Discussion on the overestimated magnitude of the 1920 Haiyuan earthquake
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摘要: 1920年海原大地震作为史料记载以来中国大陆震级最高、伤亡最多的极具破坏性的地震之一,开启了我国用现代地震学方法研究大地震的新篇章,在我国地震研究史上具有里程碑的意义。最新研究结果表明,1920年海原大地震的矩震级为MW(7.9±0.2),与文献和大众广泛接受的M8${\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}} $的数值相差较大。本文通过对震级标度及其演化历史的总结和梳理,阐述了仪器记录早期阶段基于地震波波形振幅和频率的震级标定存在系统偏差的问题,这与仪器限制、台站稀疏、标定不统一等因素有关,也使得1920年海原大地震和同时期世界上其它一些重要大地震的震级不同程度被高估。在各种震级标度中,矩震级MW与地震破裂面积和位移等物理参数关联,是地震震级的最佳标定方法。震级作为表述地震大小和能量的重要参数,被广泛地用于评估断层未来的地震潜势;震级的偏差对地震活动时空分布样式的研究会产生重要影响,并造成基于历史地震资料的地震危险性评价和灾害评估等产品的可信度降低。因此,本文倡导对历史地震震级进行检验和修订,并建议1920年海原大地震的震级采用矩震级MW(7.9±0.2)表示,修正后的1920年海原大地震的震级与2008年汶川地震(MW7.9,MS8.0)和2001年昆仑山大地震(MW7.8,MS8.1)相当。Abstract: The great 1920 Haiyuan earthquake, resulting in tremendous casualties, ranks as one of the largest and most devastating earthquakes in China. This significant event marks the start of investigating earthquakes through modern scientific approaches in China. Recent studies show that the moment magnitude of the 1920 Haiyuan earthquake is MW(7.9±0.2), prominently smaller than the widely known and often cited magnitude M8${\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}} $. This paper reviews the re-calibration and conversion of different types of magnitude in the early developing phase of seismometers and analogue seismographs. Similar to the 1920 Haiyuan earthquake, the magnitude of many large shallow earthquakes that occurred in this period are systematically overestimated due to factors such as developing technology, sparse instrumentation and data, and diverse calibration functions. The moment magnitude, linked to physical parameters of earthquake rupture, is the best magnitude scale. For magnitude is the most commonly used parameter in describing an earthquake’s size and energy and is an essential factor in seismic hazard assessment, bias and errors in magnitude conversion have significant consequences in understanding the spatio-temporal pattern of historical seismicity and the reliability of various products of seismic potential and hazard evaluation. We thus advocate citing revised moment magnitude MW(7.9±0.2) for the 1920 Haiyuan earthquake in future studies and re-evaluating the magnitude of historical earthquakes in general. With a revised magnitude, the 1920 Haiyuan earthquake is similar in size to the 2008 Wenchuan earthquake (MW7.9, MS8.0) and the 2001 Kunlun earthquake (MW7.8, MS8.1).
