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南海位于欧亚、印澳和太平洋三大板块的汇聚中心,是西太平洋最大的边缘海。滨海断裂带位于南海的北部陆缘,是华南沿海地震活动强度最大、频度最高的地震带(图1;刘以宣,1981, 1985;钟建强,1987;赵明辉等,2003;詹文欢等,2004;徐龙辉等,2010;曹敬贺等,2012, 2014a, 2014b;李志刚等,2022)。滨海断裂带走向NEE,大体沿着30~50m等深线展布,从东北部的南澎列岛延伸至西南部的北部湾涠洲岛,全长超过1200km(图1;赵明辉等,2003;徐辉龙等,2006)。滨海断裂带是华南次级板块和南海次级板块的分界断裂(詹文欢等,2004;徐辉龙等,2010),也是珠江口盆地新生代含油气盆地的控盆断裂(龚再升等,1997, 2004)。因其独特的构造环境、复杂的地壳结构、特殊的地理位置和严重的地质灾害,长期以来受到学术界的关注(李志刚等,2022)。利用重力、磁力和地震等手段,前人对滨海断裂的不同段落开展了大量的探测研究,揭示了滨海断裂带是华南大陆正常陆壳向南海微板块减薄的一个分界断裂(詹文欢等,2004;徐辉龙等,2006;熊成等,2018)。据史料记载,滨海断裂带历史上发生多次7级以上大地震(图1;刘以宣,1981;魏柏林等,2001;徐辉龙等,2006):1600年南澳7级地震,1604年泉州8.0级地震,1605年琼州7.5级地震和1918年南澳7.5级地震,其中1918地震伴有海啸的相关记录,造成了严重的人员伤亡和经济损失。如果滨海断裂带再次发生大型地质灾害(地震、海啸等),将会对包括珠三角在内的沿海经济发达、人口密集地区造成更为严重的损失。
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图1 南海北缘滨海断裂带与华南沿海地区地震分布(地震数据来自广东省地震局)
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Fig.1 Regional map of the Littoral Fault Zone and seismicity in the northern South China Sea and South China (the seismic data are from the Seismological Bureau of Guangdong Province)
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P1~P3—地震剖面的位置
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P1~P3—Locations of seismic profiles
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综合大洋钻探计划(Integrated Ocean Drilling Program, IODP) 被誉为是最成功的国际科学合作计划之一,极大地推动了地球科学的发展,提高了人类对于地球演变历史及其资源、环境和灾害等效应的认识,并明确指出地震、海啸等自然灾害是未来大洋钻探探索地球的五大旗舰计划之一。自2014年以来,IODP在南海成功实施了4次大洋钻探航次(IODP Expedition 349、367、368、368x)(Expedition 349Scientists, 2014; Li Chunfeng et al., 2014; Jian Zhimin et al., 2018; Sun Zhen et al., 2018; Larsen et al., 2018),在南海东部、西南次海盆以及北部的洋-陆过渡带首次获得了火成岩基底样品;同时,实施了多次的多道地震、深拖地磁、地热、大地电磁和海底地震仪(OBS)等地球物理实验(Hsu et al., 2004; Ding Weiwei et al., 2013, 2020; Lester et al., 2014; Yan Pin et al., 2014; Gao Jinwei et al., 2015; Yu Junhui et al., 2018; Zhao Minghui et al., 2018; Zhang Jie et al., 2020);研究成果极大提高了对南海晚中生代以来,从陆到洋的转换过程和南海海盆张开-扩张的动力学机制的认识(Lin Jian et al., 2019; Sun Zhen et al., 2019; Wang Pinxian et al., 2019)。南海IODP钻探航次主要聚焦在新生代以来(65Ma)的南海演变历史,但是对于全新世(约1万年)以来的南海重大灾害历史的研究十分缺乏,目前尚不清楚滨海断裂带大地震的次数、分布和周期,严重影响了对于该断裂带地质灾害风险的认识。因此亟需针对这一问题开展新的大洋钻探,为海洋防灾减灾提供关键的理论依据和科学支撑。
