# Crustal Structure of Hainan Island and Surrounding Seabed Based on High-Resolution Airborne Gravity  
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1 School of Ocean Sciences, China University of Geosciences, Beijing 100083, China; [EMAIL_ADDRESS]  
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Footnote 1: [[https://doi.org/10.3390/app15105564](https://doi.org/10.3390/app15105564)]([https://doi.org/10.3390/app15105564](https://doi.org/10.3390/app15105564))  
Received: 25 February 2025  
Revised: 27 April 2025  
Accepted: 5 May 2025  
Footnote 11: https:which leads to the Leiqiong volcanic area [7; 8; 9; 10; 11]. The Cenozoic extension of the South China Sea caused a large-scale extension around Hainan Island [12; 13; 14]. This unique tectonic setting has led to the complex geological environment of Hainan Island, and it has relatively intense volcanic activity and seismicity, such as the 1605 M7.6 Qionghai earthquake [15]. The significant volcanic rock and seismic activity are obvious responses to the deep structure and tectonic transition in the interior of Hainan Island. Research on the crustal structure of this area is important for understanding the continental accretion and the Genesis of the seismic activity.  
The deep structure of Hainan Island has attracted substantial attention, and many researchers have investigated the crustal structure under the Hainan region mainly based on receiver functions, seismic tomography, and wide-angle seismic exploration [16; 17; 18; 19; 20; 21; 22; 23]. The main conclusions of geophysical studies are as follows: the crustal thickness bins from the mainland of the South China Block to Hainan Island, and several studies suggest that the Moho depth of Hainan Island and the surrounding seabed is 25-30 km [24]. The upper crust has been through intense stretching, and three basins have formed around Hainan Island, with thicker sediment from north to south. Beneath the Qiongzhou Strait, the Pn anisotropic velocity model reveals a low-V anomaly, which is inferred to be the upwelling mantle under the lower crust [11]. However, there are ongoing debates about the Moho depth in the study area. [PERSON] [16] found that the crust is 40-45 km thick around the northeastern side of the Qiongzhou Strait.  
The geophysical research on the crustal structure in this area mainly focuses on the crust-mantle scale, and most of these studies are based on several limited seismic observation stations, only covering the onshore section of the research area. The crustal structure is an important area of research in terms of the seismogenic mechanism and tectonic deformation between the South China Craton and South China Sea. Recently, a joint airborne gravity survey was conducted by the Chinese Geologic Survey in the area (Figure 1). The airborne gravity data improve the resolution of medium-long-wavelength gravity anomalies and provide more information about the deep crust. Based on these airborne gravity data and satellite gravity data, this paper aims to elucidate the structure and deep deformation of Hainan Island and its surrounding seabed using the gravity anomaly separation inversion method and provide a multi-dimensional density structure under the constraint of seismic research results and geological information. Then, we explain the tectonic significance of the crustal structure and its relationship with seismogenic mechanisms. Our result also provides new insights into the crustal structure of the northwestern SCS.