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引言
据中国地震台网中心(2019)正式测定,北京时间2019年10月28日1时56分,甘肃省甘南州夏河县(35.10°N,102.69°E)发生MS5.7地震,震源深度为10 km (下文简称为夏河地震)。夏河地震发生在西秦岭北缘断裂与临潭—宕昌断裂之间(图1)。该区历史上中强地震较少,仅1936年康乐M6¾地震和1964年和政M5地震有记载,这两次地震与西秦岭北缘断裂有关(国家地震局震害防御司,1995;张波等,2015)。
图 1 区域地形地貌、活动构造和历史地震分布图F1:东昆仑断裂;F2:西秦岭北缘断裂;F3:龙门山断裂;F4:六盘山断裂;F5:塔藏断裂;F6:白龙江断裂;F7:光盖山—迭山断裂;F8:临潭—宕昌断裂;F9:哈南断裂;F10:武都—康县断裂;F11:两当—江洛断裂;F12:礼县—罗家堡断裂;F13:龙日坝断裂;F14:岷江断裂;F15:虎牙断裂. 底图为SRTM1 (shutter radar topographic mission)的数字高程模型(digital elevation model,缩写为DEM) 数据(分辨率为30 m);震源机制解来源于GCMT (2019)Figure 1. Regional topography,active tectonics and historical earthquakesF1:East Kunlun fault;F2:West Qinling fault;F3:Longmenshan fault;F4:Liupanshan fault;F5:Tazang fault;F6:Bailongjiang fault;F7:Guanggaishan-Dieshan fault;F8:Lintan-Dangchang fault;F9:Ha’nan fault;F10:Wudu-Kangxian fault;F11:Liangdang-Jiangluo fault;F12:Lixian-Luojiapu fault;F13:Longriba fault;F14:Minjiang fault;F15:Huya fault. The base map is SRTM1 (shutter radar topographic Mission) DEM (resolution is 30 m),and the focal mechanism solutions are from GCMT (2019)临潭—宕昌断裂东段中强地震活跃,发生过2013年岷县—漳县MS6.6地震、1837年岷县M6地震、1573年岷县M6¾地震、2003年岷县MS5.2地震、2006年岷县—卓尼MS5.0地震等(国家地震局震害防御司,1995;郑文俊等,2005,2007a,b,2013a,b;何文贵等,2006,2013);而原认为与临潭—宕昌断裂西段有关的公元842年碌曲M7地震,经史料考证和发震构造调查,被认为是光盖山—迭山断裂西段的构造活动(袁道阳等,2014)。临潭—宕昌断裂的强震活动表现为“东强西弱”,其原因和构造机理尚不清楚。
2019年夏河MS5.7地震位于中强地震稀少的临潭—宕昌断裂西段附近,对于研究临潭—宕昌断裂西段的构造活动以及分析临潭—宕昌断裂“东强西弱”的分段活动差异具有十分重要的意义。本文拟通过研究震中周边活动断裂的空间展布图像和新活动特征,结合刘旭宙等(2021)通过震源机制解和地震重定位而判定的发震节面,分析其与区域构造的关系,建立夏河地震的构造模型。
1. 区域构造背景
夏河地震位于南北地震带中段的松潘—西秦岭构造结,是青藏高原与四川盆地、鄂尔多斯地块相互作用而强烈变形的区域,也是东昆仑断裂与西秦岭北缘断裂的左阶过渡区,阶区内的构造活动受控于东昆仑断裂和西秦岭北缘断裂,阶区内发育弧形次级断裂系,从西向东断裂走向由WNW转向NE,形成南凸的弧形断裂系,弧形断裂系共同分配和调节了东昆仑断裂和西秦岭北缘断裂的差异活动速率(袁道阳等,2004;郑文俊等,2016)(图1)。晚第四纪以来,弧形构造系活动强烈,发生了1654年礼县M8、1879年武都南M8、公元前186年武都M7、公元842年碌曲M7等大地震和数十次中强地震(顾功叙,1983;国家地震局震害防御司,1995)。
