5:15 PM - 6:30 PM
[SSS11-P13] Modeling of subsurface velocity structures from seismic bedrock to ground surface for Kumamoto region
Keywords:Underground structure model, Microtremors, Strong Ground Motion Prediction
1. Introduction
The NIED has created a shallow and deep integrated ground structure model using the microtremor array exploration observed in the Kumamoto Plain (Senna et al., 2018). In this study, the range of the shallow and deep integrated underground structure model was expanded by significantly adding microtremor array exploration to the northern part of the Kumamoto Plain and the Aso area. In this study, we report the results of expanding the scope using the method proposed in the above paper.
2. Creation of initial ground model
Regarding the expansion range, of the initial ground models, the J-SHIS_V2 model of the NIED was basically used as the initial model for the deep underground model (deeper than Vs350 (m/s) defined as the engineering bedroock). For shallow underground, a ground model was connected to make it an initial model. After that, the upper surface of the engineering bedrock of the shallow underground was used as the upper surface of the first layer of the deep underground, and the two were integrated to form the initial integrated underground model. For the shallow underground model (shallow than Vs350 (m/s)), the geomorphologic classification of J-SHIS was reviewed and revised based on the flood control topography classification map, and Borehole data (about 12,000) were organized and a 250m mesh model was created based on the modified geomorphologic classification.
3. Microtremor array observations and seismic records used
Microtremor array observations are about 5 km of large array observations (three points with array radii of 100, 200, 400 m and L-shape with sides 25 to 75 m) in the lowlands and plateaus indicated by the geomorphologic in the northern part of Kumamoto Prefecture and the Aso area. 16 points were added at intervals. Including the 26 large array points in the past, there are a total of 42 points. On the other hand, extremely small and irregular array observations (4 points with an array diameter of 60 cm and triangles with a side of 5m to more than 10 m) were carried out at approximately 1 km intervals and in the Aso area at approximately 500 m intervals (2021. As of March). Including the 473 points of the existing microtremor array, the total number is 1327 points. The phase velocity and H/V spectrum ratio for each microtremor observation point were calculated. For the seismic records, the R/V spectrum ratio for each station was calculated using the K-NET and KiK-net seismic stations of the NIED, and the seismic records of local governments and the JMA. Based on these ground vibration characteristic data and the initial underground model, joint inversion was performed to obtain a one-dimensional S wave velocity structure at each observation point.
4. Modification of ground model based on microtremor array observations and seismic observation results
Using the one-dimensional S-wave velocity structure obtained in Chapter 3, the initial deep underground model at each observation point was modified, and a three-dimensional S-wave velocity structure model was created by spatially interpolating each velocity layer in the horizontal direction. Based on the estimated S-wave velocity structure, the deep underground velocity structure (J-SHIS_V2) was modified for the extension. Compared with the J-SHIS_V2 model created in this study, the top depth of the Vs900 layers (Figure1) more reflects the shape of the Aso caldera. There is. The top depth of the Vs2700 layers (Figure2) is deeper near the central crater hills of Aso volcano. This is in harmony with the results of the distribution map of the upward connection residuals of gravity. In the vicinity of Shimabara Bay, the model is based on the results of sound wave exploration and gravity. As a result of the above, we believe that we were able to construct a model that more clearly reflects the structure of the "Beppu-Shimabara Graben" from Oita to Aso, Kumamoto, and the Shimabara Peninsula. The modification of the shallow ground model was made to match the AVS30 (C40 and S-wave velocity structure by inversion) obtained from the microtremor exploration at the point where the microtremor exploration was carried out.
5. Summary
In this paper, we have extended the shallow and deep integrated ground structure model around the Kumamoto Plain that we have created so far, and showed the results. In the future, we plan to acquire past data such as active fault priority surveys, add microtremor array observations, build a underground model for the entire Kumamoto prefecture, and verify it with seismic records.
The NIED has created a shallow and deep integrated ground structure model using the microtremor array exploration observed in the Kumamoto Plain (Senna et al., 2018). In this study, the range of the shallow and deep integrated underground structure model was expanded by significantly adding microtremor array exploration to the northern part of the Kumamoto Plain and the Aso area. In this study, we report the results of expanding the scope using the method proposed in the above paper.
2. Creation of initial ground model
Regarding the expansion range, of the initial ground models, the J-SHIS_V2 model of the NIED was basically used as the initial model for the deep underground model (deeper than Vs350 (m/s) defined as the engineering bedroock). For shallow underground, a ground model was connected to make it an initial model. After that, the upper surface of the engineering bedrock of the shallow underground was used as the upper surface of the first layer of the deep underground, and the two were integrated to form the initial integrated underground model. For the shallow underground model (shallow than Vs350 (m/s)), the geomorphologic classification of J-SHIS was reviewed and revised based on the flood control topography classification map, and Borehole data (about 12,000) were organized and a 250m mesh model was created based on the modified geomorphologic classification.
3. Microtremor array observations and seismic records used
Microtremor array observations are about 5 km of large array observations (three points with array radii of 100, 200, 400 m and L-shape with sides 25 to 75 m) in the lowlands and plateaus indicated by the geomorphologic in the northern part of Kumamoto Prefecture and the Aso area. 16 points were added at intervals. Including the 26 large array points in the past, there are a total of 42 points. On the other hand, extremely small and irregular array observations (4 points with an array diameter of 60 cm and triangles with a side of 5m to more than 10 m) were carried out at approximately 1 km intervals and in the Aso area at approximately 500 m intervals (2021. As of March). Including the 473 points of the existing microtremor array, the total number is 1327 points. The phase velocity and H/V spectrum ratio for each microtremor observation point were calculated. For the seismic records, the R/V spectrum ratio for each station was calculated using the K-NET and KiK-net seismic stations of the NIED, and the seismic records of local governments and the JMA. Based on these ground vibration characteristic data and the initial underground model, joint inversion was performed to obtain a one-dimensional S wave velocity structure at each observation point.
4. Modification of ground model based on microtremor array observations and seismic observation results
Using the one-dimensional S-wave velocity structure obtained in Chapter 3, the initial deep underground model at each observation point was modified, and a three-dimensional S-wave velocity structure model was created by spatially interpolating each velocity layer in the horizontal direction. Based on the estimated S-wave velocity structure, the deep underground velocity structure (J-SHIS_V2) was modified for the extension. Compared with the J-SHIS_V2 model created in this study, the top depth of the Vs900 layers (Figure1) more reflects the shape of the Aso caldera. There is. The top depth of the Vs2700 layers (Figure2) is deeper near the central crater hills of Aso volcano. This is in harmony with the results of the distribution map of the upward connection residuals of gravity. In the vicinity of Shimabara Bay, the model is based on the results of sound wave exploration and gravity. As a result of the above, we believe that we were able to construct a model that more clearly reflects the structure of the "Beppu-Shimabara Graben" from Oita to Aso, Kumamoto, and the Shimabara Peninsula. The modification of the shallow ground model was made to match the AVS30 (C40 and S-wave velocity structure by inversion) obtained from the microtremor exploration at the point where the microtremor exploration was carried out.
5. Summary
In this paper, we have extended the shallow and deep integrated ground structure model around the Kumamoto Plain that we have created so far, and showed the results. In the future, we plan to acquire past data such as active fault priority surveys, add microtremor array observations, build a underground model for the entire Kumamoto prefecture, and verify it with seismic records.