbars and the lack of clayey soil, but on the other hand, a relatively large amount of liquefaction damage has been observed due to the large amount of sandy soil.
Summary
In the results of this study, the AVS30 (amplification factor) in the central part of Wajima City (Fugeshi and Kawai areas) was found to be no less than that in the low-lying areas of Kanto, where amplification is higher. The Kanto Plain is a very large plain and is known to have thick alluvial sedimentary layers. However, the fact that the alluvium in lowland areas of about 1 km width surrounded by mountains, such as Wajima City and Anamizu Town, had large amplification rates suggests that many such areas may exist in other areas throughout Japan. The current ground amplification factor based on microtopography classification does not sufficiently consider the amplification factor of such narrow lowland areas.
In order to compare damage trends and ground amplification characteristics in cities and towns in the northern Noto Peninsula that suffered severe damage to residential areas from the 2024 Noto Peninsula earthquake, microtremor array surveys (250 m mesh intervals) were conducted at about 423 points mainly in lowland areas in the northern Noto Peninsula, including Wajima City, Suzu City, Shiga Town, Anamizu Town, Noto Town, etc.The objectives of this study are confirm the amplification characteristics of the Noto Peninsula earthquake of 2024 in detail, mainly in the shallow areas below the engineering foundation, because the degree of damage differs in each of the above-mentioned areas, especially the correlation with the surface soil, and construct a detailed integrated shallow and deep ground structure model to improve earthquake ground motion prediction.
Observation and analysis methods
The microtremor array survey consists of 6 to 8 units per site, each of which is basically a 60 cm array, set up in irregular shapes from 10 m to 25 m, with a minimum of 15 minutes of simultaneous observation.When the quality of the microtremor data was not sufficient with the accelerometer, the velocity meter was also utilized to obtain records that sufficiently confirmed AVS30 and engineering base depth (top surface around Vs 400). We have confirmed that the dispersion curve is generally around (Vs700) and the period can be observed from 0.5 to 1.0 second. Since these observations were made with three components for both acceleration and velocity, the H/V spectral ratio was also calculated. The ground amplification factor is obtained from AVS30 by Fujimoto and Midorikawa (2006).
Analysis Results and Discussion
In Wajima City (central area), AVS30 ranged from 108 to 410 (surface ground amplification factor was 0.98 - 3.05), and the average amplification factor in low-lying areas was about 1.85 times. In particular, in the Kawai to Fugeshi area of Wajima City, where the damage to houses was concentrated, soft ground with an amplification factor of 2.5 or higher and a period of ground excitability of 0.75 seconds or higher was observed in some places.
In the central part of the Anamizu district, areas with an amplification factor of about 2.5 were locally observed, mainly in K-NET Anamizu. In addition, areas showing large amplification ratios of 2.0 to 2.5 times were observed in the hinterland wetlands and valley floor lowlands of Machino District, Wajima City, and in the inland delta and coastal lowlands of Shoin District and Iida District, Suzu City. On the other hand, in the Monzen area of Wajima City and the Tomirai and Takahama areas of Shiga Town, the amplification was found to be relatively small, generally less than 2 times (average of about 1.35 times in the lowland areas).
It is thought that the amplification is not so large in this area due to the development of dunes and sandbars and the lack of clayey soil, but on the other hand, a relatively large amount of liquefaction damage has been observed due to the large amount of sandy soil.
Summary
In the results of this study, the AVS30 (amplification factor) in the central part of Wajima City (Fugeshi and Kawai areas) was found to be no less than that in the low-lying areas of Kanto, where amplification is higher. The Kanto Plain is a very large plain and is known to have thick alluvial sedimentary layers. However, the fact that the alluvium in lowland areas of about 1 km width surrounded by mountains, such as Wajima City and Anamizu Town, had large amplification rates suggests that many such areas may exist in other areas throughout Japan. The current ground amplification factor based on microtopography classification does not sufficiently consider the amplification factor of such narrow lowland areas.