地球物理学报  2014, Vol. 57 Issue (9): 3069-3079   PDF    
丽江—清镇剖面上地壳速度结构及其与鲁甸MS6.5级地震孕震环境的关系
徐涛1, 张明辉1,2, 田小波1, 郑勇3, 白志明1, 武澄泷1,2, 张忠杰1, 滕吉文1    
1. 中国科学院地质与地球物理研究所, 岩石圈演化国家重点实验室, 北京 100029;
2. 中国科学院大学, 北京 100049;
3. 中国科学院测量与地球物理研究所, 大地测量与地球动力学国家重点实验室, 武汉 430077
摘要:2014年8月3日,云南省昭通—鲁甸地区发生MS6.5级地震,造成了重大的人员伤亡和财产损失.鲁甸震区位于扬子块体的西缘,小江断裂带的东侧北东向的昭通—莲峰断裂带内.由于至今没有穿越该断裂带的人工源深地震测深剖面,而丽江—攀枝花—清镇650 km长深地震测深剖面距离鲁甸主震区不超过50 km,利用宽角地震资料的初至波震相,通过有限差分反演揭示该地区上地壳速度结构,可以为鲁甸震区的地震定位、地震孕育机制等提供深部速度模型.速度剖面显示:剖面结晶基底厚度平均为2 km左右;小江断裂带速度较低,东西两侧的速度较高;因此小江断裂带区域地壳强度比较低,加上断裂两侧的应变速率很高,所以小江断裂带和旁边的鲁甸—昭通断裂带,未来具有发生较大地震的可能,值得关注.
关键词鲁甸地震     人工源地震     初至波成像     有限差分法     上地壳速度结构     昭通—莲峰断裂     小江断裂    
Upper crustal velocity of Lijiang-Qingzhen profile and its relationship with the seismogenic environment of the MS6.5 Ludian earthquake
XU Tao1, ZHANG Ming-Hui1,2, TIAN Xiao-Bo1, ZHENG Yong3, BAI Zhi-Ming1, WU Cheng-Long1,2, ZHANG Zhong-Jie1, TENG Ji-Wen1    
1. State key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. State Key Laboratory of Geodesy and Earth's Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China
Abstract: On August 3, 2014, an MS 6.5 earthquake occurred in Ludian County, Yunnan. The Ludian earthquake ruptured along the NE-oriented Zhaotong-Lianfeng fault zone, western margin of the Yangtze block. However, there was no seismic sounding profile across the Zhaotong-Lianfeng fault zone up to now. Since the distance from Lijiang-Panzhihua-Qingzhen active source seismic sounding profile to the Ludian earthquake source region is within 50 km, we can handle the wide-angle seismic data to reveal the upper crustal velocity structure in this area, and provide the velocity model of upper crust for the earthquake locating and focal mechanism inversion of Ludian earthquake sequence, etc. The velocity profile shows that the crustal velocity around the Xiaojiang fault is lower than its periphery, thus the rheological strength of the upper crust beneath the Xiaojiang fault is speculated to be weaker than its surrounding areas. Considering the high strain rate along the Xiaojiang fault, it is possible to accumulate great strain and strong stress in the Xiaojiang fault zone and its surrounding faults. Thus we should pay attention to the potential risk of strong earthquakes on the Xiaojiang fault and its nearby Ludian-Zhaotong fault.
Key words: Ludian earthquake     Active seismic source     First-arrival seismic tomography     Finite-difference method     Upper crust velocity structure     Zhaotong-Lianfeng fault     Xiaojiang fault    
1 引言

2014年8月3日16时30分,云南省鲁甸县发生MS6.5级地震,震中位置位于北纬27.1°,东经103.3°,震源深度12km(据中国地震局).截至2014年8月6日10时30分,地震已经造成589人死亡、9人失踪,造成了重大的人员伤亡和财产损失.地震震区位于小江断裂带东侧的昭通—鲁甸断裂带之间.昭通—鲁甸断裂是由两条北东向断裂带组成,平行展布于川滇交界东段、四川大凉山南部至云南昭通之间的地区(图 1)(闻学泽等,2013).近十年来,该构造带及其邻区发生了一系列的中强地震,如:2003年云南鲁甸MS5.0和5.1级地震、2004年鲁甸MS5.6级地震、2006年云南盐津两次MS5.1级地震、2012年云南彝良MS5.6和5.7级地震(吕坚等,2013)以及2014年云南永善MS5.3级地震.因此,地震发生区域是一个地震活动非常活跃的地区.截至8月12日,本次主震之后已经发生了1471个余震(图 1)(房立华等,2014).因此,震区所处的深部构造背景和孕震环境值得关注和研究.