由于西秦岭地形起伏大、海拔高、侵蚀改造强烈等原因,很多研究人员在研究区开展了活动构造和历史地震研究,但是东昆仑—西秦岭阶区内弧形断裂系的几何展布仍然不明,断裂活动性质、活动性定量参数和历史地震等研究一直比较薄弱(韩竹军等,2001;袁道阳等,2004;冯希杰等,2005;侯康明等,2005;贾伟等,2012,2018;俞晶星等,2012;何文贵等,2013;刘兴旺等,2015;杨晓平等,2015;郑文俊等,2016;Zheng et al,2016;张波等,2021)。
2019年夏河MS5.7地震震中附近的已知活动断裂包括北侧的西秦岭北缘断裂锅麻滩段和南侧的临潭—宕昌断裂西段。西秦岭北缘断裂锅麻滩段西起甘加以西,向东经切龙沟、大槐沟、松香滩、羊毛沟、锅麻滩至莲麓以东,长约165 km,由多条雁列排布的次级断裂组成。该断裂的新活动以左旋走滑为主,全新世以来该段发生过两次古地震事件,分别距今(1 932±54)年和(3 602±89)—(5 880±115)年(张波等,2012)。1936年康乐M6¾ 地震也发生在该断裂上,形成了长达14 km的地表破裂带,现今仍残留地震陡坎、地裂缝、大型基岩崩塌等地表破裂和次生灾害,最大同震左旋和垂直位移为2.5 m和0.6 m (张波等,2015)。
临潭—宕昌断裂西起合作以西,向东经临潭、岷县至宕昌以东,走向由WNW逐渐转变为NNW,形成向北东凸出的弧形,延伸长达300 km (袁道阳等,2004;何文贵等,2013;郑文俊等,2013a,b;张波等,2021)。该断裂几何结构复杂,由多条规模不等、相互平行或斜接的次级断裂组合而成,自西向东可分为三段:西段由南、北两支组成,南、北支又可进一步细分,南支断裂走向为NE−ENE,北支断裂走向为WNW;中段各分支断裂的几何形态收敛,走向为WNW;东段断裂分散成临潭—宕昌断裂主断裂、禾驮断裂、木寨岭断裂和柏林口断裂等四条分支断裂,其中,临潭—宕昌断裂主断裂可能是2013年岷县漳县MS6.6地震的发震构造(何文贵等,2013;郑文俊,2013b)。临潭—宕昌断裂主断裂的运动性质以左旋走滑为主,局部段表现为向南逆冲,最新活动时代以晚更新世为主。张波等(2021)在中段的贡恰村一带首次发现临潭—宕昌断裂的全新世活动。夏河地震震中南侧为临潭—宕昌断裂的西段北支,目前尚未见关于该段的研究成果,断裂的几何展布和活动性质也不明。
在临潭—宕昌断裂西段与西秦岭北缘断裂锅麻滩段之间,地质图上还标绘了两条活动性未知的断裂,这两条断裂也未见前人研究成果,本文暂将这两条断裂命名为夏河断裂和达麦—合作断裂(图2)。夏河断裂发育于三叠系内部,而达麦—合作断裂位于二叠系与三叠系之间。夏河断裂距离夏河地震震中北侧仅2.6 km,可能与夏河地震存在密切的关联。
图 2 2019年夏河MS5.7地震周边构造分布地层、基岩断层来源于1 ∶ 20万地质图(合作幅)①,地震烈度引自甘肃省地震局(2019)Figure 2. Active tectonics around the epicenter of the Xiahe MS5.7 earthquake in 2019Stratigraphy and bedrock fault data are from 1 ∶ 200 000 geological map (Hezuo district). The earthquake intensity field refers to Gansu Earthquake Agency (2019)2. 研究方法
2.1 遥感图像选取和解译
遥感图像解译是研究活动断裂的基础工作。在研究区附近,张波等(2018)通过解译多种遥感图像,结合野外地质地貌调查验证,研究了光盖山—迭山断裂的几何展布图像和新活动特征。解译夏河地震震中附近的断裂时主要使用谷歌地球图像(亚米级),但极震区位于几幅图像的拼接部位,存在大片阴影区和云区,而考虑到其它时相的图像分辨率又过低,解译中还使用了印度遥感卫星IRS-P5影像(全色分辨率为2.5 m)。在宏观地貌解译中,使用分辨率为12.5 m的日本对地观测卫星ALOS (Advanced Land Observing Satellite)的数字高程模型(DEM)。
