## 3 Activity Characteristic of Hetan-Guotan Buried Fault
### Composite Drilling Section Exploration in Xietan Village
#### 3.2.2 Comparison of Stratigraphic Lithology  
Based on the core logging of boreholes, we compared the lithological columns of each stratum where the borehole was located. The thick layer of variegated pebble and gravel is distributed over 30 m, and the sand layer and cohesive soil layer alternately appear below the pebble and gravel layer. The color and lithology can be distinguished. There is a significant boundary 50 m below the Earth's surface, dividing the gravelly sand from the cohesive soil. The boundary is broken between Z1-2 and Z1-8, with a fault displacement exceeding 10 m. Due to the rapid lateral variation of the fluvial facies' sedimentary strata, it is difficult to make stratification only by lithology. It is necessary to consider the characteristics of color, stratification and structure to divide the drilled strata of Xietan Village into 6 strata (Figure 4 1 - 6). The strata from top to bottom are as follows:  
Figure 3: The time profile of Xietan Village’s survey line and the layout of boreholes on composite drilling section in Xietan Village. (**a**) A time-stacked section of the shallow seismic line in Xietan Village. Dashed black box shows the location of (**b**). (**b**) The layout of boreholes on composite drilling section in Xietan Village. 1. The upper part of the Stratum 1 is filled with earthly yellow plain fill, which contains plant roots and is moist and loose. The middle part consists of black-red and gray-white sandy clay, the bottom part has much sandy and is filled with light yellow clayey sand. This stratum is distributed 0-5 m below the Earth's surface and the sediment thickness is 4-5 m.
2. The Stratum 2 is a layer of variegated pebble and gravel, which is mainly gray-green and blue-green, followed by maroon and milky white. The pebble and gravel have fine psephicity. The maximum gravel diameter exceeds the drill core diameter by 10.8 cm. This stratum is distributed 5-30 m below the Earth's surface, its thickness decreases from north to south. 3. The upper part of the Stratum 3 is light yellow with gravel-bearing coarse sand and gravelly sand. Some area is dark yellow or dark red. However, the upper parts of Z1-7, Z1-9 and Z1-2 have 1-3 m of dark red cohesive soil, silt and fine sand, its thickness decreases from north to south and finally thins out. The bottom parts of Z1-7, Z1-9, Z1-2 and Z1-10 gradually turn light red. The lower part is filled with light-red or dark-red cohesive soil and some areas have light red medium-fine sand. The bottom boundary of this stratum is different from the upper part of Stratum 4 in terms of gravelly sand's color and lithology. There is a dip of around 30\({}^{\circ}\) at the bottom boundary of Z1-2. This stratum is distributed 30-43 m below the Earth's surface; its thickness decreases from south to north. Figure 4: Stratigraphic lithology of composite drilling section in Xietan Village. 4. The upper part of Stratum 4 mainly consists of light-yellow gravelly sand, interbedded with medium-fine sand, medium-coarse sand or gravel-bearing coarse sand. The bottoms of Z1-8, Z1-4, Z1-3 and Z1-1 are black. The lower part of this stratum is light-red or dark-red cohesive soil sandwiched with thin layers of fine (silty) sand. The bottom boundary of Stratum 4 is obviously different from the light-yellow gravelly sand or light-yellow medium-fine sand in the upper part of Stratum 5 in color and lithology. The sandy clay interbedded with clay has a dip angle of about 20\({}^{\circ}\). This stratum is distributed 43-50 m below the Earth's surface, thick in the middle and thin in the south and north sides. 5. Stratum 5 mainly consists of light-yellow gravelly sand, with dark yellow mud in some areas. Its gravel content is between 25% and 50%, with poor sorting and psephicity. The maximum gravel diameter is about 8 cm. It has medium-fine sand, (medium) coarse sand and gravel-bearing coarse sand interbed.