震区所处的川滇地区位于我国大陆的西南部,青藏高原东南部,地处南北地震带的南段,是我国大陆内部地震活动最强的地区之一(滕吉文,1994; Wang et al., 2003; 徐锡伟等,2003).根据其断裂的展布特征,川滇地区可划分为四个一级块体,即马尔康块体、川滇菱形块体、保山—普洱块体和密支那—西盟块体等(徐锡伟等,2003).川滇菱形块体由东部的安宁河断裂、则木河断裂、小江断裂,西部的金沙江断裂、红河断裂,以及北部的鲜水河断裂所围限(王椿镛等,2002; 徐锡伟等,2003).鲁甸处于川滇菱形块体东侧,发育有北东向鲁甸—昭通断裂带(丰明伦等,2006).鲁甸主震目前推测发生在小江断裂带东侧昭通—莲峰断裂带内,北西向的包谷垴—小河断裂上,震源机制为走滑型破裂(房立华等,2014付芮等,2014刘成利等,2014郑勇等,2014).包谷垴—小河断裂是隐伏断裂,属于次级断裂;从位置和断层性质上看,可能和昭通—鲁甸断裂为一对共轭断裂,其具体的孕震环境目前还不是很清楚(房立华等,2014刘成利等,2014).

云南地区深大断裂发育,而且该地区的地震一般受控于大的断裂或断层,主要分布在剑川—丽江地震带、红河地震带、小江地震带、腾冲—龙陵地震带、普洱—思茅地震带以及澜沧—耿马地震带(白志明和王椿镛,2003).因此,该地区已经开展了大量的地球物理研究工作,为该区的深部物理场特性、地震孕育环境等提供了重要的深部资料(胡鸿翔等,1986; 熊绍柏等,1993苏有锦等,1999; 刘建华等,2000Huang et al., 2002; Wang et al., 2003; Li et al., 2008; Chen et al., 2010; Hu et al., 2011; Wu and Zhang, 2012; 吴建平等,2013张恩会等,2013).为了更好的理解川滇地区跨峨眉山大火成岩省的深部地球物理场特征,中国科学院地质与地球物理研究所实施了一条丽江—攀枝花—清镇的人工源宽角地震测深剖面,全长约650 km.由于至今还没有穿越昭通、莲峰断裂带的深地震测深剖面,而本测线距离鲁甸主震区不超过50km,利用宽角地震数据揭示该地区上地壳速度结构,可以为鲁甸震区的地震定位、地震孕育机制等提供深部速度模型.

2 区域构造背景概况

丽江—清镇人工源深地震测深剖面跨越我国境内唯一被国际学术界所认可的大火成岩省,即中-晚二叠世之交喷发的峨眉山玄武岩.峨眉山玄武岩分布于扬子克拉通西缘,位于我国西南地区四川、贵州和云南三省境内.峨眉山玄武岩主要由亚碱性及偏碱性的基性火山熔岩及火山碎屑岩组成,出露面积2.5×105km2,其西界以哀牢山—红河断裂为界,西北则以龙门山—小金河断裂为界(徐义刚和钟孙霖,2001; Xu et al., 200120042007; He et al., 2003; Peate and Bryan, 2008; 徐义刚等, 20072013; Wu and Zhang, 2012; Zhang et al., 2013a).

剖面位于扬子块体的西缘,呈东西向展布,西端由峨眉山大火成岩省内带的云南省丽江市,穿过中带至外带的贵州省清镇市.剖面地处云贵川高原,海拔一般在1500~2000 m以上,多为中高山侵蚀地形,属新构造运动强烈上升区,沿线的地质构造及地貌都比较复杂.测线穿过攀西裂谷,自西向东主要穿过的断裂有:丽江—小金河断裂、程海断裂、元谋—绿汁江断裂、安宁河断裂、小江断裂、威宁—水城断裂等(图 1).丽江—小金河断裂南北两侧海拔高度存在明显差异,晚第四纪以来,其水平差异升降运动(逆)速率达1.0~1.3 mm/a,且可能是青藏高原东南缘的重要深部边界(Chen et al., 2013).程海断裂带,北自宁蒗,南至弥渡,长逾200 km,沿断裂带有超基性岩零星出露.安宁河断裂位于青藏高原东缘、横断山脉中段的东部,是一条很重要的大型走滑断裂带,是青藏块体与华南块体的边界断裂之 一,总体走向近南北向,东西向挤压缩短速率约1.7~4.0 mm/a(何宏林等,2007).小江断裂为一条大型活动断裂,空间分布总体呈南北展布,其南段则与北西走向的曲江断裂和红河断裂相互交汇,构成一特殊而复杂的楔形断块构造,控制着该区的地震活动性,该断裂带的左旋滑动速率(10±2)mm/a(何宏林等,1993; 俞维贤等,1997).威宁—水城断裂位 于贵州威宁—水城一带,长约80 km,平均宽18 km左右,是构成黔中地层小区与黔西北地层小区分区的断裂.该断裂发育有比较完整的逆冲推覆构造,断裂构造主要由断面倾向南西、倾角≥60°的类叠瓦式高角度逆冲断层组合而成(张荣强等,2009).

