## 4 Results
### Erjiayan Village (Site 2)
#### 4.2.2 Evidence for Fault Activity and Parameters  
At Site 2, a trench was excavated at the eastern end of the scarp, where the surface linearity is most distinct, to obtain information about the fault profile. The EJY trench intersects the surface fault scarp, which trends SE152\({}^{\circ}\), and is located at GPS coordinates 41\({}^{\circ}\)08\({}^{\prime}\)57.21\({}^{\prime\prime}\) N, 113\({}^{\circ}\)00\({}^{\prime}\)28.60\({}^{\prime\prime}\) E.  
Six units were defined within the west wall of the HQB trench (Figure 9), and the specific descriptions of the stratigraphic units are as follows:  
1. U1: Alluvial layer, orange-yellow to brown, containing coarse to very large gravels, poorly sorted and poorly rounded. Local mud content shows slight horizontal bedding, with relatively strong consolidation.
2. U2: Alluvial deposits, yellow-brown, composed of muddy coarse sand with fine gravels. Poorly rounded, lacking stratification, and relatively well consolidated.
3. U3: Alluvial layer, gray-white to earthy yellow, exhibiting some directional features. Medium-sized gravels, poorly rounded, with slight horizontal bedding visible in local muddy areas. Relatively well consolidated.
4. U4: Alluvial layer, reddish-brown to gray-brown, moderately sorted, poorly rounded, with large-sized gravels, and loosely consolidated. This layer shows evidence of sediment deformation, overlying U1, U2, and U3, likely caused by paleoseismic activity.
5. U5: Loess layer, earthy yellow, silt-like, lacking stratification, and tightly consolidated.
6. U6: Paleosol layer, dark brown, containing medium to fine gravels, with a soil-like texture, and loosely consolidated.  
The trench profile revealed evidence of multiple alluvial deposition events, characteristic of alluvial fan sedimentary layers (U1, U2, U3, and U4). Although no distinct fault outcrop was observed, significant deformation of sedimentary layers was identified in front of the scarp (Figure 9).  
No clear fault plane traces were observed in the trench profile, but the sedimentary characteristics of U1 and U3 alluvial layers are consistent, with both exhibiting some stratification. However, stratification dips of U1 are more steeply southward than U3. Sedimentary units U1, U2, and U3 exhibit deposition gaps, likely caused by fault activity burying the southern strata within the basin. U4 shows significant flexural deformation, with changes in sedimentary morphology that may have been caused by paleoseismic events. U5 also exhibits typical scarp-front sedimentary characteristics, with a sudden increase in thickness at the front edge of the scarp. Although no distinct fault plane was identified in the trench, the deformation caused by fault activity is clearly recorded in the trench stratigraphy. To further refine the timing of fault activity, this study collected an OSL  
Figure 8: Orthomosaic image (**A**) and DEM (**B**) at Site 2 based on UAV data.  
sample (WOSL-11) from the bottom of the U5 loess layer (Figure 9), which can define the most recent age of the activity of the fault. OSL dating results indicate that the sediment age of the U5 loess layer is 15.5 \(\pm\) 0.9 ka B.P., before the Holocene.  
Since no fault outcrop was revealed in the trench, a GPR survey line was deployed perpendicular to the linear scrap on the east side of the trench to identify the morphological and structural features of the fault in the bedrock layer. At 160 m along the GPR profile (EJY trench location), a poorly coherent reflection anomaly was observed, interpreted as structural deformation caused by fault activity. The fault's upper break point is at a depth of 3-4 m, without disturbing the near surface alluvial deposits. The vertical offset along the fault trace is approximately 1 m (Figure 10). This confirms the existence of the JRNF, with its most recent activity likely occurring before the deposition of U5 in the EJY trench.