5:15 PM - 6:30 PM
[SSS11-P12] Modeling of subsurface velocity structures from seismic bedrock to ground surface for Yamanashi region
Keywords:Underground structure model, Microtremors, Strong Ground Motion Prediction
1. Introduction
The NIED has constructed a shallow and deep integrated underground structure model in the Kanto, Tokai, and Kumamoto regions in order to improve the accuracy of strong motion prediction. In this study, we will focus on the construction of a shallow and deep integrated underground structure model for the Yamanashi region, which is adjacent to the Kanto and Tokai regions. The model construction procedure basically follows the method of the Kanto region (for example, Senna et al., 2017) in which modeling is carried out in advance. Specifically, first, an initial underground structure model was created based on the existing underground model, borehole data and geological information collected so far. Then, the initial underground model was adjusted using the ground vibration characteristics obtained by conducting a dense microtremor array exploration. In this paper, we report the above contents and the features of the modified underground structure model.
2. Initial ground model
Of the initial underground models, the J-SHIS_V2 model of the NIED was basically used as the initial model for the deep underground structure model (deeper than Vs350 (m/s) defined as the engineering bedrock). For shallow underground, a underground 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 structure model (shallower than Vs350 (m/s)), the geomorphologic classification of J-SHIS was reviewed and revised based on the hydraulic topography classification map, and then the borehole data in Yamanashi prefecture (about 3,000). This was 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 large array observations (three points with array radii of 100, 200, 400m and L-shape with sides 25 to 75 m) at intervals of about 5 km in the lowlands and plateaus indicated by the geomorphologic divisions throughout Yamanashi prefecture. Microtremor observations (4 points with an array diameter of 60 cm and triangles with a side of 5 m to more than 10 m) were carried out at about 500 points at intervals of about 1 km (as of February 2021). The phase velocity and H/V spectrum ratio for each observation point were calculated. Seismic records are recorded using the K-NET and KiK-net seismic stations of the NIED and the JMA recorded in the Metropolitan Area Strong Motion Network (SK-net) of the Earthquake Research Institute, the University of Tokyo. The R/V spectrum ratio for each observation point was calculated. Based on these ground vibration characteristic data and the initial ground model, joint inversion was performed to obtain a one-dimensional S wave velocity structure at each observation point.
4. Modification of initial ground model based on microtremor array observations and seismic observation results
Chapter 3. using the one-dimensional S-wave velocity structure obtained in the above, the initial deep ground 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 ground velocity structure (J-SHIS_V2) was modified. Since J-SHIS _V2 is uniformly shallow up to the S wave velocity of 1,000 m/s, the result is deep after correction (Figure 1). After the S-wave velocity of 1,200 m/s, it became shallower on the northwest side of Yamanashi prefecture after the model modification and deeper on the east side. How to modify the shallow underground model. It was created to match the AVS30 (C40 and S-wave velocity structure by inversion) obtained from the microtremor exploration at the point where the microtremor exploration is being carried out.
5. Summary
In this sutudy, we show an example of the construction and results of a shallow and deep integrated underground structure model for the Yamanashi region. In the future, we plan to verify the underground model using seismic records, such as the period and amplification characteristics at seismic stations.
The NIED has constructed a shallow and deep integrated underground structure model in the Kanto, Tokai, and Kumamoto regions in order to improve the accuracy of strong motion prediction. In this study, we will focus on the construction of a shallow and deep integrated underground structure model for the Yamanashi region, which is adjacent to the Kanto and Tokai regions. The model construction procedure basically follows the method of the Kanto region (for example, Senna et al., 2017) in which modeling is carried out in advance. Specifically, first, an initial underground structure model was created based on the existing underground model, borehole data and geological information collected so far. Then, the initial underground model was adjusted using the ground vibration characteristics obtained by conducting a dense microtremor array exploration. In this paper, we report the above contents and the features of the modified underground structure model.
2. Initial ground model
Of the initial underground models, the J-SHIS_V2 model of the NIED was basically used as the initial model for the deep underground structure model (deeper than Vs350 (m/s) defined as the engineering bedrock). For shallow underground, a underground 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 structure model (shallower than Vs350 (m/s)), the geomorphologic classification of J-SHIS was reviewed and revised based on the hydraulic topography classification map, and then the borehole data in Yamanashi prefecture (about 3,000). This was 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 large array observations (three points with array radii of 100, 200, 400m and L-shape with sides 25 to 75 m) at intervals of about 5 km in the lowlands and plateaus indicated by the geomorphologic divisions throughout Yamanashi prefecture. Microtremor observations (4 points with an array diameter of 60 cm and triangles with a side of 5 m to more than 10 m) were carried out at about 500 points at intervals of about 1 km (as of February 2021). The phase velocity and H/V spectrum ratio for each observation point were calculated. Seismic records are recorded using the K-NET and KiK-net seismic stations of the NIED and the JMA recorded in the Metropolitan Area Strong Motion Network (SK-net) of the Earthquake Research Institute, the University of Tokyo. The R/V spectrum ratio for each observation point was calculated. Based on these ground vibration characteristic data and the initial ground model, joint inversion was performed to obtain a one-dimensional S wave velocity structure at each observation point.
4. Modification of initial ground model based on microtremor array observations and seismic observation results
Chapter 3. using the one-dimensional S-wave velocity structure obtained in the above, the initial deep ground 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 ground velocity structure (J-SHIS_V2) was modified. Since J-SHIS _V2 is uniformly shallow up to the S wave velocity of 1,000 m/s, the result is deep after correction (Figure 1). After the S-wave velocity of 1,200 m/s, it became shallower on the northwest side of Yamanashi prefecture after the model modification and deeper on the east side. How to modify the shallow underground model. It was created to match the AVS30 (C40 and S-wave velocity structure by inversion) obtained from the microtremor exploration at the point where the microtremor exploration is being carried out.
5. Summary
In this sutudy, we show an example of the construction and results of a shallow and deep integrated underground structure model for the Yamanashi region. In the future, we plan to verify the underground model using seismic records, such as the period and amplification characteristics at seismic stations.