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1 滨海断裂带的构造特征
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由于受周缘板块的汇聚碰撞(印澳、欧亚和菲律宾三大板块)、青藏高原的侧向挤出以及南海的张开-扩张作用,滨海断裂带的构造变形和动力学机制十分复杂(徐辉龙等,2006;曹敬贺,2014a, 2014b)。自东向西,滨海断裂以NEE走向为主,在阳江-湛江海域形成分支,一条沿SWW走向经过琼州海峡直到北部湾盆地,另外一条沿着海南岛东侧,以SSW走向延伸直到三亚以南海域(图1)。滨海断裂带在形成和演化过程中,受到不同构造应力的影响,现今以张性变形为主,局部存在构造反转(刘以宣,1985;姚伯初,1993;曹敬贺等,2014a, 2014b;熊成等,2018)。在闽南-粤东沿海与珠江口盆地担杆列岛区域,滨海断裂带表现为一条宽6~20km,自海底向东南倾的连续低速带,断裂带内部沉积层纵波速度为1.8~3.5km/s,上地壳速度5.2~6.1km/s,下地壳速度为6.3~6.6km/s,在深地震探测剖面和重、磁异常特征上都是切割深度超过莫霍面的重要构造边界(图2;赵明辉,2003, 2004;徐辉龙等,2006;Xia Shaohonget al., 2010;孙金龙等,2012;曹敬贺等,2014a, 2014b;熊成等,2018)。作为华南正常陆壳向南海减薄型陆壳的分界断裂,滨海断裂带两侧的地形变化、沉积地层和地壳结构差异明显(图2;刘以宣,1981, 1986;赵明辉等,2004;徐辉龙等,2006;夏少红等,2010;曹敬贺等, 2014a, 2014b;熊成等,2018;李志刚等,2022):北侧地形起伏较大,缺少新近系沉积地层,第四系沉积厚度一般小于30m,莫霍面深度约29~30km,整体呈现出正负交替的重力异常及显著变化的高磁异常带,上地壳下部存在速度为5.5~5.9km/s,厚度约3~4km,埋深10~18km的低速层;南侧地形起伏小,以海底正地形为主,新生界沉积厚度约5~7km,其中第四系地层厚度一般大于150m,整体表现为正重力异常和稳定的负磁异常,莫霍面深度约25~28km,上地壳下部的低速层逐渐减薄直至尖灭。近年来的深部探测结果在滨海断裂带孕震机制方面取得了重要进展,发现滨海断裂带与NW向断裂带和上地壳下部的低速层的交接构造部位,形成南海北部重要的应力应变集中带,为地震的孕育和爆发提供了深部动力学条件(徐辉龙等,2006, 2010)。但是由于断裂带较长,所处水深较浅,目前针对其深部和浅部结构的探测十分有限,致使对其精确的空间位置、深-浅构造耦合关系等问题知之甚少(李志刚等,2022)。目前针对滨海断裂带相关的海域活动构造研究也很不足,导致对其晚第四纪活动特征、变形强度,古地震历史、空间分布和地震空区的断层滑动性质等关键问题认识不清。另外,滨海断裂带是位于被动大陆边缘的板内断裂带,其地震发生机制、地震破裂特征与活动大陆边缘俯冲带有何不同?
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图2 南海北缘滨海断裂带构造剖面与地震速度剖面(据熊成等,2018)
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Fig.2 Seismic profile and seismic velocity profile through the Littoral Fault Zone in the northern South China Sea (after Xiong Cheng et al., 2018)
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(a)~(c)—对应图1中的剖面P1,P2和P3
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(a)~(c)—Corresponding to the profiles 1, 2and 3in Fig.1
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2 滨海断裂带的主要历史大地震
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有历史记载以来,滨海断裂带曾发生过至少4次7级以上大地震,部分地震还触发了海啸(顾功叙等,1983;魏柏林等,2001)。这些历史大地震包括1600年广东南澳7级地震、1605年海南琼州M 7.5级大地震、1918年广东南澳7.5级地震,1604年福建泉州M 8级大地震。由于这些地震发生的年代、位置和破坏程度不同,历史地震记载的详细程度有所差别。下面以记录和研究程度较高的3次地震为例,介绍滨海断裂带的历史大地震灾害的影响。
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2.1 1605年琼州M 7.5大地震
