# Crustal Structure of Hainan Island and Surrounding Seabed Based on High-Resolution Airborne Gravity
## 2 Geological Setting  
Hainan Island is located in the southwest of South China, which lies at the conjunction of the Eurasian, Indian-Australian, and Pacific plates [2, 27, 28]. The geodynamic process and geological evolution history is complicated, and there are three typical geological provinces in Hainan Island. The first are amphibolite speisses, granulites, and charnockites, making up the Paleoproterozoic basement, and they are represented by the Shilu Group and Baoban Group [29, 30]. The second is the granitoid of the Permo-Triassic and Jurassic-Cretaceous ages intruded into the basement, which is thought to record continental accretion events [2, 27, 28, 31, 32]. The formation of Jurassic-Cretaceous gynantioids is associated with the subduction of the paleo-Pacific plate beneath Eurasian plates [31, 32]. Overlying the other two are Leiqiong volcanics in the northern part of island. Those volcanoes were composed of basaltic lava [33, 34] that came from the deep mantle related to the activity of the Hainan Plume.  
Figure 1: The geological location of the study area and the topography of Hainan Island. The solid line represents faults modified from [25, 26]. The circle represents the location of the epicenter of the natural earthquake, and the bigger circle represents the magnitude. The green patches indicate the distribution of the Leiqiong and Hainan volcanic rocks. YGHF, YingGeHai Fault; WXNF, WeiXiNan Fault; YGHB, YingGeHai Basin; QDNF, QiongDongNan Fault; WWWJF, WangWu–WenJiao Fault; BSF, BaiSha Fault; JFDLF, JianFeng–DiaoLuo Fault; CQHF, ChangJiang–QiongHai Fault; JSLSF, JiuSuo–LingShui Fault; BSF, BaiSha Fault; QDNB, QiongDongNan Basin; BBGB, BeiBu Gulf Basin; WPRMB, West part of Pearl River Mouth Basin; SCP, South China Plate; SCS, South China Sea; PP, Philippine Plate; IP, Indosinian plate; HI, Hainan Island.  
The tectonic division of Hainan Island has been widely discussed by many experts. Three trending faults exist in our study area, which are NE-trending faults, such as the Baisha, Weixinan, Gezhen, and Qiongdongnan faults; EW-trending faults, such as the Wangwu-Wenjiao, Changjiang-Qionghai, and Jiusuo-Lingshui faults; and NW-trending faults, the Red River Fault, such as the Changliu-Xiangou and Puqian-Qinglan faults. The EW-trending faults are considered to have formed first. The NE-SW trending faults are the dominated tectonic faults in Hainan Island, whose formation is inferred to be related to the NW-trending subduction of the paleo-Pacific plate beneath the Eurasian plate. The NW-trending faults are a group of seismogenic faults, whose formation is related with the strike-slip of the Yinggehai Fault in Cenozoic influenced by the remote squeezing-out effect of the Tibetan Plateau collision. Surrounding Hainan Island are three important Cenozoic sedimentary basins, including the Qiongdongnan Basin, Beibuwan Basin, and Yinggehai Basin [26; 35; 36; 37].  
Located at the margin of three plates and influenced by the plume from deep mantle [9; 10], Hainan Island is also a seismically active region. According to the seismic data from the China Earthquake Administration, about 820 earthquakes have occurred since 2001. Analysis of those data shows that 7% of earthquakes in our study area are larger than magnitude 3, and the depth of the earthquake's center is between 1 km and 30 km. The famous 1605 Qiongshan Earthquake occurred in the north of Hainan Island. [PERSON] et al. [38] divided these earthquakes into four distinct earthquake swarms and inferred that seismic activity in Hainan Island is affected by the deep crust-mantle magmatic fluid movement.