5:15 PM - 6:45 PM
[SSS11-P08] Drilling surveys across the estimated fault scarp of Tatsutayama fault passing the Kumamoto Castle Park, Kumamoto Prefecture, southwest Japan
Keywords:Tatsutayama fault, boring core, active fault, paleoseismology, Kumamoto Prefecture
The Tatsutayama Fault is an active fault that extends in a northeast-southwest direction in Kumamoto City, Kumamoto Prefecture, causing the northwest side of the fault to relatively subside. This fault was described by Watanabe (1984, 1987) in terms of fault traces and fault outcrops, and an active fault investigation was conducted by Kumamoto Prefecture in 1996. According to Watanabe (1984), the Tatsutayama Fault is an active fault approximately 14 km long. However, the Tatsutayama Fault shown as in the Active Fault Map of GSI (Kumahara et al., 2017) is limited to an area of about 2.6 km at the western foot of Tatsutayama. It is important to obtain data on the fault's past activity at multiple locations and examine the fault's length and distribution to evaluate and mitigate the seismic hazards of the Tatsutayama Fault.
In the northern part of Kumamoto Castle Park, there is a cliff terrain estimated to be a fault cliff caused by the Tatsutayama Fault. Reports by Kumamoto Prefecture (1996) and the Kumamoto Castle Research Center (2019) indicate that on the north side of the cliff terrain, the elevation of Quaternary deposits and pyroclastic materials cumulatively decreases (Ogami et al., 2024). Such geological structures suggest that this cliff terrain is estimated to be a fault cliff. Following this background, we conducted drilling surveys at two locations across the cliff terrain in the northern part of Kumamoto Castle Park in 2023FY (Ota et al., 2024).
Two drilling core samples were collected: 74 m long at the south side (uplifted side, GS-KMJ-1) and 114 m long at the north side (subsided side, GS-KMJ-2). We measured a X-ray CT scan, and photographed, observed macroscopically for lithofacies, and sampled for various analyses.
The stratigraphy at both locations is generally consistent, dividing into five major lithofacies units (A to E) (Ota et al., 2024). Unit A, forming the surface, consists of tuffaceous sand to silt and fill affected by human modifications since the construction of Kumamoto Castle. Unit B is composed of Aso-4 pyroclastic flow deposits, with the upper part weathered to brown and the lower part fresh black to dark gray. Unit C is made up of Aso-3, showing similar coloration patterns to Unit B. Unit D, found only on the north side (GS-KMJ-2), consists of mudstone deposits with parallel laminations and sandy deposits showing cross-laminations and upward coarsening. Unit E comprises gravels and pyroclastic material.
Similar to existing drilling data, the base levels of Units B and C cumulatively decrease on the north side (subsided side). Mudstone deposits corresponding to Unit D have been identified in existing drilling samples on the south side of GS-KMJ-1, suggesting a sporadic distribution of Unit D. Although existing drilling surveys have identified the Togawa lava flow below Unit D, it was not found at the two sites drilled in this study, suggesting topographical undulations around the survey locations at the time of the Togawa lava flow.
The new drilling surveys at two locations have corroborated the geological structures around the cliff terrain developed in the northern part of Kumamoto Castle Park, based on existing survey results. This presentation discusses the depositional environments and formation periods of each lithofacies unit based on volcanic ash analysis, microfossils analysis, and elemental analysis.
References
Kumahara et al. (2017) 1:25,000 Active Fault Map “Kumamoto”.
Kumamoto Castle Research Center (2019) “Report of Geological survey for the Ninomaru and other areas of the Kumamoto castle.”
Kumamoto Prefecture (1996) “Report of Active fault survey of Futatagwa fault zone and Tatsutayama fault.”
Ogami et al. (2024) JpGU2024.
Ota et al. (2024) Annual Report on Active Fault and Paleoearthquake Researchs.
Watanabe (1984) Journal of the Kumamoto Geoscience Association.
Watanabe (1987) Journal of the Kumamoto Geoscience Association.
In the northern part of Kumamoto Castle Park, there is a cliff terrain estimated to be a fault cliff caused by the Tatsutayama Fault. Reports by Kumamoto Prefecture (1996) and the Kumamoto Castle Research Center (2019) indicate that on the north side of the cliff terrain, the elevation of Quaternary deposits and pyroclastic materials cumulatively decreases (Ogami et al., 2024). Such geological structures suggest that this cliff terrain is estimated to be a fault cliff. Following this background, we conducted drilling surveys at two locations across the cliff terrain in the northern part of Kumamoto Castle Park in 2023FY (Ota et al., 2024).
Two drilling core samples were collected: 74 m long at the south side (uplifted side, GS-KMJ-1) and 114 m long at the north side (subsided side, GS-KMJ-2). We measured a X-ray CT scan, and photographed, observed macroscopically for lithofacies, and sampled for various analyses.
The stratigraphy at both locations is generally consistent, dividing into five major lithofacies units (A to E) (Ota et al., 2024). Unit A, forming the surface, consists of tuffaceous sand to silt and fill affected by human modifications since the construction of Kumamoto Castle. Unit B is composed of Aso-4 pyroclastic flow deposits, with the upper part weathered to brown and the lower part fresh black to dark gray. Unit C is made up of Aso-3, showing similar coloration patterns to Unit B. Unit D, found only on the north side (GS-KMJ-2), consists of mudstone deposits with parallel laminations and sandy deposits showing cross-laminations and upward coarsening. Unit E comprises gravels and pyroclastic material.
Similar to existing drilling data, the base levels of Units B and C cumulatively decrease on the north side (subsided side). Mudstone deposits corresponding to Unit D have been identified in existing drilling samples on the south side of GS-KMJ-1, suggesting a sporadic distribution of Unit D. Although existing drilling surveys have identified the Togawa lava flow below Unit D, it was not found at the two sites drilled in this study, suggesting topographical undulations around the survey locations at the time of the Togawa lava flow.
The new drilling surveys at two locations have corroborated the geological structures around the cliff terrain developed in the northern part of Kumamoto Castle Park, based on existing survey results. This presentation discusses the depositional environments and formation periods of each lithofacies unit based on volcanic ash analysis, microfossils analysis, and elemental analysis.
References
Kumahara et al. (2017) 1:25,000 Active Fault Map “Kumamoto”.
Kumamoto Castle Research Center (2019) “Report of Geological survey for the Ninomaru and other areas of the Kumamoto castle.”
Kumamoto Prefecture (1996) “Report of Active fault survey of Futatagwa fault zone and Tatsutayama fault.”
Ogami et al. (2024) JpGU2024.
Ota et al. (2024) Annual Report on Active Fault and Paleoearthquake Researchs.
Watanabe (1984) Journal of the Kumamoto Geoscience Association.
Watanabe (1987) Journal of the Kumamoto Geoscience Association.