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1605年7月13日午夜,在海南琼州北部发生7.5级大地震,震中位于东寨港西侧,烈度为10度(图1;徐起浩,2007)。重震区为海口(琼山)、澄迈、临高、文昌四地。震感北达600余千米外的湖南临武县,6度破坏范围达300余千米外的广西陆川、博白和广东阳江一线。这次大地震使整个沉陷体块垂直下降,导致陆地沉陷的幅度3~4m,陆陷成海的最大幅度为10m左右,震区内除100多平方千米的陆地深入海底外,还有上千平方千米的陆地也有不同程度的下沉,死亡3300多人。据《琼州府志》、《琼山县志》等史料记载,这场大地震过后,有72座村庄(也有说100多个村庄)和千顷田野颓然陷落,山化海、人为鱼,伤者十之八九,小溪般的东寨河变成了一片汪洋,形成世界罕见的“海底村庄”。这是我国地震历史上唯一导致陆陷成海的一次大地震,这种情况在国内外地震史上也极其罕见。唯一“幸存”的是距文昌铺前镇1.5km的琼山区北港岛、浮水墩沉而不灭,孤浮海面。在现今的琼山市东北海岸的东营港、北创港、东寨港和文昌县辅前港等地的波涛之下,隐蔽着72个“海底村庄”,即琼州大地震遗址,位于海口市美兰区东寨港至文昌市铺前镇一带的海湾海底。
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2.2 1918年南澳M 7.5级大地震
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1918年2月13日下午14时07分,在广东南澳东北侧海中发生大地震,震中在南澳外海南澎列岛附近,南澳全县绝大部分房屋倒塌,南澳岛东部云澳发生上崩和滑坡,人员死伤十之有八九(图1;陈恩民等,1984, 1985;黄日恒等,2002)。地震强度达到里氏7.5级,震中烈度10度。1918年南澳大地震发生在滨海断裂带地震活动最活跃的闽南-粤东区域(孙金龙等,2012)。这是20世纪我国东南地区除台湾省外的最大地震,也是继《潮州府志》记载唐代潮州地区发生8.0级大地震以来的第二大地震。南澳旧县城——深澳被地震殃及,繁华的深澳城遭到了摧毁,遂荒废。地震还波及粤、闽、赣、浙、台、苏、沪、皖、鄂、湘、桂等省市自治区。南澳大地震也是南海北部少数几个触发海啸的地震之一(李琳琳等,2022a)。历史文献中明确记载了地震发生后福建和广东沿海出现的一些海啸现象,这其中的描述包括“民国七年二月十三日, 福建、广东沿海地震。其震中区域在泉州至汕头一带,地裂土崩,海水腾涌,房舍倾覆,死亡者以数百计”(翁文灏,2001)和“1918年2月13日14时07分(民国七年正月初三)广东南澳附近海域(23.60°N,117.3°E)发生7.3级地震;福建同安大地震,海潮退而复涨,鱼船多遭没;广东汕头:当时湾泊在码头的一艘船,其船底竟至与海底接触”(谢毓寿等,1983)。
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2.3 1604年泉州M 8级大地震
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1604年12月29日泉州以东海域发生8级大地震,震中位于119.4°E,24.9°N,震源深度24~28km,震中烈度XI(图1;雷土成等,1985)。泉州大地震是我国东南沿海最大的一次地震,这次地震强度大,频率高,地震破坏严重,最远距震中达220km,有感范围达1000km(许振栋,2007;刘中良等,2011)。地震顷刻,在兴化、泉州、漳州-龙海之间等平原地带,还出现地裂、喷水冒砂的地震形变带,其联线呈NE方向。福建、江西、浙江3省22个县(市)记载了不同程度的震害。关于1604年大地震的发震构造一般认为是滨海断裂带的长乐-诏安段,滨海断裂带位于台湾海峡西侧,是福建沿海水下岸坡带与台湾中央盆地之间的边界断裂,该段总长500km(黄昭等,2006)。据《泉州府志》记载,29日夜发生大震,山石海水皆动;泉州城内外楼房店铺全都倾倒;开元寺东塔顶盖南部的椽石有两条毁坏,东南角有8条毁坏;洛阳桥被破坏;多处出现地裂缝;在清源山,裂开的地缝中还涌出砂、水,气若硫磺。泉州沿海覆舟甚多。莆田城墙崩塌数处,城中高大建筑多倾塌,乡间房屋倾倒无数。田地皆裂,并冒黑砂还带硫磺臭味,池水亦因地裂而干涸。漳浦南门外的田陷一穴,宽五丈余,深约二丈,水涌出,中有黑砂泥。福宁大震时听到如雷的响声等。城内外楼房店铺悉数倾倒。在台湾新竹,也有海水位异常变化的记录,记录波高约20~30cm。
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3 国际上大洋钻探研究大地震的实例
