# Crustal Structure of Hainan Island and Surrounding Seabed Based on High-Resolution Airborne Gravity
## 4 Results
### Decomposed Gravity Anomalies  
Using the multiscale wavelet analysis method, we can separate shallow small-scale source bodies from deep and large-scale source bodies [51]. Based on Equation (1), we performed an eighth-order wavelet decomposition on the gravity data in the study area, and the decomposition results are shown below (Figures 2-6). We also collected modern earthquake data from the National Earthquake Data Center website and plotted the earthquake epicenter in each decomposed result. By calculating the power spectrum, we estimated the depth of each decomposed gravity anomaly wavelet, as shown in Table 1. The calculated depth of the shallow layer D3 is 3 km, and D4 is 5 km. The middle layer, D5, is 11 km, and D6 is 19 km. The deep layer, D7, is 27 km. The shallow layers are located on the upper crust. The middle is located in the middle crust, while the deep layer is located in the lower crust, and the upper mantle is related to the shape of Moho. The gravity anomaly of the D1-D3 wavelet detailed field mainly reflects the top of the upper crust, as shown in Figure 2. The map shows the density disturbance of sedimentary rocks and the small-scale density anomaly with an average depth of 3 km. The negative anomaly relates to the Cenozoic sedimentary depression belt, such as the BBGB, YGHB, and QDNB. In BBW and northern Hainan Island, small-scale NW-trending liner gravity anomalies exist. As can be seen from Figure 2, these NW-trending anomalies terminate to the Wangwu-Wenjiao fault. The NW-trending fault is mainly affected by the large-scale left-lateral displacement of the YGHF during Neogene. The decomposition results of D4 and D5 mainly correspond to depths of 5 km and 11 km, respectively. The maximum and minimum values of D5 are \(-20\) mgal and 45 mgal. With the depth increasing, the decomposition results show that the density inhomogeneity in the upper crust decreases. D4 and D5 represent the density anomaly of the middle crust. As can be seen from Figure 3, D4 shows that the main fault in the study area corresponds with the gravity gradient belt. The YGHF is located on the southwest of Hainan Island with a positive-negative-positive gravity gradient belt. The YGHB shows a chaotic NW-trending negative gravity anomaly. The \"Two depressions and one uplift\" structure of the BBGB is well expressed too. The Qixi uplift zone of the BBGB is expressed as a nearly NE-trending positive anomaly belt. And the QDNF is an NE-trending gravity gradient belt on the southeast of Hainan Island. The gravity anomaly of the QDNB is nearly NE-trending. Figure 4 clearly delineates the depressions and uplifts of the QDNB. Central Hainan Island displays distinct gravity anomalies with NE-trending and WE-trending belts. This geophysical contrast suggests that Central Hainan Island constitutes a unique tectonic unit distinct from surrounding marine terrains. The D6 and D7 wavelet details are smoother with an increasing source depth. According to previous research results in the study area, the mean Moho depth is about 28 km. The D7 wavelet detail with an average source depth of 27 km is interpreted as the gravity anomaly caused by the Moho discontinuity. From D6 (Figure 5), there are a series of NE-trending positive-negative-positive gravity anomaly stripes in the central part of Hainan Island. This gravity anomaly combination represents the density anomaly in the middle and lower crust, induced by tectonic deformation. According to the stress direction, it may be affected by the ancient Pacific NW subduction and form a series of NE-trending tectonic patterns [52; 53]. The Qiongzhou Strait fault zone exhibits a NE-trending high gravity anomaly belt in D4-D6 (Figures 3-5). The gravity anomaly of this fault gradually increases continuously from D4 to D6, indicating that the fault has cut through the deep crust. The QDNF is manifested as a set of positive-negative-positive gravity gradient zones.