# Crustal Structure of Hainan Island and Surrounding Seabed Based on High-Resolution Airborne Gravity
## 4 Results
### Moho Topography  
This study collects the Moho depth from published studies [17; 18] as constraints in the Moho inversion. Utilizing deep-reflection seismic velocity profiles within the study area, we calculated the density difference between the upper mantle and lower crust. The lower crustal density is defined as 2.84 g/cm\({}^{3}\), whereas the upper mantle density is defined as 3.28 g/cm\({}^{3}\). The density difference is approximately 0.36 g/cm\({}^{3}\). Previous seismic investigations delineate an average Moho depth ranging from 25 to 30 km. The D7 decomposition results (Figure 6) corresponds to a field source depth of 27 km (Table 1), which is selected as the anomaly data for the Moho inversion. Using the Parker-Oldenburg inversion method, the Moho depth map is shown in Figure 7.  
The Moho inversion results (Figure 7) reveal that the average depth of the Moho in the study area is approximately 25 km. The Moho varies significantly with a change range of 16.2 to 31.4 km. The Moho depth becomes shallower from the northwest part to the southeast part in the study area, indicating that the crust thins from northwest to southeast. Two Moho depression zones within Hainan Island are apparently reflected. The first is located along the line of Wuzhi Mountain-QiongZhong-BaoTing. This Moho depression is distributed in an elliptical shape with an NE-trending axis, showing a negative correlation with the terrain. The other Moho depression zone is located in the eastern coastal zone and does not show a negative correlation with the terrain. Three distinct Moho uplift anomalies are delineated to be surrounding Hainan Island:  
1. The first uplift is along the line of Lingshui-Wanning-Qizhou Island in the southeast of Hainan Island. The depth of the first Moho uplift decreases significantly from approximately 28 km to 23 km in the southeast part.  
Figure 5: D6 decomposition results of gravity anomaly.  
2. The second Moho uplift region is located 30 km off the west coast of Hainan Island beneath the YGHB, with the shallowest Moho depth ranging from 23 km to 25 km.
3. Located on the north part of Hainan Island, the third Moho local uplift zone is EW-trending with slow depth variation. The shallow part of this Moho uplift corresponds to the Hainan volcanic rock area.  
These three uplift zones form roughly discontinuous circular rings, and Hainan Island is separated from the surrounding plates.  
The Moho topography beneath the YGHB, BBGB, and QDNB is different, indicating that the three basins might have different origins. From the Qiongzhou Strait northwestward, the Moho discontinuity depth deepens markedly to 26-31 km. In the BBGB, the shape of the basin basement geometry inversely correlates with Moho topography, indicating that the basin originated in tectonic extension. In the southwest, the average Moho depth in the YGHB is 26 km, ranging from 24 km to 29 km. The YGHF cut through the Moho interface. The Moho depths gradually deepen southwestward in the YGHB, accompanied by pronounced local Moho uplifts. In the southeastern part of the study area, the Moho discontinuity in the QDNB is the shallowest, and the Moho depth ranges from 18 km to 23 km.  
Figure 6: D7 decomposition results of gravity anomaly.