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大地震发生在地表以下几千米至数十千米,因此,目前仍然无法实现断层周围的原位测量和监测,这也是导致无法预测地震的主要原因之一。然而,断层区域的岩石物理属性、温压条件、流体和应力状态等,是控制断层活动性、地震爆发机制的主要控制因素(林间等,2017),而获得该区域岩石样品的唯一方式是钻探。和陆地断层相比,深海大洋受制深海探测技术和经济情况的限制, 大洋钻探极具困难与挑战,但确是认识海底地震大断裂的唯一途径。海底断裂,尤其是俯冲带大断裂一般都在4~6km深的海域,而断层面在海底以下几百米到数千米,具体取决于断层顶部海底沉积物的厚度。因此,需要穿透4~6km的海水,打入海底以下几百米至数千米的深度,本身就是一项非常困难的技术。目前日本的JAMSTEC(Japan Agency For Marine-Earth Science and Technology)在全球大洋钻探技术方面处理领先的地位。2011年日本东北3月11日M w 9.1大地震之后,日本联合国际联合大洋钻探航次(IODP Expedition 343and 343T)发起日本海沟快速钻探计划(Japan Trench Fast Drilling Project)在日本俯冲带断层浅部开展了大洋钻探工作。该钻探过程中C0019钻孔在水深约6900m,离M w 9.1地震震源大概93km,离海沟大概6km,穿透浅部断层820m直到滑脱层(décollement)。该钻探在断层周围获取了重要岩石样品,测得了温度、压强等重要岩石环境物理参数,为系统研究断层的摩擦特性、应力状态和揭示地震成因,提供了至关重要的原位数据(Fulton et al., 2013; Lin Weiren et al., 2013; Ujiie et al., 2013)。
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苏门答腊和Cascadia俯冲带是世界上最活跃的俯冲带之一,前者自2004年以来已经发生三次特大地震并引发海啸:2004年M w 9.2苏门答腊-安达曼地震、2005 M w 8.6尼亚斯-锡默卢地震、2007 M w 8.4明古鲁地震和2010年M w 7.8级明打威海啸地震(Chlieh et al., 2007; Konca et al., 2007, 2009; Hill et al., 2012; 赵旭等,2017; Philibosian et al., 2020; Wirth et al., 2022),造成了巨大的人员伤亡和经济损失;后者历史上大地震多发,其中1700地震事件引发了跨洋大海啸,影响到日本及整个太平洋区域,给沿海地区造成了严重的灾难。俯冲带大地震造成的强地面震动容易触发海底滑坡和垮塌,在地震破裂区域产生广泛的海底浊流沉积。自1990年代以来,科学家已开始利用海底浊流沉积研究俯冲带的古地震。具体而言,通过对海底浊流沉积物的结构、粒度、古生物、化学性质、年龄、物源、搬运路径和空间分布等参数的约束,并将其与已知历史大地震的时间进行对比,可以用来确定浊流沉积是否是大地震产生的。Patten et al.(2013)通过对苏门答腊俯冲带109个钻孔沉积的分析,发现了2004年9.2级大地震造成的浊流沉积证据,包括非常年轻沉积物、浆糊状、近海底浊流,近零的放射性测年。利用类似的研究方法和手段,前人对Cascadia、日本海沟和新西兰俯冲带的浊流沉积进行了研究,利用浊流沉积层的年龄重建了研究区的晚更新世和全新世以来的古地震历史(图3),并根据浊流沉积的空间分布特征,提出了可能的地震破裂分段性,极大提高了对于这些俯冲带古地震的时间、周期和破裂特征的认识(Adams, 1990; Nakajima et al., 2000; Nelson et al., 2000; Goldfinger et al., 2012, 2013; Pouderoux et al., 2012; Patton et al., 2013)。
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4 南海大洋钻探建议
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以粤港澳大湾区为代表的华南沿海地区是我国人口最为密集、经济最为发达的地区之一。滨海断裂带是靠近我国华南沿海最大的一条活动断裂带。该断裂带西段(北部湾-阳江)和东段(粤东-福建)历史地震比较活跃(图1)。在东段的1918年南澳7.5级地震附近,现今微震不断,形成一个明显的地震高发区,该区将来很可能发生类似1918地震。西段靠近海口的区域曾发生过1605年琼州7.5级大地震,导致了大面积地面沉陷。西段不仅是大地震发生的区域,还是滨海断裂带分支的位置,而且现今微震活动强烈,在阳江及海域形成了一个明显的地震高发区(图1)。东西段历史文献仅记录到4次大地震,那么全新世以来大地震数量有多少,周期有多大,下一次大地震可能什么时候爆发,都无法回答。此外,珠江口盆地目前是大地震空区(孙金龙等,2012),有仪器记录以来一直没有爆发5级以上的地震,历史上也没有记录,目前小震也不活跃,该段的断层是在蠕滑还是在孕育大地震,其大地震周期是否比东、西段的长。如果是后者的话,该区域断层岩石属性和东、西段有何不同?而且与活动大陆边缘的俯冲带不同,位于被动大陆边缘的滨海断裂带发生8级以上大地震是十分罕见的,这里的大地震机制是什么,地震空间分布和破裂分段有何特征?
