10:00 AM - 10:15 AM
[SSS11-10] Update of Deep Subsurface Structural Model around Morimoto-Togashi Fault Zone, Hokuriku, Japan
Keywords:Morimoto-Togashi Fault Zone, Deep subsurface velocity struture model, Strong motion prediction, Microtremor array survey
The Morimoto-Togashi fault zone is a major reverse fault zone forming the boundary between the Kaga Plain and hills in Ishikawa Prefecture. The probability of an M7.2 earthquake occurring within 30 years is 2-8% in the long-term evaluation by Earthquake Research Committee (2025). A wide area of the surrounding region, including the Tonami Plain as well as the Kaga Plain, is expected to be hit by strong ground motions. According to NIED J-SHIS, the population that will encounter ground motions of intensity 5+ or higher is assessed to be approximately 1.5 million (NIED, 2024).
DPRI of Kyoto University, in collaboration with Kanazawa University, the ERI of the University of Tokyo, GSJ/AIST and NIED, and researchers from affilated institutions, is conducting a three-year research project on the Morimoto-Togashi Fault Zone from FY2022. In sub-theme 3 of this project, we have been conducting temporary strong-motion observations around this fault zone (Asano, et al., 2024) and developing a shallow / deep integrated subsurface structure model for Ishikawa and western Toyama prefectures, with the aim of improving strong-motion prediction. In this presentation, we report on the update of the deep subsurface velocity structure model in the depth range from the seismic bedrock to the engineering bedrock (VS = 350 m/s).
Microtremor array surveys with large radii up to about 2,000 m have been conducted at 11 sites. Single-station microtremor observations were conducted at all strong-motion or seismic intensity stations in all of Ishikawa Prefecture and western Toyama Prefecture. Data from microtremor array surveys and H/V observations conducted in other previous studies (Kanno et al., 2003; Yamanaka et al., 2008; Asano et al., 2009; Horikawa et al., 2010, Asano et al., 2020; Suzuki et al., 2021; Iwata et al., 2022) were also collected and were used for updating the velocity model. Reflection and refraction seismic survey data and reanalyzed data of existing survey lines conducted by ERI in this project and other projects were also referenced.
The deep subsurface velocity structure model was updated by referring to the J-SHIS V4 model as the initial model. First, the phase velocity dispersion curves, which were modeled as the fundamental mode of the Rayleigh wave, and H/V spectral ratios were used to adjust the layer thickness of each velocity layer in the 1D S-wave velocity structure model at each site. For example, a layer with S-wave velocity of 0.7 km/s was added around the Kaga Plain to better explain the phase velocity dispersion curves, although the existing J-SHIS V4 model has layers with S-wave velocities of 0.6, 1.1, 1.7, 2.1, and 2.7 km/s. This resulted in layered velocity models that adequately reproduced the phase velocity and H/V spectral ratio at many points. Based on the modified model at each point, we updated the 3D deep subsurface velocity structure model by interpolating the modifications areally while also considering gravity anomalies and surface geological information. The model was verified and adjusted by comparing it with velocity profiles from reflection and refraction surveys. As a result, the velocity structure model above the seismic bedrock is now more detailed than before.
We also plan to verify the model by simulating earthquake ground motions observed at temporary strong-motion stations and seismic intensity stations in the area.
Acknowledgments: This study was conducted in the Comprehensive Research Project for the Morimoto-Togashi Fault Zone funded by MEXT. Microtremor array observations were conducted with the participation of graduate students from Kyoto University. Single-station microtremor observations were conducted with the cooperation of the Geo-Research Institute. We would like to express our sincere thanks to the researchers who provided phase velocity dispersion curves and other data, and to local governments, local meteorological offices, and local residents for their cooperation in the field observations.
DPRI of Kyoto University, in collaboration with Kanazawa University, the ERI of the University of Tokyo, GSJ/AIST and NIED, and researchers from affilated institutions, is conducting a three-year research project on the Morimoto-Togashi Fault Zone from FY2022. In sub-theme 3 of this project, we have been conducting temporary strong-motion observations around this fault zone (Asano, et al., 2024) and developing a shallow / deep integrated subsurface structure model for Ishikawa and western Toyama prefectures, with the aim of improving strong-motion prediction. In this presentation, we report on the update of the deep subsurface velocity structure model in the depth range from the seismic bedrock to the engineering bedrock (VS = 350 m/s).
Microtremor array surveys with large radii up to about 2,000 m have been conducted at 11 sites. Single-station microtremor observations were conducted at all strong-motion or seismic intensity stations in all of Ishikawa Prefecture and western Toyama Prefecture. Data from microtremor array surveys and H/V observations conducted in other previous studies (Kanno et al., 2003; Yamanaka et al., 2008; Asano et al., 2009; Horikawa et al., 2010, Asano et al., 2020; Suzuki et al., 2021; Iwata et al., 2022) were also collected and were used for updating the velocity model. Reflection and refraction seismic survey data and reanalyzed data of existing survey lines conducted by ERI in this project and other projects were also referenced.
The deep subsurface velocity structure model was updated by referring to the J-SHIS V4 model as the initial model. First, the phase velocity dispersion curves, which were modeled as the fundamental mode of the Rayleigh wave, and H/V spectral ratios were used to adjust the layer thickness of each velocity layer in the 1D S-wave velocity structure model at each site. For example, a layer with S-wave velocity of 0.7 km/s was added around the Kaga Plain to better explain the phase velocity dispersion curves, although the existing J-SHIS V4 model has layers with S-wave velocities of 0.6, 1.1, 1.7, 2.1, and 2.7 km/s. This resulted in layered velocity models that adequately reproduced the phase velocity and H/V spectral ratio at many points. Based on the modified model at each point, we updated the 3D deep subsurface velocity structure model by interpolating the modifications areally while also considering gravity anomalies and surface geological information. The model was verified and adjusted by comparing it with velocity profiles from reflection and refraction surveys. As a result, the velocity structure model above the seismic bedrock is now more detailed than before.
We also plan to verify the model by simulating earthquake ground motions observed at temporary strong-motion stations and seismic intensity stations in the area.
Acknowledgments: This study was conducted in the Comprehensive Research Project for the Morimoto-Togashi Fault Zone funded by MEXT. Microtremor array observations were conducted with the participation of graduate students from Kyoto University. Single-station microtremor observations were conducted with the cooperation of the Geo-Research Institute. We would like to express our sincere thanks to the researchers who provided phase velocity dispersion curves and other data, and to local governments, local meteorological offices, and local residents for their cooperation in the field observations.