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断裂作为地球内部动力的表层响应,不仅常作为盆地构造单元的边界,控制着断陷盆地的形成演化和地层发育(Lei Chao et al.,2015; 谢玉洪等,2015; 刘雨晴等,2020;占华旺等,2021);而且还可以诱发形成地震、滑坡等地质灾害,尤其是一些区域性深大断裂是发生强烈地震的常见诱因,如龙门山断裂带、南海北部滨海断裂带(以下简称滨海断裂带)(赵明辉等,2004; Xia Shaohong et al.,2020;杨宜海等,2021; 李志刚等,2022)。因此开展断裂构造特征分析不仅有助于了解盆地的形成机制及动力学过程,也可为相关部门防灾减灾提供重要支撑。
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琼东南断裂带,又称琼东南盆地5号断裂,位于海南岛南侧、琼东南盆地北侧,为琼东南盆地北部凹陷带的控边断裂,控制了琼东南盆地北部凹陷带的沉积发育(于俊峰等,2008; 尹新义等,2010)。目前有关琼东南盆地断裂系统研究以琼东南盆地1号断裂和2号断裂为主(于俊峰等,2008; 谢玉洪等,2015),仅有部分学者对琼东南盆地5号断裂活动性进行了简单分析(尹新义等,2010),缺少对该断裂地质地球物理特征系统性研究。另外,有关该断裂带与滨海断裂带的关系也一直存在争议,一些学者认为琼东南断裂为滨海断裂带在南海西北部的主要分支断裂(徐辉龙等,2010;熊成等,2018; 张馨予等,2019);也有学者认为该断裂与滨海断裂无关联。
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本文基于广州海洋地质调查局近年在海南岛及邻域1∶100万和1∶25万海洋地质区调工作获得的多道地震及区域自由空间重力异常等地球物理资料,对琼东南断裂带地质-地球物理特征进行详细解释和分析,并探讨该断裂带与滨海断裂带的构造关系,这不仅对南海西北部油气资源勘探有重要意义,还可为南海西北部沿海地区防灾减灾工作提供重要参考。另外,鉴于目前滨海断裂带在南海西北部研究程度较低,以及滨海断裂带在晚更新世以来活动规律以及该断裂带的发震构造和孕震机制认识还存在一些不足, 本文对我国未来科学钻探试验靶区的选择给出了初步建议。
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1 区域地质背景
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琼东南盆地位于南海西北部、海南岛东南侧,呈北东东向延伸(图1),大体上围绕海南岛南部呈一弧形展布(谢文彦等,2009)。盆地西以1号断裂与莺歌海盆地为界,东接珠江口盆地,北部以琼东南断裂带与海南隆起区分割,南临中建南盆地(朱荣伟等,2020, 2021)。琼东南盆地被认为是一个发育在强烈减薄陆壳上方的新生代伸展型盆地(Lei Chao et al.,2016; Zhao Zhongxian et al.,2018; 雷超等,2022; 夏少红等,2022),是南海西北部重要的含油气盆地。
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在大地构造方面,琼东南盆地地处欧亚板块、印-澳板块和太平洋板块的交汇处,属华夏地块最南端的延伸部分,其形成演化过程复杂,受到了印-澳板块与欧亚板块碰撞、太平洋板块的俯冲和南海扩张等多种地质作用的影响(尹新义等,2010; 朱荣伟等,2020, 2021; 杜文波等,2022)。受新生代复杂的构造演化历史影响,琼东南盆地断裂十分发育,其中西侧的1号断裂为红河断裂在南海西北部的延伸(刘海龄等,2015; 雷超等,2022),盆地陆架区发育了5号、3号、6号等断裂,深水区发育了2号、12号、18号、17号、10号、19号、16号和11号等断裂(Lei Chao et al.,2016, 2020;雷超等,2022)。断裂的走向主要为NE,NW和近EW向。其中,以NE向断裂为主,为盆地主要控凹断裂;NW向断裂通常为盆地调节断裂,具有一定的走滑性质(尹新义等,2010)。
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Fig.1 Geographical location map of Qiongdongnan basin (a) (the location of the Binhai fault zone in Fig.1b refers to Xu Huilong et al., 2010) and distribution map of major faults in Qiongdongnan basin (b) (the division of structural units and distribution of main faults in Fig.1b refer to Song Guangzeng et al.,2014; Lei Chao et al.,2022)
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在地层发育方面,该盆地基底主要由前新生代的火成岩、变质岩及沉积岩组成(雷超等,2011; 谢玉洪等,2015),盆地的充填序列由裂陷层系(古近系)和坳陷层系(新近系+第四系)组成(图2),自下而上分别是始新统(Tg~T8)、下渐新统崖城组(T8~T7)、上渐新统陵水组(T7~T6)、下中新统三亚组(T6~T5)、中中新统梅山组(T5~T3)、上中新统黄流组(T3~T2)、上新统莺歌海组(T2~T1)和更新统乐东组(T1~T0)(雷超等,2011, 2022),新生代沉积厚度可达10000m以上(谢玉洪等,2015)。
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图2 琼东南盆地地层综合柱状图(红河断裂活动特征参考安慧婷等, 2012)
