# Crustal Structure of Hainan Island and Surrounding Seabed Based on High-Resolution Airborne Gravity
## 5 Discussion
### Crustal Structure Features and Seismic Activity  
Located on the southern margin of the South China Block, Hainan Island is situated within a stress environment influenced by the collisional extrusion of the Tibetan Plateau and the westward subduction of the Philippine Sea Plate. The decomposed Bouguer gravity anomaly map and Moho depth map (Figure 7) reflect the crustal mass distribution. To further investigate the relationship between the crustal density anomaly and seismic activities in the study area, we collected natural seismic data from 2000 from the official website of the Hainan Earthquake Bureau. The distribution of focal depths relative to the Moho surface indicates that deeper earthquakes predominantly occur along Moho gradient zones. Figure 7 characterizes these zones by significant vertical shear stress and strong vertical differential motion. Decomposed gravity anomalies D4 (Figure 3) in the BBGB and northern Hainan Island exhibit NW-trending dislocation trace earthquake epicenters mainly concentrated along the NE boundary faults in the basin. On the D5 and D6 decomposed results, NE-trending faults appear as gravity anomaly gradients, indicating that seismic activity in the BBGB occurs due to the interaction of NE-trending and NW-trending faults. In the QDNB, [PERSON] et al. [23] have demonstrated deep material upwelling beneath the Qiongdongnan earthquake cluster. Our study also reveals high-density anomalies consistent with [PERSON] et al.'s findings, indicating that the earthquake cluster is mainly driven by deep material activities.  
The high gravity anomaly observed in the northern part of Hainan Island is likely attributable to the upwelling of deep material driven by the Hainan mantle plume. The high gravity anomaly is related to seismic activity in northern Hainan Island, as shown in Figure 8. D5 (Figure 4) and D6 (Figure 5) reveal a prominent nearly NNE-trending high gravity anomaly zone beneath the volcanic earthquake cluster in the northern part of Hainan Island. This feature indicates the presence of a high-density body that protrudes relative to its surroundings, evidenced by both the D5 and D6 wavelet decomposition results. To interpret the results, we built a conceptual model in Figure 8. The Moho interface also exhibits pronounced uplift in this region, suggesting the potential upward intrusion of mantle material into the lower crust. Extensive Quaternary volcanic rocks are observed in the shallow subsurface [57; 58], indicating an intense mantle-crust interaction in this region. The deep structure of northern Hainan Island is prone to stress accumulation, leading to significant historical events such as the 1605 Qiongshan earthquake and frequent contemporary seismic activity in this region. The existence of the Hainan mantle plume has been confirmed by various geophysical studies [9; 10; 17; 58], and its active location is situated between northern Hainan Island and the Leizhou Peninsula. The horizontal wavelet decomposition results (Figures 3 and 5) reveal density anomalies within the crust caused by mantle plume activity, showing that high-density bodies exist in the deep crust. Specifically, the D5 and D6 decomposed results indicate that 20-25 km deep high-density anomalies are primarily concentrated in this region, while the D7 (Figure 6) wavelet at 30 km shows an expanded range trending approximately northeast. This pattern likely results from deep magma ascending along pre-existing faults, leading to Moho uplift and large-scale magmatic activity.