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图3 新西兰Hikurangi俯冲带钻孔沉积识别的浊流沉积事件及其年龄(据Pouderoux et al., 2012)
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Fig.3 The turbidite events and ages identified from the core sediments in the Hikurangi subduction zone, New Zealand (after Pouderoux et al., 2012)
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(a)—钻孔MD06-3002;(b)—钻孔MD06-3003
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(a)—The core MD06-3002; (b)—the core MD06-3003
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因此,本文提议利用高分辨率的多道地震剖面,准确理清滨海断裂带各段的几何结构特征和精准空间位置,在海口-阳江段,珠江口段和汕头段开展海底钻探研究,获取穿过断层带和地震破裂区的关键岩石和沉积样品,探索这三段的岩石物理属性、环境温压、流体运移方式和应力状态等;为系统研究断层的摩擦特性和地震成因,提供至关重要的原位数据 (Fulton et al., 2013; Lin Weiren et al., 2013; Ujiie et al., 2013)。尽管滨海断裂所处的南海北缘的浅水区很少发育浊积层,但是大地震爆发的一瞬间,可以导致断层位错,形成陡坎并产生垮塌,在断层附近产生同震的楔形的混杂堆积体,称之为地震沉积。因此,通过对各段钻孔沉积物的精细分析,厘定与大地震相关的沉积层(比如楔形的混杂堆积),并对各层的粒度、古生物、化学性质、年龄、物源、搬运路径和空间分布等参数进行定量研究。这些研究结果可以有效解决东段和西段历史大地震的准确数量、空间分布、复发周期和大地震破裂分段特征等;解决中段大地震空区的成因机制(地震复发周期过长还是断层滑动以蠕滑为主),并有望揭示被动大陆边缘的活动断裂带的发震机制和破裂特征与主动大陆边缘俯冲带大断裂的异同。研究成果和经验积累为下一步在南海、东海所面临的马尼拉和琉球俯冲带开展的大洋钻探,提供关键的科学和技术支撑,为我国华南沿海区域地震和海啸灾害评估提供重要科学数据。
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5 展望
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除了南海北缘的滨海断裂带,南海周边的俯冲带,包括安达曼-苏门答腊-爪哇、菲律宾、马尼拉和西太平洋等,都是地震、火山和海啸等地质灾害的频发区,这些灾害将影响我国东部和南部的海岸地段。
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5.1 南海的海啸灾害
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南海东部的太平洋板块和菲律宾板块向西俯冲,以及西边、南边的印澳板块向北和北东俯冲,形成了独特的东南亚超级环形俯冲汇聚系统(Li Jiabiao et al., 2021)。该超大汇聚系统是全球构造最活跃,大地震海啸最频发的区域之一。在环形俯冲带中,琉球和马尼拉是重点威胁我国沿海经济带的俯冲带。
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在南海的东北部,欧亚板块俯冲于菲律宾板块之下,形成从我国台湾省南部一直到菲律宾明都洛西部上千千米长的马尼拉俯冲带大断裂(图4)。该断裂是南海内部规模最大,致灾极强的活动大断层。南海区域古老的海啸沉积物主要分布在东北部、西北部、南部以及东部菲律宾吕宋岛,其沉积物的定年大约在1000CE(Yu Kefu et al., 2009; Sun Likuang et al., 2013; Ramos et al., 2017; Yang Jie et al., 2019)。从沉积物的空间分布来推断,南海大概在1000多年前曾发生过影响整个南海海域的超大海啸,如果这次地震导致马尼拉俯冲带整体断裂产生9级地震,其引发的海啸波在南海海岸线尤其是我国华南沿海部分区域超过10m,以粤港澳大湾区最为严重(图4;Qiu Qiang et al., 2019; 李琳琳等, 2022b)。如果从1000多年前开始计算,有历史记录500多年以来该断层没有爆发过矩震级超过7.6级的地震,同时,过去几十年综合地球物理资料、地表形变观测表明断层在积累能量,将来发生M w 8~9超大地震的风险极高(Megawataii et al., 2009; Hsu et al., 2012, 2015)。如果按照1000年为地震周期(Hsu et al., 2015),目前该断层已经到达地震爆发的临界点,大地震随时爆发的可能性很高。南海海域较为狭窄,因此产生的海啸波能量几乎都在南海内部消耗,加上超宽的大陆架和陆坡对海啸波的俘获和折射效应,会在南海大范围内引发高幅度和超长时间的波浪流,将给我国华南沿海,尤其是粤港澳大湾区,带来严重的威胁。