通过收集研究区附近活动断裂研究的相关文献(张波等,2012,2018;何文贵等,2013),结合笔者在研究区附近的工作经验,归纳了研究区活动断裂的解译标志。活动断裂最明显的标志是清晰连续的线性特征,线性特征在图像上多表现为线性色调差异,如线性亮色、暗色条带以及色差分界带(张波等,2018)。根据研究区活动断裂的运动特征,可能还存在冲沟左旋、断层泉、断塞塘、地形坡折等识别标志。①
2.2 野外考察
在详细解译的基础上,在野外对典型点进行踏勘和详细研究,修正解译结果,总结断错地质地貌表现。根据断错地质地貌特征,分析断裂的活动性质;根据被断错的典型地貌面的年龄对比,分析断裂的最新活动时代;根据地形地貌和断裂形迹,基于地质学中的“V”字形法则,判断断面的倾向。最后,初步获得了夏河地震震中附近的断裂展布和新活动特征。
3. 结果
3.1 夏河断裂新活动的发现
通过遥感解译和野外考察,发现夏河断裂西接贵德断裂,向东经官秀寺、交隆务、多哇镇南、拉卜楞寺、塘乃合村、洒易囊村南、当浪咱村南,终止于合作西北的多朗囊一带(图3)。夏河断裂西段主要位于拉卜楞寺以西,走向近EW,断裂长度为54 km (图3a);夏河断裂东段主要位于拉卜楞寺以东,总体走向为ESE,断裂长度为39 km (图3b)。
夏河断裂东段控制了夏河东南侧的宏观地貌边界,在较大几何尺度上表现为地形的坡折,穿过断裂的多条大冲沟和山脊显示同步左旋(图3)。东段沿线可见较典型的线性地貌,包括断层陡坎、左旋、断层沟槽和断层垭口等连续出露的断层地貌(图4)。塘乃合南侧,断层连续错动山脊和冲沟,形成断层陡坎和断层沟槽等地貌(图4a,b);一条小冲沟左旋6.5 m,冲沟左岸的二级阶地左旋6 m (图4c),二级阶地拔河高度为2.5—5 m,根据阶地拔河高度和形成年龄的经验公式(袁道阳等,1999)推算,二级阶地形成于全新世中期。该冲沟东侧200 m发育一条大冲沟,冲沟二级阶地上保留两条断层陡坎,断层陡坎高均为1.5 m (图4d),该阶地拔河高度为7—9 m,根据阶地拔河高度和形成年龄的经验公式(袁道阳等,1999)推算,该阶地形成于晚更新世—全新世。形成于晚更新世—全新世的微地貌(冲沟阶地)被断错,说明夏河断裂东段为全新世活动断裂。
图 4 夏河断裂东段塘乃合村南断层地貌(a) 塘乃合南侧断层卫星影像,红色箭头代表断层;(b) 塘乃合南侧断层线性地貌,红色箭头代表断层,镜向E;(c) 塘乃合南侧小冲沟及二阶阶地左旋位错,红色实线代表断层,黄色实线代表阶地前缘,蓝色实线和箭头代表冲沟和流向,镜向SES;(d) 图(c)冲沟东侧200 m处的大冲沟东侧断层地貌,红色箭头代表断层,黄色虚线代表阶地面上的地形陡坎,蓝色实线和箭头代表冲沟和流向,镜向EFigure 4. Offset geomorphologies to the south of the Tangnaihe village along the east segment of the Xiahe fault(a) Remote sensing image of fault trace,where red arrows represent fault trace;(b) Linear fault trace to the south of the Tangnaihe village,where red arrows represent fault trace,view to E;(c) Left-lateral offset of a small gully and its terrace,red solid line represents fault,yellow solid lines represent terrace edge,blue solid line and arrow represent gully and flow direction,view to