图 1 丽江—清镇人工源深地震测深观测系统;红色实心五角星为人工源炮点,蓝色三角形为纵测线接收器;红色空心五角星为鲁甸主震震中位置,绿色圆圈为余震分布;红色空心圆圈为云南省历史上主要地震活动性分布,不同大小代表不 同震级范围;紫色方框为小江断裂带附近主要地震区域.主要断裂:LJ-XJHF,丽江—小金河断裂;CHF,程海断裂;YM- LZJF,元谋—绿汁江断裂; ANHF,安宁河断裂; ZMHF,则木河断裂;WN-SCF,威宁—水城断裂;ZTF,昭通断裂;LFF,莲峰断裂Fig. 1 Geophysical surveys in Lijiang-Qingzhen profile; Red solid stars denote active seismic sources,blue triangles denote receivers; Red hollow stars denote the location of Ludian main shock,green circles denote the location of aftershock. Red hollow circles denote the main historical seismicity distribution; Purple rectangular box denotes particular seismic region along Xiaojiang fault. Main Faults: LJ-XJHF,Lijiang-Xiaojinhe fault; CHF,Chenghai fault; YM-LZJF,Yuanmou-Lüzhijiang fault; ANHF,Anninghe fault; ZMHF,Zemuhe fault; WN-SCF,Weining-Shuicheng fault; ZTF,Zhaotong fault; LFF,Lianfeng fault
3 地震剖面数据和初至波Pg震相走时反演 3.1 地震剖面数据

2012年6月份,中国科学院地质与地球物理研究所沿丽江—攀枝花—清镇近东西向纵测线,实施了6炮人工源爆破(总数达17.5吨TNT,单炮药量2.4~3.9吨),采取井下组合爆破激发地震波场的方式,炮点间距60~90 km.纵测线布设323台人工地震测深专用便携式三分量数字地震仪同时记录观测,接收器间距1.5~2.0 km,剖面长约650 km,记录来自地壳上地幔顶部不同深度范围、不同属性的深层地震波场信息(图 1).

3.2 初至波Pg震相拾取

初至波Pg震相,一般被认为是来自壳内结晶基底的折射波,接收段几十至一百多公里.长接收段的初至波,可以来自上地壳的回折波和反射波等.炮点附近视速度迅速增大,随炮检距增大,视速度稳定在6.0~6.3 km·s-1,反映地表沉积盖层厚度和地壳结晶层顶部介质速度结构性质.局部到时的超前、滞后一般与地表局部隆起、凹陷构造相关.

丽江—清镇剖面6炮初至波Pg震相清晰,信噪比高,在东西方向平均能追踪60~120 km左右(图 2图 7).以6.0 km·s-1的折合速度,Pg震相在60 km处的折合走时为0.5 s左右(如四川盆地为1.0~1.2 s左右).震相近似水平,表明视速度接近于6.0 km·s-1.初步表明该剖面地表覆盖层速度大约在4.0 km·s-1以上,覆盖层不会太厚.

图 2 丽江—清镇纵剖面Sp01炮地震记录,蓝色十字为拾取的Pg震相Fig. 2 Wide-angle seismic profile at shot Sp01. Blue crosses denote picked Pg phases

图 3 丽江—清镇纵剖面Sp02炮地震记录,蓝色十字为拾取的Pg震相Fig. 3 Wide-angle seismic profile at shot Sp02. Blue crosses denote picked Pg phases

图 4 丽江—清镇纵剖面Sp03炮地震记录,蓝色十字为拾取的Pg震相Fig. 4 Wide-angle seismic profile at shot Sp03. Blue crosses denote picked Pg phases

图 5 丽江—清镇纵剖面Sp04炮地震记录,蓝色十字为拾取的Pg震相Fig. 5 Wide-angle seismic profile at shot Sp04. Blue crosses denote picked Pg phases