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琉球俯冲带在我国东南部海域,是菲律宾板块俯冲于欧亚板块之下形成。该俯冲带历史上曾爆发过矩震级7~8级以上的致灾性大地震。其中1771年M w 8.0地震,在当地产生的最大海啸爬高接近30m,同时在海啸波巨大能量的带动下,大量巨石被推到离海岸较远的内陆区域 (Goto et al., 2010)。巨石的年龄定年表明,该俯冲带很活跃,150~400a会发生一次大海啸事件。长时间尺度(2000a)的古海啸沉积成功记录了1771海啸事件,同时也揭示了相同级别的历史海啸事件重复多次发生(Goto et al., 2010; Ando et al., 2018; Fujita et al., 2020)。如果类似1771事件的再次爆发,其产生的海啸波将在我国东南沿海海域产生1~2m的波高和强流,很可能给沿岸港口、重要经济设施和高人口密度的沿海大城市带来破坏性灾难和巨大经济损失(图5)。琉球俯冲带长度约1200km和2004年M w 9.2级苏门答腊-安达曼特大地震破裂区域相当(Lay et al.2005; Chlieh et al.2007),如果发生类似的特大地震,我国东南沿海大城市,如上海、杭州、宁波和温州等将直接面临灾难性的海啸威胁。
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图4 南海马尼拉断层整体断裂(a)(断层滑移分布)和发生M w 9级超大地震产生的最大海啸波高分布(b)(据李琳琳等, 2022b)
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Fig.4 The complete rupturing of Manila megathrust (a) (fault slip distribution) and the distribution of maximum tsunami wave height triggered by M w 9giant earthquake (b) in the South China Sea (after Li Linlin et al., 2022b)
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图5 琉球1771年M w 8级地震所产生的最大海啸波高模拟结果(断层模型据Nakamura, 2009)
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Fig.5 The maximum tsunami wave triggered by the Ryukyu 1771 M w 8earthquake (fault model after Nakamura, 2009)
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5.2 南海的火山灾害
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中国海域及周边的海底火山分布主要在第一岛链(日本、菲律宾),另外在大陆架(韩国济州岛、越南)以及南海海盆也有零星的海底火山。第一岛链上的海底火山属于岛弧火山,与日本、琉球、马尼拉以及菲律宾俯冲带相关。这类型火山数量多,而且动力强,富含挥发组分,爆炸式喷发次数多,影响大,例如2022年1月汤加火山喷发(Carvajal et al., 2022; Wright et al., 2022; Omira et al., 2022; Kubota et al., 2022; 张锦昌等,2022)。大陆架上的海底火山属于板内火山,是地幔柱或者热点的成因,例如雷琼火山区(毛建仁等,1999;胡久常等,2007;鄢全树等,2007;Lei Jianshe et al., 2009; Huang Jinli, 2014; Xia Shaohong et al., 2016)。这些火山零星分布在大陆架上,数量不多,而且属于地幔柱尾的火山喷发,影响小于第一岛链的火山。南海海盆里的海底火山主要有两种:第一种是海底扩张时期产生的洋脊火山;第二种是板内热点产生的海山(陈洁等,2012;Hui Gege et al., 2016; Zhang Jie et al., 2020)。这两种火山已经不再活跃了,寂静无声。
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6 结论
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南海北缘的滨海断裂带是一条大型的控震和孕震断裂,其位置靠近人口密集和经济发达的华南沿海地区。滨海断裂带全长超过1200km,包括西段(北部湾-阳江),中段(珠江口)和西段(粤东-福建),其东段和西段历史上至少曾发生过4次大地震 (M 7+),中段目前存在一个大地震空区。如果滨海断裂带再次发生大地震并触发海啸,必将对我国华南沿海地区的人员安全和经济发展造成严重威胁。由于缺乏完整的历史地震和古地震记录以及断层带的岩石物理性质,目前尚不清楚滨海断裂带的大地震的准确次数、空间分布、复发周期和断层滑动性质(蠕滑或黏滑)。为解决上述问题,本文建议未来大洋钻探在滨海断裂带西段、中段和东段进行钻探研究,获取断层各段的关键岩石和沉积资料,利用浊流沉积古地震和岩石物理性质研究等手段,重建滨海断裂带古地震历史、复发周期和空间分布,揭示地震空区的断层摩擦性质,为华南沿海的防灾减灾提供重要的科学依据。
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致谢:感谢中国科学院南海海洋研究所深海地球动力学学科组成员对本文的帮助和讨论!