SES;(d) Offset geomorphologies 200 m east of the gully in Fig. (c), where red arrows represent fault trace,yellow dashed lines indicate topographic scarps on gully terrace,blue solid line and arrow represent gully and flow direction,view to E另外,从断裂的地表展布和地形地貌(图3)来看,断层在冲沟内更靠南,在山坡上更靠北,地表形迹为向北东凸出的弧形,基于“V”字形法则判断夏河断裂东段具有向北的逆冲分量(仰冲),断层面倾向SW。数字高程模型(SRTM3 DEM和ALOS DEM)显示(图2,7),夏河断裂一线表现为南高北低的宏观差异构造地貌,支持“V”字形法则的判断结果。因此,夏河断裂东段兼具逆冲和左旋走滑分量,断面倾向南。
图 7 2019年夏河MS5.7地震的发震构造模型(a) 重定位后的夏河地震序列及沿剖面BA和DC的地震深度分布(刘旭宙等,2021);(b) 夏河地震与周边断裂的平面关系;(c) 夏河地震与周边断裂的深部关系,图中红色虚线为刘旭宙等(2021)认为的发震断层Figure 7. Seismotectonic model of the MS5.7 Xiahe earthquake in 2019(a) The relocated earthquake sequence,and depth distribution of earthquakes along profiles BA and DC (after Liu et al,2021); (b) Plane view on the relationship between the Xiahe earthquake and surface faults;(c) Relationship between the Xiahe earthquake and surrounding faults in deep where the red dashed line is the potential seismogenic fault supported by seismic sequence relocation and focal mechanism presented by Liu et al (2021)3.2 达麦—合作断裂
达麦—合作断裂起自达麦西北,向东南延伸至合作市,在合作东南被临潭—宕昌断裂截断(图2)。该断裂走向为N30°−50°W,长约60 km,倾向NE,倾角为65°。断裂主要发育在二叠系灰褐色砂岩、灰岩和三叠系灰黄色板岩之间,局部段发育在下三叠统灰黄色板岩与燕山期花岗闪长岩之间,表现为二叠系灰褐色砂岩和燕山期花岗闪长岩向南西逆冲至下三叠统灰黄色板岩之上,破碎带宽10 m左右(图5)。经多种遥感图像综合解译,结果显示该断裂在地表无线性显示。野外考察表明,断层在扎油沟东侧山坡通过处无明显断错地貌,山脊和冲沟无同步变形。因此,达麦—合作断裂应为一条前第四纪断裂。
3.3 临潭—宕昌断裂西段
通过遥感解译和野外考察,完善了临潭—宕昌断裂西段北支的几何展布图像(图2)。西段北支可细分成两条次级断裂:① 合作南断裂,控制合作新近系盆地的南边界,形成盆山地貌;② 临潭—宕昌断裂西段的主体部分,向西延伸至旦尔道一带后分散成近平行的两条断裂,在桑科盆地南边界附近,两条断裂收敛并终止。
合作南断裂构成盆山边界,北侧为新近系(N),南侧为古生界(Pz)。微地貌断错包括冲沟和阶地的同步左旋,阶地拔沟高度低于5 m [据袁道阳等(1999)的经验公式,估计形成于晚更新世至全新世 ] ,说明合作南断裂自晚更新世以来活动过,且具有明显的左旋走滑分量。临潭—宕昌断裂西段的主体部分在国道合作界牌处揭露断层剖面,剖面上显示逆断层,倾向南,倾角30°,断裂由南向北逆冲(图6)。同时,地形上发育坡向北、高数米的陡坎,支持断层由南向北的逆冲活动。地质地貌现象表明,临潭—宕昌断裂西段北支的运动性质兼具逆冲和左旋走滑。
