## 3 Activity Characteristic of Hetan-Guotan Buried Fault
### Composite Drilling Section Exploration in Xietan Village
#### 3.2.3 Marker Bed and Fault Location  
During the composite drilling section exploration, the marker bed is the main basis for judging the fault location, fault displacement and the burial depth of the uppermost point. It has a stable horizon and its lithology and color are easily identified. The composite drilling section in Xietan Village revealed four stable and comparable marker beds, which are used as the basis for studying the fault location and the burial depth of the uppermost point of the fault. The burial depth and thickness are shown in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Marker Bed**} & \multirow{2}{*}{**Buried Depth and Thickness**} & \multirow{2}{*}{**Z1-7**} & \multirow{2}{*}{**Z1-9**} & \multirow{2}{*}{**Z1-2**} & \multirow{2}{*}{**Z1-10**} & \multirow{2}{*}{**Z1-8**} & \multirow{2}{*}{**Z1-6**} & \multirow{2}{*}{**Z1-4**} & \multirow{2}{*}{**Z1-3**} & \multirow{2}{*}{**Z1-1**  
Marker Bed B1 consists of light-yellow gravel-bearing coarse sand and gravelly sand. Some areas are dark yellow or dark red. Its gravel content is about 25%-50%, with poor sorting and psephicity. The maximum gravel diameter is around 8 cm. It has a thin layer compositing light yellow medium-fine sand or coarse sand. This marker layer is predominantly yellow, with coarse sand and gravelly sand as the main lithology. Its thickness ranges from 5.96-12.33 m, decreasing from south to north. The thickness of Z1-1 is only 5.96 m, which is probably due to the erosion of the upper pebble and gravel layer. The thickness of Z1-6 is 7.99 m, which is probably due to the deformation of the stratum by the lower fault. The thickness of the other drill holes is between 8.21-12.33 m.  
Marker Bed B2 consists of light red/dark red cohesive soil, with a thin layer of light red medium-fine sand in some areas. The upper part of Z1-7, Z1-9, Z1-2 and Z1-10 has light red gravelly sand or gravel-bearing coarse sand. Given that the depositional environment in the same period is the same, it is appropriate to include them in this stratum. Its thickness is 2.95-5.70 m in the boreholes on the hanging wall of the fault and 1.18-4.22 m in the boreholes on the football of the fault. In general, the thickness of the hanging wall is greater than that of the football and the mark bed in Z1-8 and Z1-6 is (dark) black-red. Stratum 4 is regarded as the Marker Bed B3, mainly due to the significant thickening of the light red/dark red cohesive soil layers in the lower part of Z1-7, Z1-9, Z1-2 and Z1-10 and little difference between the hanging wall and foot wall of B3's bottom boundary. Its thickness ranges from 6.17 m to 9.68 m in boreholes, increasing slightly from south to north. The boundary between the gravelly sand layer of Z1-8, Z1-6 and Z1-4 and cohesive soil is black. It can be taken as a typical feature of the same stratum and the football is slightly thicker than the hanging wall. Marker Bed B4, i.e., Stratum 5, which is mainly light yellow and partially dark yellow, with distinct color and lithology. Its thickness ranges from 12.72 m to 14.35 m in the Z1-7, Z1-9 and Z1-2 on the hanging wall of the fault and from 4.51 to 6.64 m in Z1-6, Z1-4, Z1-3 and Z1-1 on the football of the fault. The hanging wall is thicker than the football, which facilitates the fault location. The comparison results of the lithological column indicate that Marker Bed B1 is not broken. The top boundary of Marker Bed B2 is unbroken horizontally, while its bottom boundary is broken between Z1-8 and Z1-6. The bottom boundary of Marker Bed B3 is broken between Z1-8 and Z1-6. The fault is presumed to pass through the gravelly sand in Marker Bed B4 of the Z1-8 and the fault surface of this section is not preserved. In Z1-10, the fault may pass right through the boundary (60.80 m below the Earth's surface) between the gravelly sand and clayey sand. Only 63% of the cores were extracted due to abrasion of the gravels in the gravelly sands. Taken together, the apparent dip of the fault is 78\({}^{\circ}\) and dips to the south.