图 6 丽江—清镇纵剖面Sp05炮地震记录,蓝色十字为拾取的Pg震相Fig. 6 Wide-angle seismic profile at shot Sp05. Blue crosses denote picked Pg phases

图 7 丽江—清镇纵剖面Sp06炮地震记录,蓝色十字为拾取的Pg震相Fig. 7 Wide-angle seismic profile at shot Sp06. Blue crosses denote picked Pg phases
3.3 初至波Pg震相有限差分反演上地壳速度结构

在6炮初至波Pg震相拾取的基础上,通过有限差分反演上地壳的速度结构(Vidale,1988; Hole,1992Zhao et al., 2004;Lan and Zhang, 2013a2013b; 兰海强等, 2012a2012b刘一峰等,2012赵烽帆等,2014Ma and Zhang, 2014).设置01炮东侧附近一点(26.5583°N,106.9668°E)为起始桩号,每个接收器到该点的距离为一维投影的坐标.考虑到实际测线是二维的,不是直线,炮点附近的接收器位置对浅层速度结构反演影响很大,因此 我们通常不考虑1.5 s之内的接收器拾取到时(徐果明,2003).反演过程中,接收器水平向和垂直向的网格间距均为1 km,通过25次迭代,获得图 8所示的上地壳速度结构.

图 8 纵剖面上地壳速度结构;红色空心五角星表示主震在剖面的投影.主要断裂:LJ-XJHF,丽江—小金河断裂; CHF,程海断裂;YM-LZJF,元谋—绿汁江断裂;ANHF,安宁河断裂;XJF,小江断裂;WN-SCF,威宁—水城断裂.Fig. 8 Upper crust velocity structure along the profile; Red hollow star denotes the projection location of main shock; Main Faults: LJ-XJHF,Lijiang-Xiaojinhe fault; CHF,Chenghai fault; YM-LZJF,Yuanmou-Lüzhijiang fault; ANHF,Anninghe fault; XJF,Xiaojiang fault; WN-SCF,Weining-Shuicheng fault
3.4 走时拟合和射线覆盖

图 9为丽江—清镇剖面6炮最终的走时拟合结果(徐涛等,2004李飞等,2013Xu et al., 200620102014).图 10为射线覆盖图,可以看出,地震射线的探测深度可以达到近12km,最高射线覆盖达80多次.由于实际地质情况差异,剖面的射线覆盖密度在水平方向存在较大的差异.在有限差分反演过程中,会对某个节点及其周围节点的速度结构 进行插值和光滑处理,因此没有射线覆盖的区域,速度结构也有一定的参考价值(图 8).图 11为仅显示存在射线覆盖区域、可靠性最高的速度分布.

图 9 丽江—清镇纵剖面6炮Pg震相拟合结果Fig. 9 Comparison between observed(blue crosses) and calculated(red circles)travel times for all Pg phases of six shots along “Lijiang-Qingzhen” profile

图 10 丽江—清镇纵剖面6炮Pg震相反演的射线覆盖Fig. 10 Ray coverage of the whole upper crust along “Lijiang-Qingzhen” profile

图 11 丽江—清镇纵剖面射线覆盖内的上地壳速度结构,黑色圆圈为余震在剖面的投影,红色空心五角星为主震到剖面的投影.主要断裂:LJ-XJHF,丽江—小金河断裂;CHF,程海断裂;YM-LZJF,元谋—绿汁江断裂;ANHF,安宁河断裂;XJF,小江断裂;WN-SCF,威宁—水城断裂Fig. 11 Upper crust velocity structure along the profile in the ray coverage region; Red hollow star denotes the position of main shock;Black circles denote projection of aftershock distribution. Main Faults: LJ-XJHF,Lijiang-Xiaojinhe fault; CHF,Chenghai fault; YM-LZJF,Yuanmou-Lüzhijiang fault; ANHF,Anninghe fault; XJF,Xiaojiang fault; WN-SCF,Weining-Shuicheng fault
3.5 丽江—清镇纵剖面上地壳速度结构及鲁甸地震孕震环境

从上地壳速度来看(图 8图 11),速度小于5.8 km·s-1的区域,其速度等值线较密集,而速度大于5.8 km·s-1的区域,其速度等值线较为稀疏;参考中国大陆和该地区的地壳速度结构(Teng et al., 2013; Zhang et al., 2013b; 张恩会等,2013),可以得到该剖面结晶基底的速度应该在5.8 km·s-1 左右.图中可以看出,剖面结晶基底的厚度在2 km左右,由于剧烈的区域构造活动,呈现出明显的起伏变化特征.