4. 讨论
4.1 夏河地震的发震构造模型
上述各断裂展布、活动性质和断层产状如图7所示。临潭—宕昌断裂西段包含3条次级分支,其中:合作南断层、西段主体(震中南侧18 km)均向南倾,断面往深部延伸(下延)时会远离震中;达麦—合作断裂位于震中北东侧,倾向NE,断面下延时同样远离震中;西秦岭北缘断裂位于震中北侧20 km外,断层面倾向南,倾角为45°—65°,虽然断面下延时会靠近震中,但距离震中仍然较远;夏河断裂距离震中北侧仅2.6 km,断面倾向南,断面下延时会靠近震源和震中,震中向深部的投影(震源)可能与断裂相交。从震中与断裂位置、断层产状的几何关系可知,夏河断裂与2019年夏河MS5.7地震可能存在密切关联。
刘旭宙等(2021)通过gCAP方法和P波初动求解夏河地震的震源机制解(表1),并利用双差定位方法对夏河地震序列进行了重新定位(图7a),认为发震断层的节面参数为:走向312°,倾角48°,滑动角48°,该节面与GCMT (2019)和USGS (2020)的节面Ⅰ大体一致(表1)。发震断层无法与地表断裂对应,可能是一条隐伏断裂,该断裂走向为312°,在平面上与夏河断裂东段(走向110°)呈小角度(夹角22°)斜交;发震断层倾向北东(42°),而夏河断裂倾向200° (SW),二者在深部可能相交(图7b,c);发震断层和滑动角48°与夏河断裂的运动性质(兼具逆冲和左旋走滑)相对应。夏河断裂在地表形成了显著的宏观构造地貌,反映其长期的构造活动历史,并且夏河断裂本身具有较大的规模(夏河断裂东段长39 km),所以该断裂应该是临潭—宕昌断裂与西秦岭北缘断裂之间的一条主干断裂,对区域地形地貌、构造活动起控制作用。据此推测,发震断层向深部延伸时可能归并到夏河断裂上,可能是夏河断裂的一条隐伏分支。因此,2019年夏河MS5.7地震可能是夏河断裂东段派生的一条隐伏分支活动的结果,该分支走向312°,倾向42°,倾角48°,平面上与夏河断裂小角度相交,在深部归并到夏河断裂。
表 1 不同研究机构发布的2019年夏河MS5.7地震的震源机制解Table 1. Focal mechanism solutions of the 2019 Xiahe MS5.7 earthquake issued by different institutions资料来源 节面Ⅰ 节面Ⅱ MW 震源深度
/km走向/° 倾角/° 滑动角/° 走向/° 倾角/° 滑动角/° GFZ (2019) 336 57 84 166 34 99 5.2 10 USGS (2020) 307 37 40 184 66 120 5.3 14 GCMT (2019) 307 48 40 188 62 131 5.3 16.3 刘旭宙等(2021) 312 48 48 185 56 127 5.36 5.9 4.2 夏河断裂的构造归属
本文初步查明了夏河地震周边断裂的几何图像和最新活动特征,发现夏河断裂具有全新世活动,活动性质兼具左旋走滑和逆冲。结合刘旭宙等(2021)判定的发震断层分析夏河地震的发震构造模型,认为发震断层可能是夏河断裂东段派生出的一条隐伏分支(走向312°,倾向42°,倾角48°),2019年夏河MS5.7地震可能是夏河断裂东段构造活动的结果。
夏河断裂东段倾向南,断面下延在趋势上可能归并到临潭—宕昌断裂或滑脱面上(图7c)。因此,临潭—宕昌断裂西段除了南支(ENE走向)和北支(EW−WNW走向)以外,还包含更北侧的夏河断裂东段。临潭—宕昌断裂整体形成中间收敛、两端发散的几何形态,其西段与东段类似,分散成多条分支断层,但发散形态与东段不同:西段从中部向南、北两侧散开,而东段主要呈现向南的帚状散开。
4.3 西秦岭西端的三维构造模型与夏河地震发震机制
