## 4 Activity Characteristic of Zhenjing-Zhenbei Buried Fault
### Composite Drilling Section Exploration in Lizui Village
#### 4.2.3 Marker Bed and Fault Location  
The marker bed refers to the same stratum that is continuously distributed in the section and is significantly different from the upper and lower strata. The cohesive soil (including clay, clayey sand and sandy clay) in the fluvial strata of Zhongwei Basin is generated under weak hydrodynamic force and a stable depositional environment. It is hard with a high and reliable core recovery percentage. Therefore, we selected a cohesive soil layer with a certain thickness and stable distribution as the marker bed.  
We figured out the location of the fault of the composite drilling section in Lizui Village by locating two obvious marker beds in Stratum 2 (Table 4). The brief introduction is as follows:  
Marker Bed B1 mainly consists of light red sandy clay and clay. Some areas have a thin layer of clayey sand. The thickness of Z2-1 and Z2-3 is between 4.30 to 4.52 m and the thickness of Z2-4 and Z2-5 near the footwall of the fault is 1.19-1.30 m. This marker bed of Z2-2 in the hanging wall of fault drops by about 3.0 m and its thickness increases slightly. Z2-6 is about 2.0 m higher than Z2-2, but about 0.5 m lower than Z2-5.  
Marker Bed B2 mainly consists of light red clay, with thin layers of sandy clay and fine sand in some areas. Z2-1 and Z2-3 are thick, ranging from 1.93 m to 2.16 m, while Z2-4 and Z2-5 near the footwall of the fault are thin, ranging from 0.71 m to 1.42 m. This marker bed of Z2-2 in the hanging wall of fault drops by about 4.0 m and its thickness increases significantly. Z2-6 is about 3.0 m higher than Z2-2, but about 1.5 m lower than Z2-5.  
The stratigraphic comparison shows that the bottom boundary of Stratum 2 (the top boundary of Stratum 2) is not broken and overlaid on the fault. The two marker beds in Stratum 2 (\(\hat{\texttt{\char 3}}\) are broken and reverse faults are developed. The main fault is located between Z2-5 and Z2-2 and the reverse secondary fault is located between Z2-2 and Z2-6. The marker beds of Z2-2 descends and thickens, indicating that the fault is normal. According to Table 3, the bottom boundary of Stratum 2 (\(\hat{\texttt{\char 3}}\) is 71.01-72.91 m, so the position of the uppermost point of the fault is not higher than 71 m.  
Taken together, the apparent dip of the main fault is 67\({}^{\circ}\) and the normal fault dips to the north. The uppermost point of the fault is buried at a depth of more than 71 m. The uppermost point is projected to the surface between Z2-5 and Z2-2, approximately 50 m to the north of the position identified by the shallow seismic exploration.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Marker Bed** & **Bureied Depth** & **Borehole Number** & **Z2-1** & **Z2-3** & **Z2-4** & **Z2-5** & **Z2-2** & **Z2-6** & **Z2-7** & **B** \\ \hline \multirow{4}{*}{Stratum 2} & top boundary/m & 68.10 & 66.75 & 67.41 & 67.53 & 67.61 & 68.22 & 67.66 & 69.22 \\  & bottom boundary/m & 71.50 & 72.05 & 72.83 & 72.91 & 72.56 & 72.65 & 71.01 & 73.17 \\  & thickness/m & 3.40 & 5.30 & 5.42 & 5.38 & 4.95 & 4.43 & 3.35 & 3.95 \\ \hline \multirow{4}{*}{B1} & top boundary/m & 71.84 & 73.18 & 75.21 & 74.79 & 77.30 & 75.14 & \(-\) & \(-\) \\  & bottom boundary/m & 76.14 & 77.70 & 76.40 & 76.09 & 79.24 & 76.75 & \(-\) & \(-\) \\  & thickness/m & 4.30 & 4.52 & 1.19 & 1.30 & 1.94 & 1.61 & \(-\) & \(-\) \\ \hline \multirow{4}{*}{B2} & top boundary/m & 79.38 & 79.89 & 79.16 & 78.71 & 82.07 & 80.17 & \(-\) & \(-\) \\  & bottom boundary/m & 81.54 & 81.82 & 79.87 & 80.13 & 84.47 & 82.44 & \(-\) & \(-\) \\ \cline{1-1}  & thickness/m & 2.16 & 1.93 & 0.71 & 1.42 & 2.40 & 2.27 & \(-\) & \(-\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: The buried depth and thickness of Stratum 2 and marker beds revealed by composite drilling section in Lizui Village.