## 4 Results
### Spatial Distribution of relocated Earthquakes  
Current seismic monitoring data indicate that this region is seismically active, with numerous events occurring in the middle and lower crust [18; 64]. This suggests that the deep structures of the Linfen Basin exert a significant influence on seismic activity.  
Figure 5: Deep seismic profile A–A’ and the structural explanation. (**a**) Deep seismic profile A–A’ and (**b**) interpretation scheme for the deep seismic profile. The interpretation was modified based on the solutions provided by [PERSON] et al. (2020) and [PERSON] et al. (2020). The red solid lines represent faults, and the red arrows indicate the direction of fault movement. The blue dashed line represents the boundary between the middle and lower crust, the pink dashed line represents the Moho, and the pink band represents the crust–mantle transition zone. The purple line represents the Quaternary bottom boundary (TQ), the blue line represents the Neogene bottom boundary (TN), and the black line represents the basement of the sedimentary cover (Tg). F3: Luoyunshan Frontal Fault; F6: Fenxi Fault; F7: Quting Fault; F9: Dayang West Fault; F10: Dayang East Fault; F2: Fushan Fault; FX: Xizuo Fault.  
In this study, three velocity profiles (B-B', C-C', and D-D') were selected. Profiles C-C' and D-D' pass through the epicentral locations of the 1303 Hongtong and 1695 Linfen earthquakes. Profile B-B' crosses the entire basin in a nearly N-S direction (Figures 3 and 4). The detection depth is \(\sim\)50 km, which shows the distributions of velocity structures at different depths. A distinct low-velocity body (Vp = 6.2 km/s) is present at a depth of 10-20 km in the southern basin. This low-velocity body is distributed between the Linfen Sag and Xiangfen Uplift (Figure 6).  
Most small earthquakes in this region occurred in the Linfen Basin, and only a few occurred in mountainous areas (Figure 3). With the Hongtong-Subao Fault (F4) as a boundary, small earthquakes were concentrated north of the fault, accounting for \(\sim\)36.5% of all earthquakes. Small earthquakes were more dispersed south of the fault, accounting for \(\sim\)63.5% of all earthquakes (Figure 3).  
We projected all small earthquakes within the Linfen Basin onto the velocity structure profile (B-B') to investigate the depth distribution of the earthquakes (Figure 6). The 1695 Linfen earthquake occurred in the Linfen Sag in the southern part of the basin. Earthquakes were relatively infrequent in this region, and small earthquakes were more dispersed. The predominant rupture layer was located at a depth of 15-25 km, and some clustering was observed below the low-velocity body (Figures 6 and 7). The 1303 Hongtong earthquake occurred in the Hongtong Sag in the northern part of the basin. Earthquakes in this area occurred more frequently, with small earthquakes being more densely distributed. The predominant rupture layer for the earthquakes was located at a depth of 25-35 km (Figures 6 and 7). The two large earthquakes occurred in different velocity structural zones. Based on the spatial distributions of small earthquakes and velocity variations, it can be inferred that the northern and southern parts of the basin have different seismogenic environments.  
Figure 6: P-wave velocity tomographic imaging of profile B–B’. The brown spheres represent small earthquakes in the region from 1981 to 2018, the green spheres represent the 1695 Linfen \(M\) 7.8 earthquake, and the yellow spheres represent the 1303 Hongtong \(M\) 8 earthquake. F4: Hongtong–Subao Fault.