郑文俊等(2013b)在研究2013年岷县漳县MS6.6地震时指出,临潭—宕昌断裂是西秦岭北缘断裂花状构造的组成部分,且在深部归并到西秦岭北缘断裂上(图7c)。唐克—合作—临夏深地震反射剖面(王海燕等,2014;Gao et al,2014;Xu et al,2017)跨越了东昆仑—西秦岭过渡区的西端,其结果显示:东昆仑—西秦岭过渡区的深反射层的变形情况大致可以分成南北两段,南段的迭部—白龙江断裂带以强烈褶皱变形为主,北段的西秦岭北缘断裂和临潭—宕昌断裂(碌曲—麻当)构成一个相对宽缓的变形带;自东昆仑断裂往北,向北倾的下地壳内反射层清晰可见,而到合作一带,下地壳内反射层变为向南倾,北倾与南倾的分界线贯穿整个地壳,同时将临潭—宕昌断裂与迭部—白龙江断裂系(白龙江断裂和光盖山—迭山断裂)截然分开。因此,临潭—宕昌断裂与西秦岭北缘断裂的关系更加密切,二者对应的上地壳反射层变形也不同于东昆仑—白龙江变形带。南段上地壳的反射层变形由于受到东昆仑断裂的直接影响,活动速率大,褶皱变形强烈,地形发生强烈隆起,河流急剧下切,形成了极高的地形起伏度(张波等,2018);北段临潭—宕昌断裂受西秦岭北缘边界断裂的直接影响,活动速率相对较小,上地壳反射层相对平缓,仅局部位置发育强烈变形带。夏河地震是临潭—宕昌断裂西段的构造活动直接受到西秦岭北缘断裂控制的反映。
横跨西秦岭西段的两条大地电磁剖面(赵凌强等,2015)显示西秦岭阶区地壳内形成倒梯形的高电阻体,说明若尔盖盆地和陇中盆地均向西秦岭深部俯冲,两条边界断裂相向俯冲,形成西秦岭深部结构的边界条件。因此,东昆仑断裂和西秦岭北缘断裂从地表和深部均控制了西秦岭阶区的构造变形。前人研究表明,南边界东昆仑断裂两盘的相对运动速率大(van der Woerd et al,2002;Kirby et al,2007;Harkins et al,2010;李陈侠等,2011;Ren et al,2013;胡朝忠等,2017),北边界西秦岭北缘断裂两盘的相对速率小(滕瑞增等,1994;李传友等,2007),阶区南北边界左旋速率的差异必然导致阶区内部的变形调整,调整主要发生在阶区内的次级断裂上,变形方式包括断错和褶皱。
在这样的构造环境下,与边界断裂平行的阶区内WNW向断裂兼具左旋走滑和逆冲的性质,逐步吸收和分配了边界断裂的差异变形(袁道阳等,2004)。临潭—宕昌断裂西段北支和夏河断裂的运动性质以及夏河地震的发生均与区域构造变形分析一致。由于临潭—宕昌断裂东段的走向发生顺时针旋转,且该段具有左旋走滑的运动分量,因此构造应力容易在该段发生重新分配和快速释放,所以历史上3次M6—7地震和多次M4—6中强地震主要发生在临潭—宕昌断裂东段(国家地震局震害防御司,1995;郑文俊等,2005,2007a,b,2013a,b;何文贵等,2006,2013)。本研究发现临潭—宕昌断裂西段呈张开状,分支断层较多,各分支断层的走向也有差异,一些分支断层未出露地表。与东段类似,西段的构造应力由于没有稳定的积累环境,容易发生重新分配和快速释放,在释放的过程中形成了2019年夏河MS5.7地震。
5. 讨论与结论
本文通过全面调查2019年甘肃夏河MS5.7地震震中周边的断裂,发现了夏河活动断裂,在此基础上结合地质和地球物理资料,综合分析并构建了夏河地震的发震构造模型,提出夏河地震可能是夏河断裂东段隐伏分支活动的结果。震中周边完整的断裂图像是深入分析地震构造、建立发震构造建模的基础,也是地震灾害风险普查的关键。
宏观构造地貌和“V”字形法则指示夏河断裂倾向南,是临潭—宕昌断裂西段的分支,而临潭—宕昌断裂在深部归并到西秦岭北缘断裂,所以夏河地震代表临潭—宕昌断裂西段和西秦岭北缘断裂西段的构造活动,是调节西秦岭北缘断裂与东昆仑断裂之间差异构造变形的体现。
2019年甘肃夏河MS5.7地震的发生,说明临潭—宕昌断裂的西段与东段一样,也具有孕育中强地震的能力。下一步我们将深入研究震中周边断裂(尤其是夏河断裂)的几何学和运动学特征,获取震中周边活动断裂的古地震和滑动速率等定量活动参数,完善区域构造模型和地球动力学模型,为区域中长期地震危险性分析提供基础资料。
感谢审稿老师严谨认真的修改意见,笔者获益良多。
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图 2 1920年海原大地震地表破裂几何形态平面展布(修改自国家地震局地质研究所,宁夏回族自治区地震局,1990)(a)和同震左旋位移沿断裂分布的不同研究结果对比(b),图(b)右侧为位移量直方图