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Fig.2 Integrated stratigraphic column of Qiongdongnan basin (activity characteristics of Red River fault refer to An Huiting et al.,2012)
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2 琼东南断裂地质-地球物理特征
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本文利用Geoframe4.5软件平台对海南岛东南部海域高分辨率多道地震资料进行了综合解释,并在Geoframe4.5软件平台上对断裂平面展布特征进行了组合,综合分析了琼东南断裂平面展布特征和剖面构造特征。
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2.1 平面展布特征
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基于海南岛东南部海域二维多道地震剖面解释结果,通过对海南岛东南部海域基底断裂(地震剖面中切穿Tg层位的断裂)进行平面组合,从而获得了海南岛东南部海域基底断裂平面分布图(图3)。由图3可知,海南岛东南部海域基底断裂展布方向与琼东南盆地的断裂展布方向具有一致性,主要为NE、NW和近EW向,其中以NE向为主。由图3可以看出,在海南隆起区与琼东南盆地之间存在一条边界断裂,该断裂位置与前人提到的琼东南断裂(即5号断裂位置)相当,推测该断裂属于琼东南断裂的一部分。该断裂大致从海南岛南侧开始,呈NE走向,向SE方向倾斜,基本上沿着琼东南盆地北部坳陷的北侧边界分布,并且向东北方向一直延伸。另外,由海南岛东南部海域基底断裂平面组合结果可知,琼东南断裂在平面上并不连续,被一条近EW向断裂(f1)错开,分成东西两段,两段断裂在平面上的走向存在细微差异。其中,琼东南断裂东段呈NE走向,琼东南断裂西段走向为NEE向。
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2.2 剖面构造特征
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为研究琼东南断裂带剖面构造特征,本文在琼东南断裂带东西两端各选取了一条多道地震剖面(测线编号分别为L1、L2,测线位置见图3)横穿该断裂。多道地震剖面解释结果(图4、图5)表明,琼东南断裂带为一条向SE方向倾斜的伸展断裂,对琼东南盆地北部坳陷的沉积具有一定的控制作用,尤其是对其南侧松西凹陷的沉积控制作用明显。该断裂在新生代初期活动强度较大,之后活动强度逐渐减弱,约至中新世晚期(对应地震反射界面为T2)停止活动。基于多道地震剖面计算得到的垂直断距和水平偏移距均表明该断裂东西两段表现出明显的差异,其中断裂西段的活动强度明显要大于断裂东段的活动强度(图4、图6)。例如,在断裂活动最强的始新世时期(Tg~T8),断裂西段的垂直断距经时深转换后(时深转换公式参考朱荣伟等,2020,下同),结果约为2.43km,水平偏移距约为1.74km(图4);而断裂东段在该时期的垂直断距经时深转换后,结果约为0.26km,水平偏移距约为0.16km(图6)。另外,从过断裂西段的多道地震剖面解释结果可知,该断裂在渐新世(T8~T6)伴随发育有花状构造(图4、图5),这可能与其西侧的红河断裂在该时期正经历快速的左旋走滑活动有关(孙龙涛等,2005;安慧婷等,2012)。
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图3 海南岛东南部海域基底断裂平面分布图
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Fig.3 Plane distribution of faults in the basement of the southeast sea area of Hainan Island
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L1—过琼东南断裂带西段多道地震剖面;L2—过琼东南断裂带东段多道地震剖面;f1—错开琼东南断裂带的EW向断裂
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L1—Multi-channel seismic profile passing through the western segment of the Qiongdongnan fault zone; L2—multi-channel seismic profile passing through the eastern segment of the Qiongdongnan fault zone; f1—the EW-trending faults dividing the Qiongdongnan fault zone
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图4 海南岛东南部海域过琼东南断裂西段的多道地震剖面(L1)特征(L1测线位置见图3)
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Fig.4 Characteristics of multichannel seismic profile (L1) passing through the western section of Qiongdongnan fault zone in the southeast sea area of Hainan Island (the location of L1shown in Fig.3)
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d1—琼东南断裂西段Tg~T8时期垂直断距,约2.43km;s1—琼东南断裂西段Tg~T8时期水平偏移距,约1.74km
