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探槽附近的地貌学和地质学沿冈村断裂迹线的右旋水系断错和构造隆起显示出位移以左旋分量为主。同时,北盘的晚第四纪阶地累积下沉。我们在冈村断层中段的一个小断层陡坎的麓部横跨断层进行了开挖,这个陡坎致使一个更新世的河流阶地发生断错。虽然这个小断层陡坎在现今冲积扇上没有任何的地形显示,但是,我们根据它的直线连续性推测出冈村断层迹线的位置。该探槽地点以南的地质学由白垩系的和泉组(砂岩)、上新统的冈村建造(含有结晶片岩砾石的砂质粉砂)、第四系中阶地沉积物(以砂充填的片岩砾石—巨砾)、和第四系低阶地沉积物(以砂富集的砂岩砾石—大砾) (图1)。冈村建造和第四纪中阶地沉积物中的砾石层组成物为片岩砾石,它们来源于地质上特殊的中央构造线以南的三波川变质岩。但是,第四纪低阶地沉积物中的砾石层由和泉组砂岩砾石组成。河谷 A、B 和 C(图1)提供了不同类型的砾石,这些砾石与它们源头地质学有关。河谷 A 主要提供片岩砾石,河谷 B 提供片岩和砂岩两种砾石,以及河谷 C 主要提供砂岩砾石。因为开挖探槽的冲积扇组成物质来源于河谷A,所以,探槽的大部分沉积物中均含有片岩砾石。探槽中有一些层位含有可能来源于河谷B和 C 的砂岩砾石,它们作为参照层来计算水平滑动速度。
Geomorphology and geology near the trenches The dextral water system faults and tectonic uplift along the Okamura fracture trace show that the displacement is dominated by left-handed components. At the same time, the northern plate of the late Quaternary accumulated sink. We excavated across the fault at the foot of a small fault scarp in the middle of the Okamura fault that caused a fault in a Pleistocene river terrace. Although this small fault scarp does not show any topography on alluvial fans today, we infer the Okumura fault trace from its straight continuity. The geology south of the trenches is composed of the Cretaceous Hequan Formation (sandstone), the Upper Pleistocene Okamura (sandy silt containing crystalline schist), Quaternary sediments (sand-filled Schist gravel-boulders), and Quaternary low-grade sediments (sand-rich gravel-gravel) (Figure 1). The gravel layer in the Okamura and Quaternary sediments is schist gravel, which originated from Tri-wave rock metamorphic south of the geologically special central tectonic line. However, the gravel layer in Quaternary low-grade sediments consists of the Hequan Formation sandstone gravel. The valleys A, B and C (Figure 1) provide different types of gravel, which are related to their source geology. Valley A provides primarily schist gravel, Valley B offers both schist and sandstone gravel, and Valley C provides sandstone gravel primarily. Because alluvial fan components of the excavation trenches come from valley A, most of the sediment in the trenches contains schist gravel. Some of the trenches contain sandstone gravel that may have originated in valleys B and C and serve as a reference to calculate the horizontal slip velocity.