Figure 2. Surface rupture geometry of the 1920 Haiyuan earthquake (revsied from Institute of Geology of State Seismological Bureau,Seimological Bureau of Ningxia Hui Autonomous Region,1990)(a) and comparison of coseismic left-lateral offsets along fault strike in different studies (b). The right panel of Fig. (b) is the histogram of all measured offsets
图 3 (a) 高精度LiDAR三维地形再现干盐池唐家坡村1920年地表破裂形成的陡坎和石垒田埂(7.5±1) m的左旋错断;(b) 在该点位附近沿断裂约400 m范围内Zhang等(1987)量测多个左旋同震位移,从4.8 m到7.5 m不等
Figure 3. (a) High-resolution 3D LiDAR topography shows the stone wall being left-lateral offset (7.5±1) m near the village of Tangjiapo,Ganyanchi;(b) Near the site,Zhang et al (1987) measured multiple sinistral coseismic offsets from 4.8 m to 7.5 m over about 400 m distance along the fault
表 1 1920年海原地震震级的不同估算值及文献来源 (修改自Ou et al,2020)
Table 1 Estimates of the magnitude of the 1920 Haiyuan earthquake (modified from Ou et al,2020)
震级 计算或估算方法 文献来源 M8.5 频率约为20 s的面波振幅 Gutenberg和Richter (1941) m7.9 体波震级公式${{m}_{{\rm{B}}}=\mathrm{lg}{\left({ {A}_{{\rm{H}}} }/{T}\right)}_{{\rm{max}}}+Q ( \varDelta ) }$ Gutenberg和Richter (1956) MW8.3 基于瑞雷面波频谱密度与45°断层倾角假设 Chen和Molnar (1977) MW7.8 由Chen和Molnar (1977)给出的地震矩计算而得 Kanamori (1977) MS8.6 根据Gutenberg的笔记重新修订 Abe (1981) mB7.9 根据Gutenberg的笔记重新修订 Abe (1981) M8.5 基于地震烈度M=0.58I0+1.5 顾功叙等(1983) MW8 基于90°断层倾角假设对Chen和Molnar (1977)结果进行修正 Deng等(1984) M8.6 根据Gutenberg (1945b)的方法编译 谢毓寿和蔡美彪 (1986) MS8.4 从MS8.6修正 Pacheco和Sykes (1992) MW8.3 引用Chen和Molnar (1977) International Seismological Centre (2013) MS8.7 基于三个地震记录计算得到 International Seismological Centre (2014) MW7.8 基于地表破裂长度约240 km和同震位移最大值和平均值 Liu-Zeng等(2015) MW8.2 基于震源物理动态模拟的理论计算 Xu等(2019) MW(7.9±0.2) 将早期地震波形记录扫描并数字化,计算体波震级和面波
震级并换算,辅以体波波形进行正演拟合Ou等(2020) -
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