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d1—The vertical fault distance of the western section of Qiongdongnan fault during Tg~T8 period, about 2.43km; s1—the horizontal offset of the western section of Qiongdongnan fault during Tg~T8 period, about 1.74km
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图5 海南岛东南部海域琼东南断裂西段负花状构造发育特征(剖面位置见图4)
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Fig.5 Characteristics of negative flower-like structures in the western segment of the Qiongdongnan fault zone in the southeast sea area of Hainan Island (the location of section shown in Fig.4)
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(a)—原始地震剖面;(b)—构造解释后地震剖面
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(a)—Original seismic profile; (b)—seismic profile after tectonic interpretation
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图6 海南岛东南部海域过琼东南断裂东段多道地震剖面(L2)特征(L2测线位置见图3)
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Fig.6 Characteristics of multichannel seismic profile (L2) passing through the eastern section of Qiongdongnan fault zone in the southeast sea area of Hainan Island (the location of L2shown in Fig.3)
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d2—琼东南断裂东段Tg~T8时期垂直断距,约0.26km;s2—琼东南断裂东段Tg~T8时期偏移距0.16km
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d2—The vertical fault distance of the western section of Qiongdongnan fault during Tg~T8 period, about 0.26km; s2—the horizontal offset of the western section of Qiongdongnan fault during Tg~T8 period, about 0.16km
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3 琼东南断裂与滨海断裂带构造关系探讨
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滨海断裂带作为南海北部陆缘发育的一条规模巨大的断裂带,一直被认为是华南正常型陆壳与南海北部减薄型陆壳之间的分界(刘以宣,1986; 徐辉龙等, 2006),也是珠江口盆地新生代含油气盆地的控盆断裂(龚再升,2004; 李志刚等,2022),同时是华南沿海的主要控震和孕震构造(徐辉龙等,2010; 孙金龙等,2012),其形成演化对于了解南海北部地区的构造演化、华南沿海的防震减灾等工作具有重要意义。滨海断裂带大致与海岸线平行,北东起福建平潭牛山岛东部海域,经乌丘屿和兄弟屿,在海峡区内呈NE—SW走向,往南至广东的南澎列岛、红海湾口,经担杆列岛延伸至海南岛东部海域(郭晓然等,2019;罗新刚等,2020),断裂这一部分位置已基本确定。但有关该断裂带往西的进一步延伸存在不同认识。有学者认为该断裂带往西从海南岛北部的琼州海峡通过(刘以宣,1986; 钟建强,1987; 张菲菲等,2014);也有学者认为该断裂带截止于海南岛东部海域(陈汉宗等,2005; 罗新刚等,2020);也有学者认为该断裂带在海南岛东部分叉为两条断裂(图1),其中一分支断裂从海南岛北部通过;另一分支为主断裂带,沿海南岛东南沿岸展布(徐辉龙等,2010)。
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最新高精度空间重力异常图显示(图7),在海南岛南部沿岸存在一条明显的重力异常特征分界线,该分界线北部整体表现为重力低异常,局部表现为零星分布重力高异常,分界线南部整体表现为重力高异常,其位置大致与多道地震解释得出的琼东南断裂位置重合,推测该分界线代表的应是琼东南断裂带。通过区域重力异常对比分析,该分界线与前人解释的南海北部滨海断裂带具有较好的一致性,据此可推测琼东南断裂带应属于滨海断裂带在南海西北部的延伸(图7)。另外,海南岛北部琼中海峡也存在一条明显的重力异常特征分界线,与琼州海峡断裂位置相当,通过区域重力异常特征对比分析,推测琼州海峡断裂并不属于滨海断裂带在南海西北部的延伸,而是莲花山断裂在南海西北部的延伸(图7)。
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图7 南海北部滨海断裂带自由空间重力异常图(据杨胜雄等,2015修改)
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Fig.7 Free air gravity anomaly map of coastal fault zone in the northern South China Sea (modified after Yang Shengxiong et al.,2015)
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F1—1号断裂;F2—琼东南断裂带;F3—琼州海峡断裂;F4—滨海断裂带;F5—莲花山断裂
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F1—No.1fault; F2—Qiongdongnan fault zone; F3—Qiongzhou Strait fault; F4—Binhai fault zone; F5—Lianhuashan fault
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4 未来科学钻探建议
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滨海断裂带由于特殊的构造位置及研究的重要性,不同学者对其开展过大量研究工作,极大地提升了对该断裂带的认识。自20世纪90年代以来,不同学者在滨海断裂带位置、构造属性、深部结构探测特征等方面取得了一系列成果认识(赵明辉等,2004, 2006; 徐辉龙等,2006, 2010; Xia Shaohong et al.,2010, 2020;曹敬贺等,2014a, 2014b;Wan Kuiyuan et al.,2017;Cao Jinghe et al.,2018; 杜文波等,2020; 李志刚等,2022)。但目前有关该断裂的认识还存在一些不足。如:有关滨海断裂晚更新世以来活动规律的认识还存在缺失(杨晓东等,2022),而地震地质理论证明,只有晚第四纪发生过多次错动的断裂,才会在未来再次发生错断,形成地震。前期地震剖面解释结果表明滨海断裂带在晚第四纪具有很强的构造活动性,并已错断晚更新世以来的沉积层并有可能影响到海底,但前期研究多关注滨海断裂展布特征及深部结构(徐辉龙等,2010;曹敬贺等,2014a, 2014b; Cao Jinghe et al.,2018; 熊成等,2018; Xia Shaohong et al.,2020),有关滨海断裂带新构造活动演化过程的研究较为薄弱,导致对其晚更新世以来的活动规律、变形强度及运动学性质等认识存在不足。又如:有关滨海断裂的发震构造和孕震机制认识还存在争议,不同学者从不同的角度提出了不同的观点。徐辉龙等(2006)提出NEE向的滨海断裂带与NW向相交切的断裂构造样式是地震的发震构造,而闫培等(2015)通过研究滨海断裂带的深部地壳结构和震源介质状态,认为滨海断裂带为主要的发震构造;徐辉龙等(2006)认为滨海断裂带与上地壳下部的低速层交接部位是地震孕育的深部构造,而曹敬贺等(2014a)则认为下地壳顶部的低速层、近似正交的NEE向滨海断裂带与NW向断裂三者才是地震活动的孕震构造,综合以上观点可知,虽然普遍认为南海北部的板内地震与滨海断裂带活动相关,但是对其发震构造和孕震机制认识还存在一定分歧。另外,有关该断裂带的研究多集中于东段和中段(赵明辉等,2004, 2006; 徐辉龙等, 2006, 2010; Xia Shaohong et al.,2010, 2020; 曹敬贺等,2014a, 2014b; Wan Kuiyuan et al.,2017; Cao Jinghe et al.,2018),而对其西段特征研究相对较少,导致对有关该断裂西段认识也存在诸多方面不足。
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基于以上存在问题,建议未来开展科学钻探试验时,可以在研究程度相对较低的阳江外海域滨海断裂带以及海南岛东南侧海域琼东南断裂带选择合适区域作为钻探靶区,并以钻取断裂带附近第四系完整岩芯剖面和断裂破碎带岩芯为目的。这样可通过钻探获取断裂带两盘及破碎带岩石样品,并结合补充地球物理调查测线解释结果,开展断裂带几何学特征研究,进一步验证琼东南断裂是否为滨海断裂带在南海西北部的延伸。另外,可以厘定晚更新世以来滨海断裂带周缘沉积层内的错断和古地震事件,确定断裂活动时代、断裂位错量、古地震事件时代及复发间隔等参数,从而揭示滨海断裂晚更新世以来活动规律及孕震机制,以便更加全面地开展地震危险性评价,为粤港澳大湾区的防震减灾工作提供支持。
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5 结论
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(1)通过对海南岛东南部海域多道地震剖面综合分析可知,琼东南断裂平面展布上大致从海南岛南侧开始,基本上沿着琼东南盆地北部坳陷边界分布,并且向东北方向一直延伸;由海南岛东南部海域基底断裂平面组合结果可知,琼东南断裂在平面上并不连续,被一条NEE向断裂错开,分成东西两段,其中东段呈NNE走向,西段走向为NEE向。
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(2)地震剖面上,琼东南断裂为一条向SE方向倾斜的伸展断裂,对琼东南盆地北部坳陷的沉积具有一定的控制作用,该断裂在新生代初期活动性较强,之后活动性逐渐减弱,约至中新世晚期停止活动。多道地震剖面解释结果以及基于多道地震剖面计算得到的垂直断距和水平偏移距结果均表明该断裂西段的活动强度明显要大于断裂东段的活动强度。此外,从过断裂西段的多道地震剖面解释结果可知,该断裂西段在晚始新世至渐新世伴随发育有花状构造,这可能与其西侧的红河断裂在该时期正经历快速的左旋走滑活动有关。
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(3)结合南海北部区域空间重力异常特征,推测琼东南断裂属于滨海断裂在南海西北部的延伸,而位于海南岛北侧的琼州海峡断裂应是莲花山断裂在南海西北部的延伸。
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(4)为进一步验证琼东南断裂是否为滨海断裂带在南海西北部的延伸,以及更加全面地开展滨海断裂带地震危险性评价,建议钻采船在初期开展科学钻探时,选择阳江外海域滨海断裂带和海南岛东南侧海域琼东南断裂带为钻探靶区,并以钻取断裂带两盘和断裂破碎带岩芯为目的。
