【作者】 赵盼盼；

【导师】 陈九辉；

【作者基本信息】 中国地震局地质研究所 ， 固体地球物理学， 2011， 硕士

【摘要】 2008年5月12日,四川龙门山断裂带发生Mw7.9级地震,造成巨大伤亡与损失。震后地表地质考察(Xu et al., 2009)、地震震源过程反演(张勇等,2008;Ji and Hayes,2008;王卫民等,2008; Shen et al., 2009)、余震定位(黄媛等,2008;陈九辉等,2009)等方面的研究结果显示,汶川地震是迄今为止在高角度逆冲断裂上发生的强度最大的内陆地震(Zhang et al., 2010)。地震的孕育和发生通常伴随同震应力场变化、地下流体迁移、地壳浅层破坏等事件的发生,从而导致地壳介质物理性质的变化。对地壳介质物理性质在强震前后的变化进行研究,将有助于理解强震孕育、发生和震后恢复过程,有助于地震动力学过程的研究。对汶川地震前后区域介质和应力场变化的研究对具有特殊发震构造的汶川地震尤为重要。近年来,利用地震环境噪声互相关函数尾波的被动源成像干涉测量法(PII)成为一种新兴的监测地壳介质性质变化的方法,由于具有无源优势和可持续监测的特性,使其具有广阔的应用空间。通过在地震(Wegler et al., 2009; Xu and Song, 2009; Chen et al., 2010; Cheng et al., 2010;刘志坤和黄金莉,2010),火山(Brenguier et al., 2008a),断层带(Wegler et al., 2007; Brenguier et al., 2008b)等方面的研究结果显示,该方法对于如何理解强震孕育、发生和震后恢复过程、如何认识火山内部岩浆的活动过程,进而预测其爆发的时间、强度等问题都有着十分重要的意义。自2006年10月开始,在国家973研究项目支持下,中国地震局地质研究所地震动力学国家重点实验室在川西地区(100°～105°E, 26°～32°N)布设了由297个宽频带数字地震仪组成的流动观测台阵(简称川西台阵),获得了汶川地震前后完整的地震活动观测记录,为进一步研究汶川地震提供了宝贵资料。本文利用川西流动地震台阵29°N以北地区的137个台站2007年1月至2008年10月的连续三分量地震环境噪声记录研究了汶川地震震前到震后地壳速度变化特征。借助川西台阵的密度优势,我们针对单台三分量噪声自相关函数和单台不同分量间的互相关函数,利用互谱移动窗技术,在0.33~1HZ频带范围内测量了经过50天滑动平均的相关函数与长时间平均参考相关函数的走时变化率,进而求得地壳浅部速度随时间的相对变化,并得到其空间分布特征。研究结果表明:⑴汶川Mw7.9级地震造成龙门山断裂带及其附近区域沿断裂破裂方向存在显著同震波速降低,而沿破裂反方向及其松潘-甘孜块体西部、川滇块体速度相对变化较弱,但在四川盆地内部却也存在明显速度相对降低。⑵与Chen et al.(2010)利用双台互相关函数尾波在该区域所得同震速度变化率结果比较发现,本文测量得到的同震速度变化率幅值较大,但两者变化形态基本一致,同时两者同震速度相对变化与震前扰动幅度比值分别为2和2.1。其差别应该是所用方法对该区域不同深度地壳的速度相对变化敏感度不同,因而两文所得结果并无本质差异。⑶尽管本文将同震速度变化的空间分布与同震体应变进行了对比,但是由于噪声相关函数测量所得到的同震速度变化率比汶川地震同震体应变率(约为10-6量级)高2-3个量级,说明同震体应变并非造成同震地壳速度变化的主要因素。对于浅层地壳介质,由同震应力变化造成的介质孔隙度或介质内流体的迁移可能是同震速度变化的主要因素。强地面震动对接近震源区的浅层介质造成的破坏,也可能影响噪声相关函数方法测量速度变化的结果。⑷具有更高空间分辨率的噪声自相关研究进一步发现,在鲜水河断裂和龙门山断裂交汇区康定、泸定附近存在同震速度增加区,其速度增加幅度明显高于背景速度扰动幅度。这一区域与同震库伦应力变化和地表形变观测预测的周边断层库伦应力增加区(Parsons et al., 2008;万永革等,2009;单斌等,2009)一致。这说明,高密度台站的噪声相关速度变化测量不仅可以测量大尺度的平均地壳速度变化,也可以探测得到同震应力增加。我们的研究同时发现,同震库伦应力增加效应的持续时间大约为2个月左右,在此之后,区域应力场逐渐恢复为与周边区域基本一致的应力下降。该区域的应力下降可能与在该地区发生的地震有关。⑸在汶川地震之前,各个区域,特别是龙门山地区和四川盆地地区,都表现出了类似的地壳速度随时间变化特征(图4-7),在震前约200天时间尺度内表现出了不同程度的地壳速度增加。这一速度变化与季节没有明显关系,因而很可能是地壳应力增加的体现。但是由于川西台阵在汶川震前的观测仅有约一年半的时间,尚难以给出汶川震前速度和应力场变化的确切结论。有关汶川地震之前的应力场变化及其与汶川地震的关系仍需进一步研究。更多还原

【Abstract】 On 12 May 2008, the Mw 7.9 earthquake occurred on the LMS fault in Sichuan province of China which induced huge losses. Lots of work such as geological surveys (Xu et al., 2009), inversions of surface rupture process (Zhang et al., 2008; Ji and Hayes, 2008; Wang et al., 2008), aftershock relocation (Huang et al., 2008; Chen et al., 2009), and inversion of the GPS data (Shen et al., 2009) indicate that the Wenchuan earthquake is, up to now, the largest strong intra-continental earthquake on a high angle thrust fault. There are usually many events accompanying the generation and occurrence of earthquakes, such as variations of the co-seismic stress field, migration of underground fluid, destruction of shallow crust and so on, as a result, physical properties of the crust medium will be changed. Studies on changes of physical properties of the crust medium before and after a strong earthquake will help us to understand the process of its generation, occurrence and recovery, and also it will help realize the earthquake dynamic process. It is particularly important to research variations of media and the stress field in the epicenter region of the Wenchuan earthquake which has special seismogenic structures.Recently, a technique called passive image interferometry (PII) using the cross-correlation function coda of seismic ambient noise has become a new method to monitor the changes of crust media. Because of source-free and sustainable monitoring, it could be applied to many aspects. Researches on earthquakes (Wegler et al., 2009; Xu and Song, 2009; