3:30 PM - 5:00 PM
[HDS09-P07] Modeling of Subsurface Velocity Structures from Seismic Bedrock to Ground Surface, for strong ground motion evaluation, around Byobuyama – Enasan and Sanageyama fault zone (part.2)
Keywords:underground structure model, strong ground motion, microtrmors, s-wave velocity, gravity anomaly
1.Introduction
This report describes the results of the development of the integrated shallow and deep ground structure model for strong ground motion prediction. In this report, based on the shallow and deep integrated ground structure model (hereinafter referred to as the SIP model) for the Tokai region developed by the Strategic Innovation Program (SIP) and others, we newly developed the shallow and deep integrated ground structure model using the results of borehole data collection, gravity data collection and analysis, seismic observation, microtremor array observation and sub-thema3 reflection seismic survey. The shallow and deep integrated ground structure model (hereinafter referred to as SDv2 model) was upgraded based on the SIP model.
2.Data collected
In this study, borehole data were collected (approximately 20 large-depth boreholes with a drilling length of 100 m to 1000 m and approximately 1000 shallow-depth boreholes with a drilling length of 10 m to 200 m), gravity survey data (26,595 points), seismic observations, and microtremor array observations were newly conducted. Gravity survey data were provided by the Tono Institute of Seismological Science and gravity data compiled by the Geological Survey of Japan (GSJ) of the National Institute of Advanced Industrial Science and Technology (AIST). For seismic observation records, we collected data from K-NET, KiK-net, local governments, the Japan Meteorological Agency, and seismic stations of Nagoya University, and installed seismographs at 20 elementary and junior high schools (KJCS001 to KJCS020) mainly in Aichi and Gifu prefectures. Microtremor array observations were conducted at 40 large arrays with a maximum radius of 400 m and 843 small arrays with a radius of 5 to 10 m (Figure 1).
3. analysis of the basement structure based on the results of gravity analysis and examination of the tilt angle of the fault
Conventional near-surface surveys and reflection surveys have not provided sufficient information on the dip angle of the faults that make up the Enasan fault zone to model them as epicenter faults. To examine this issue, we used the method of Komazawa (1995) to estimate the tilt angles of the faults as normal (30 and 60 degrees), 90 degrees, and reverse (30 and 60 degrees), against the gravity basement elevation estimated from Bouguer anomaly and borehole data in the vicinity of the fault zone, and assumed the structure to be a normal fault, a 90 degrees tilt fault, and a reverse fault (30 and 60 degrees). The influence of the change in the distribution of sedimentary layers on the gravity distribution in each case was calculated by two-dimensional forward calculations, and compared with the observed gravity values. As a result, it was found that the fault case that best explains the results of the gravity survey just above the fault in the Tomita area of Ena City is the reverse fault with an inclination of 30 degrees (Figure 2).
4. construction and validation of the shallow and deep integrated ground structure (SDv2) model
The SD model was developed based on the "Concept of Subsurface Structural Modeling" by the Earthquake Research Institute, using the data collected in Chapter 2. The initial model for the deep subsurface structure model was developed based on the structure of the seismic basement equivalent surface from the gravity basement analysis shown in Chapter 3, and a one-dimensional S-wave velocity structure was developed using seismic records and microtremor array observation data. /The theoretical H/V spectra of the SDv2 model are more consistent with the observed R/V spectra than those of the SIP model, especially at 20 elementary and junior high school sites where new seismic observations were made this time. The validity of the ground structure model was verified by comparing the S-wave amplification characteristics (site characteristics) obtained by spectral inversion analysis with the theoretical amplification characteristics of the SIP and SDv2 models for lowlands and plateaus that are modeled using microtremor array observation records. As a result, the theoretical amplification factor of the SDv2 model was found to be more harmonious with the S-wave amplification characteristics obtained from the spectral inversion analysis than that of the SIP model (Figure 3).
5. Summary
The SDv2 model developed in this study is a more accurate ground structure model around the fault zone than the SIP model because the model was constructed using more survey and observation data in basins and valley floor lowlands near the fault in mountainous areas than the SIP model, and it is possible to estimate the dip angle of the fault.
This report describes the results of the development of the integrated shallow and deep ground structure model for strong ground motion prediction. In this report, based on the shallow and deep integrated ground structure model (hereinafter referred to as the SIP model) for the Tokai region developed by the Strategic Innovation Program (SIP) and others, we newly developed the shallow and deep integrated ground structure model using the results of borehole data collection, gravity data collection and analysis, seismic observation, microtremor array observation and sub-thema3 reflection seismic survey. The shallow and deep integrated ground structure model (hereinafter referred to as SDv2 model) was upgraded based on the SIP model.
2.Data collected
In this study, borehole data were collected (approximately 20 large-depth boreholes with a drilling length of 100 m to 1000 m and approximately 1000 shallow-depth boreholes with a drilling length of 10 m to 200 m), gravity survey data (26,595 points), seismic observations, and microtremor array observations were newly conducted. Gravity survey data were provided by the Tono Institute of Seismological Science and gravity data compiled by the Geological Survey of Japan (GSJ) of the National Institute of Advanced Industrial Science and Technology (AIST). For seismic observation records, we collected data from K-NET, KiK-net, local governments, the Japan Meteorological Agency, and seismic stations of Nagoya University, and installed seismographs at 20 elementary and junior high schools (KJCS001 to KJCS020) mainly in Aichi and Gifu prefectures. Microtremor array observations were conducted at 40 large arrays with a maximum radius of 400 m and 843 small arrays with a radius of 5 to 10 m (Figure 1).
3. analysis of the basement structure based on the results of gravity analysis and examination of the tilt angle of the fault
Conventional near-surface surveys and reflection surveys have not provided sufficient information on the dip angle of the faults that make up the Enasan fault zone to model them as epicenter faults. To examine this issue, we used the method of Komazawa (1995) to estimate the tilt angles of the faults as normal (30 and 60 degrees), 90 degrees, and reverse (30 and 60 degrees), against the gravity basement elevation estimated from Bouguer anomaly and borehole data in the vicinity of the fault zone, and assumed the structure to be a normal fault, a 90 degrees tilt fault, and a reverse fault (30 and 60 degrees). The influence of the change in the distribution of sedimentary layers on the gravity distribution in each case was calculated by two-dimensional forward calculations, and compared with the observed gravity values. As a result, it was found that the fault case that best explains the results of the gravity survey just above the fault in the Tomita area of Ena City is the reverse fault with an inclination of 30 degrees (Figure 2).
4. construction and validation of the shallow and deep integrated ground structure (SDv2) model
The SD model was developed based on the "Concept of Subsurface Structural Modeling" by the Earthquake Research Institute, using the data collected in Chapter 2. The initial model for the deep subsurface structure model was developed based on the structure of the seismic basement equivalent surface from the gravity basement analysis shown in Chapter 3, and a one-dimensional S-wave velocity structure was developed using seismic records and microtremor array observation data. /The theoretical H/V spectra of the SDv2 model are more consistent with the observed R/V spectra than those of the SIP model, especially at 20 elementary and junior high school sites where new seismic observations were made this time. The validity of the ground structure model was verified by comparing the S-wave amplification characteristics (site characteristics) obtained by spectral inversion analysis with the theoretical amplification characteristics of the SIP and SDv2 models for lowlands and plateaus that are modeled using microtremor array observation records. As a result, the theoretical amplification factor of the SDv2 model was found to be more harmonious with the S-wave amplification characteristics obtained from the spectral inversion analysis than that of the SIP model (Figure 3).
5. Summary
The SDv2 model developed in this study is a more accurate ground structure model around the fault zone than the SIP model because the model was constructed using more survey and observation data in basins and valley floor lowlands near the fault in mountainous areas than the SIP model, and it is possible to estimate the dip angle of the fault.