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摘要
断裂作为地球内部动力的表层响应对断陷盆地的形成演化有着重要作用,也常成为地震、滑坡等地质灾害的诱发因素。本文基于已有的多道地震资料,对海南岛东南海域的琼东南断裂带的构造特征进行了综合分析。结果表明:琼东南断裂带在平面上大致沿琼东南盆地北侧边界分布,并被一条近EW向断裂错开分成东西两段;该断裂带在新生代初期活动强度较大,约至中新世晚期停止活动,且断裂西段的活动强度明显要大于东段;此外,该断裂带西段在晚始新世至渐新世伴随发育有花状构造,这可能与该时期其西侧的红河断裂正经历快速的左旋走滑活动有关。结合南海北部区域自由空间重力异常特征,推测琼东南断裂带属于滨海断裂带在南海西北部的延伸,而位于海南岛北侧的琼州海峡断裂是莲花山断裂在南海西北部的延伸。基于目前滨海断裂带的研究现状,建议未来开展科学钻探试验时,选择阳江外海域滨海断裂带和琼东南断裂带为钻探靶区,并以钻取断裂两盘和断裂破碎带的岩芯为目的,可为进一步明确滨海断裂带在南海西北部的延伸方向,以及今后更加全面地开展滨海断裂带地震危险性评价提供支撑。
Abstract
As the surface response of the earth's internal dynamics, fault play an important role in the formation and evolution of faulted basins, and they often become the inducing factors of geological disasters such as earthquakes and landslides. Based on the existing multi-channel seismic data, the structural characteristics of the Qiongdongnan fault zone on the southeast side of Hainan Island are comprehensively analyzed in this paper. The results show that the Qiongdongnan fault zone is roughly distributed along the northern boundary of the Qiongdongnan basin on the plane, and is disconnected by a EW-trending fault, dividing it into two sections. It was highly active in the Early Cenozoic and ceased activity in the Late Miocene, and the activity intensity of the western fault was significantly greater than that of the eastern segment of the fault. In addition, the western segment of the fault was accompanied by flower-like structures from the Late Eocene to the Oligocene, which may be related to the rapid sinistral strike-slip activity of the Red Rive fault zone in the west during this period. Combined with the characteristics of free air gravity anomalies in the northern part of the South China Sea, it is inferred that the Qiongdongnan fault zone belongs to the extension of the Binhai fault zone in the northwest part of the South China Sea, and the Qiongzhou Strait fault located in the north of Hainan Island should be the extension of the Lianhuashan fault in the northwest part of the South China Sea. Based on the current research status of Binhai fault zone, it is suggested that the Binhai fault zone offshore outside Yangjiang and Qiongdongnan fault zone can be selected as the drilling target area for future scientific drilling experiment, with cores drilled on both sides of the fault and the fracture zone. The drilling results can provide supports for further determining the extension direction of the Binhai fault zone in the northwest of the South China Sea, and for a more comprehensive seismic risk assessment of the Binhai fault zone in the future.