5:15 PM - 6:30 PM
[SSS10-P07] Parametric Study on Dip Angles of Fault Planes around Lake Suwa Using Finite Element Analysis
Keywords:Lake Suwa, ISTL fault zone, Finite Element Method, Parametric Study
The Suwa Basin is located between the north-central and south-central parts of the Itoigawa-Shizuoka Tectonic Line (ISTL) fault zone and is an important region for discussing both fault displacement and seismic hazard along the ISTL fault zone. The Suwa Basin has long been considered to be a pull-apart basin (e.g., Fujimori 1991), and previous works reported two types of dip angles for the north and south regions of Lake Suwa: east-dipping reverse fault for the north region (Hagiwara et al. 1986; Ikami et al. 1986; Sato et al. 2004), and west-dipping reverse fault for the south region (Hirakawa et al. 1989: Kumamoto and Ikeda 1993; Kano et al. 2004).
These studies imply that the geological structure differs significantly between the northern and southern parts of the Suwa Basin. Therefore, there is considerable scope for discussion on whether the dip angles inferred from previous studies can be applied to the faults around Lake Suwa. Namely, when the Suwa Basin is assumed to be a pull-apart basin created by fault displacement, numerical faulting simulation with appropriate tectonic stress should produce a displacement field that causes subsidence in the area including Lake Suwa. In this context, we estimate the dip angles of the fault planes that form the Suwa Basin using three-dimensional finite element analysis implemented on COMSOL Multiphysics (finite element based multiphysics simulator).
First, we modeled four-fault planes around Lake Suwa and then performed parametric studies by changing the dip angle of each of the four-fault planes from 70° to 110° in 5° increments (6561 cases in total). The results of the parametric study showed the combinations of the four dip angles that can produce subsidence resulting in Lake Suwa. Also, the parametric study revealed that the fault planes, which all have dip angles less than 90° decline toward the center of the Suwa Basin, cannot form a pull-apart basin, resulting in pop-ups.
Another example in Japan is the Median Tectonic Line Fault Zone in the central part of Shikoku. We are planning to study this fault zone as future work.
Acknowledgments:
This study was conducted as a part of "Research Project for Long-term Evaluation Methods of Active Faults related to Multi-segment Earthquakes" and "Research Project for Long-term Evaluation Methods of Multi-segment Earthquakes from Active Fault Zones" commissioned by the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors would like to thank the projects for their support.
References:
Fujimori, T. 1991. “Active Faults in the Suwa Basin, and Its Evolution as a Pull-Apart Basin on the Itoigawa-Shizuoka Tectonic Line, Central Japan” Geographical Review of Japan. Ser. A 64 (10): 665–96.
Hagiwara et al. 1986. “Gravity Observations along the Itoigawa-Shizuoka Geotectonic Line (I): An Evaluation of the Bouguer Anomaly in the Matsumoto Basin, Nagano Prefecture, Central Japan” Bulletin of the Earthquake Research Institute, University of Tokyo 61: 537-550.
Hirakawa et al. 1989. “Description of active fault at Shimotsuburai, northeastern side of the Koma -mountains, Yamanashi Prefecture, Central Japan” Japanese Society for Active Fault Studies 6: 43-46.
Ikami et al. 1986. “A Seismic-Refraction Profile in and around Nagano Prefecture, Central Japan.” Journal of Physics of the Earth 34 (6): 457–74.
Kano et al. 2004. “Pseudotachylytes of crushing origin from the Shimotsuburai fault of the Itoigawa-Shizuoka Tectonic Line active fault system, central Japan” THE JOURNAL OF THE GEOLOGICAL SOCIETY OF JAPAN 110(12), 779-790.
Kumamoto and Ikeda. 1993. “Fault Geometry and Net Slip of Low Angle Thrust Faults in the Kofu Basin, Central Japan” Journal of the Seismological Society of Japan. 2nd ser. Zisin 46 (3): 245–58.
Sato et al. 2004. “Seismological and Geological Characterization of the Crust in the Southern Part of Northern Fossa Magna, Central Japan.” Earth, Planets and Space 56 (12): 1253–59.
These studies imply that the geological structure differs significantly between the northern and southern parts of the Suwa Basin. Therefore, there is considerable scope for discussion on whether the dip angles inferred from previous studies can be applied to the faults around Lake Suwa. Namely, when the Suwa Basin is assumed to be a pull-apart basin created by fault displacement, numerical faulting simulation with appropriate tectonic stress should produce a displacement field that causes subsidence in the area including Lake Suwa. In this context, we estimate the dip angles of the fault planes that form the Suwa Basin using three-dimensional finite element analysis implemented on COMSOL Multiphysics (finite element based multiphysics simulator).
First, we modeled four-fault planes around Lake Suwa and then performed parametric studies by changing the dip angle of each of the four-fault planes from 70° to 110° in 5° increments (6561 cases in total). The results of the parametric study showed the combinations of the four dip angles that can produce subsidence resulting in Lake Suwa. Also, the parametric study revealed that the fault planes, which all have dip angles less than 90° decline toward the center of the Suwa Basin, cannot form a pull-apart basin, resulting in pop-ups.
Another example in Japan is the Median Tectonic Line Fault Zone in the central part of Shikoku. We are planning to study this fault zone as future work.
Acknowledgments:
This study was conducted as a part of "Research Project for Long-term Evaluation Methods of Active Faults related to Multi-segment Earthquakes" and "Research Project for Long-term Evaluation Methods of Multi-segment Earthquakes from Active Fault Zones" commissioned by the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors would like to thank the projects for their support.
References:
Fujimori, T. 1991. “Active Faults in the Suwa Basin, and Its Evolution as a Pull-Apart Basin on the Itoigawa-Shizuoka Tectonic Line, Central Japan” Geographical Review of Japan. Ser. A 64 (10): 665–96.
Hagiwara et al. 1986. “Gravity Observations along the Itoigawa-Shizuoka Geotectonic Line (I): An Evaluation of the Bouguer Anomaly in the Matsumoto Basin, Nagano Prefecture, Central Japan” Bulletin of the Earthquake Research Institute, University of Tokyo 61: 537-550.
Hirakawa et al. 1989. “Description of active fault at Shimotsuburai, northeastern side of the Koma -mountains, Yamanashi Prefecture, Central Japan” Japanese Society for Active Fault Studies 6: 43-46.
Ikami et al. 1986. “A Seismic-Refraction Profile in and around Nagano Prefecture, Central Japan.” Journal of Physics of the Earth 34 (6): 457–74.
Kano et al. 2004. “Pseudotachylytes of crushing origin from the Shimotsuburai fault of the Itoigawa-Shizuoka Tectonic Line active fault system, central Japan” THE JOURNAL OF THE GEOLOGICAL SOCIETY OF JAPAN 110(12), 779-790.
Kumamoto and Ikeda. 1993. “Fault Geometry and Net Slip of Low Angle Thrust Faults in the Kofu Basin, Central Japan” Journal of the Seismological Society of Japan. 2nd ser. Zisin 46 (3): 245–58.
Sato et al. 2004. “Seismological and Geological Characterization of the Crust in the Southern Part of Northern Fossa Magna, Central Japan.” Earth, Planets and Space 56 (12): 1253–59.