整条剖面所处扬子块体西缘,基本缺少第四纪沉积,主要分布早-中三叠系和中-上泥盆系的沉积地层,以及大面积出露的峨眉山玄武岩.程海断裂和小江断裂之间,上地壳部分区域呈现出高速特征,部分区域结晶基底出露到地表(水平桩号120 km左右).川西台阵的接收函数和噪声成像结果同样显示该区域上地壳整体都处于高速区(图 12,Sp04和Sp05炮之间;Liu et al., 2014).

浅层地壳低速主要分布在元谋—绿汁江断裂(YM-LZJF)和安宁河断裂(ANHF)之间、小江断裂(XJF)的东侧、威宁—水城断裂(WN-SCF)的东侧等三个区域,覆盖层速度低至4.0 km·s-1左右.在 丽江—小金河断裂(LJ-XJHF)和程海断裂(CHF)之间,以及小江断裂带上地壳的低速延伸至10 km左右,显示断裂至少切穿了上地壳(图 11),但是否往下切穿中下地壳,需要续至波,包括壳内反射和Moho面反射等多震相的联合约束来确定.川西台阵地震数据成像结果同样显示小江断裂带域呈现低速特征(图 12,Sp03和Sp04炮之间;Liu et al., 2014).此外,体波成像结果同样显示该区域上地壳呈现整体低速的特征(吴建平等,2013).因此,我们的剖面结果和面波、体波层析成像结果有良好的一致性.从1400多个余震(房立华等,2014)的剖面投影来看,余震主要集中在小江断裂带域所处的低速区内(图 11).但地震历史总结来看,强震主要孕育在地壳高速区,或高速区与低速区域交接处的高速区内(梅世蓉,1995).图 1可以看出,主震震中所处的区域构造环境复杂;川西台阵地震成像结果显示主震震中位于高速和低速交接处(图 12Liu et al., 2014).由于主震区距离测线有50 km左右,因此主震区的深部速度结构和震区在速度剖面的投影速度值存在一定的差异.对于区域构造复杂地区,用二维剖面速度结构外推代替三维速度场,仍然存在一定的近似,但可以作为该区域深部的参考速度模型.由于该区域速度变化比较剧烈,尤其是受小江断裂带的影响,我们的剖面结果可能不能直接的外推到鲁甸震源区上,对于发震构造细节的认识仍有待于近场资料的进一步完善.

图 12 鲁甸地震震中及邻区上地壳S波速度结构(修正自Liu et al., 2014); 红色空心五角星为主震震中,白色空心圆圈为余震分布Fig. 12 Upper crustal velocity of Ludian earthquake epicenter and surrounding areas(Modified from Liu et al., 2014); Red hollow star denotes the location of main shock; White hollow circles denote the location of aftershocks

从云南省历史地震活动性来看(图 1),丽江—清镇宽角剖面经过的小江断裂附近,如图 1中紫色方框所示,是地震大震易发区域,从鲁甸地震导致的同震静态库仑应力分布来看,方框所示的小江断裂活断层区域为库仑应力减小区(付芮等,2014),鲁甸地震不会增加该区域后续地震的危险性.尽管如此,由于GPS的观测表明小江断裂两侧的滑移量在1 cm/a左右(Wang et al., 2014),因此小江断裂带具备较大应变积累的可能,加上该断裂带和周边区域地壳速度偏低,强度相对较小,因此处于应力和应变的集中区域.所以小江断裂带和旁边的鲁甸—昭通断裂带,未来具有发生较大地震的可能,值得关注.

4 讨论与结论

鲁甸MS6.5级地震造成了重大的生命财产损失.本文利用丽江—攀枝花—清镇人工源深地震测深数据,采用初至波Pg震相的有限差分反演,获得剖面的上地壳速度结构,为震区提供了上地壳速度结构的参考,为鲁甸震区的地震定位、震源机制、破裂过程、地震孕育机制等后续研究工作提供了深部速度模型.速度剖面显示:小江断裂带速度较低,东西两侧的速度较高,因此小江断裂带区域地壳强度比较低,处于应力的集中区域,小江断裂带具备较大应变积累的可能,所以小江断裂带和旁边的鲁甸—昭通断裂带,未来具有发生较大地震的可能,值得关注.

致谢 对参加野外地震数据采集工作的中国地震局地球物理勘探中心及中国科学院地质与地球物理研究所的所有工作人员表示衷心的感谢.感谢中国地震局地球物理所王宝善研究员有益的讨论和建议.感谢刘启元研究员在审稿过程中良好的建设性意见,及编辑对稿件发表的推进.

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