11:00 AM - 11:15 AM
[SCG56-08] Upper crustal deformation of the high strain-rate zone insight from the topographical and geological investigations: An example of the San-in shear zone
Keywords:San-in shear zone (SSZ), Minor fault, Tectonic landform, Active fault, Intraplate earthquake, Strain rate paradox
There are two representative high strain-rate zones in the back-arc region of Honshu island, Japan: Niigata-Kobe Tectonic Zone (NKTZ) and San-in Shear Zone (SSZ). The discrepancy between the total amount of the slip rate of major active faults in the zones and geodetically obtained strain rate has been called “strain-rate paradox”, and has long been debated. Although the map scale active faults are obviously thought to contribute to the deformation of the high strain-rate zones, outcrop scale minor faults are also recognized in previous studies based on the topographical and geological investigations. This observation implies that deformation of the strain-rate zone is not explained by the major active faults alone. The authors have investigated in and outside of the NKTZ based on topographical and geological approaches, demonstrating the role of minor faults for the deformation of the NKTZ (Tamura et al., 2020). In this presentation, we report the topographical and geological characteristics of the SSZ as a next challenge, proposing a general hierarchical structure model of high strain-rate zones on the basis of the comparison between the NKTZ and the SSZ. This study is expected to not only help understanding the comprehensive view of crustal deformation, but also serve social aspects such as construction of radioactive waste disposal and seismic hazards in mobile belt.
The Shikano and the Iwatsubo faults are distributed in the central part of the SSZ. The Shikano fault is known for the major active fault which caused the 1943 Tottori earthquake. Although there are no paleoearthquake records for the Iwatsubo fault, it is one of the map scale active faults in the SSZ, inferred from aerial photograph interpretations. We found fault cores of the Shikano fault and Iwatsubo fault through the topographic analysis and geological investigations. Both fault cores being approximately 30 cm in width and show right-lateral sense of shear, which is consistent with the dextral deformation of the SSZ. Outcrop scale minor faults (a few mm to dozens of cm in width) are also found in the vicinity of map scale active faults, the same as in the case of NKTZ. Although E-W trending minor faults with steep dipping angle tend to be dextral sense of shear, NW-SE to NNW-SSE trending high angle faults with sinistral sense of shear can also be found. On the other hand, minor faults are not frequently occurred outside of the SSZ, and we only confirmed a reverse fault alone.
As for the common characteristics in the NKTZ and SSZ, the minor faults away from the map scale active faults but within the high strain-rate zones have same directions (strike) and sense of shears with those of the high strain-rate zones, which possibly explain the discrepancy between total budget of slip rate of major active faults and geodetical strain rate. Considering these findings, we propose a general hierarchical structure model of high strain-rate zones as follows: (I) fault core of major active faults, (II) damage zone of major active faults, (III) brittle shear zone which is characterized by the concentration of minor faults, (IV) inactive background (outside of high strain-rate zones). However, fault occurrences such as attitude (strike and dip) or width are not exactly the same between two zones. These differences are likely to be formed by local geologic difference/heterogeneity (e.g., lithology, preexisting structure), which may affect on formation and growth of individual fault.
The Shikano and the Iwatsubo faults are distributed in the central part of the SSZ. The Shikano fault is known for the major active fault which caused the 1943 Tottori earthquake. Although there are no paleoearthquake records for the Iwatsubo fault, it is one of the map scale active faults in the SSZ, inferred from aerial photograph interpretations. We found fault cores of the Shikano fault and Iwatsubo fault through the topographic analysis and geological investigations. Both fault cores being approximately 30 cm in width and show right-lateral sense of shear, which is consistent with the dextral deformation of the SSZ. Outcrop scale minor faults (a few mm to dozens of cm in width) are also found in the vicinity of map scale active faults, the same as in the case of NKTZ. Although E-W trending minor faults with steep dipping angle tend to be dextral sense of shear, NW-SE to NNW-SSE trending high angle faults with sinistral sense of shear can also be found. On the other hand, minor faults are not frequently occurred outside of the SSZ, and we only confirmed a reverse fault alone.
As for the common characteristics in the NKTZ and SSZ, the minor faults away from the map scale active faults but within the high strain-rate zones have same directions (strike) and sense of shears with those of the high strain-rate zones, which possibly explain the discrepancy between total budget of slip rate of major active faults and geodetical strain rate. Considering these findings, we propose a general hierarchical structure model of high strain-rate zones as follows: (I) fault core of major active faults, (II) damage zone of major active faults, (III) brittle shear zone which is characterized by the concentration of minor faults, (IV) inactive background (outside of high strain-rate zones). However, fault occurrences such as attitude (strike and dip) or width are not exactly the same between two zones. These differences are likely to be formed by local geologic difference/heterogeneity (e.g., lithology, preexisting structure), which may affect on formation and growth of individual fault.