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摘要
滨海断裂带是南海北缘的一条大型活动断裂带,其位置靠近我国华南沿海地区。滨海断裂带全长超过1200 km,包括西段(北部湾-阳江),中段(珠江口)和东段(粤东-福建)。其西段和东段历史上至少曾发生过4次大地震(M 7+),中段目前是一个大地震空区。在经济高速发展和人口高度密集的今天,如果滨海断裂带再次发生大地震并触发海啸,必将对我国华南沿海地区造成灾难性破坏。由于缺乏完整的历史地震记录和针对古地震的钻孔沉积研究,目前尚不清楚滨海断裂带大地震的准确次数、空间分布和复发周期,以及中段大地震空区的主要原因(断层蠕滑或大地震周期较长),因此无法有效评估该断裂带的大地震破裂分段和灾害风险。本研究总结了滨海断裂带的构造特征、重点描述了3次历史大地震及引发的灾害影响,和国际上针对海底大地震的钻探研究经验。根据这些信息,本文建议在断裂带的西段、中断和东段进行大洋钻探,获取穿过断层带的关键沉积和岩石样品,利用沉积古地震方法重建滨海断裂带东段和西段的大地震历史和复发周期,研究断层带的岩石物理性质,揭示滨海断裂中段大地震空区的成因,解析断层分段式破裂的原因,为我国海洋防灾减灾提供重要的科学依据。
Abstract
The Littoral Fault Zone is a major active fault zone in the northern margin of the South China Sea, close to the coastal region of the southern China. The Littoral Fault Zone is more than 1200 km long, consisting of the west segment (Beibu Bay-Yangjiang), the middle segment (Pearl River Mouth), and the east segment (Yuedong-Fujian). At least 4 large earthquakes (magnitude＞7) have occurred inthe west and east segments, while its middle segment was exempt. As the economy is rapidly growing and the present day population density is high, the littoral fault will cause enormous destruction to the coastal area of southern China if it generates great earthquakes and the ensuing tsunamis in future. Due to lack of complete historical earthquake records and oceanic drilling study on the paleoseismicity, there is lack of clarity on the precise quantity, spatial distribution and recurrence period of great earthquakes along the Littoral Fault Zone, and the nature of great earthquake quiet zone (aseismic slip or longer recurrence period of great earthquakes), which in turn leads to poor understanding of earthquake mechanism and associated risk of marine geohazards. This study summarizes the structural characteristics, describes the hazard influences of 3 large earthquakes along the Littoral Fault Zone, and introduces the international oceanic drilling studies on great earthquakes. Based on this information, we propose to carry out oceanic drilling into the west, middle and east segment of the Littoral Fault Zone, recover essential sediments and rocks through the fault zone, reconstruct the history and recurrence period of paleoseismicity along west and east parts of the Littoral Fault Zone, study the physical properties of fault zone rocks to identify the cause of earthquake quiet zone in the middle part of the Littoral Fault Zone, analyze the mechanism behavior of fault segments, finally providing critical scientific basis for the marine hazard preparedness and mitigation strategies.
Keywords
Littoral Fault Zone ； oceanic drilling ； fault structure ； core sediments ； earthquakes ； tsunami
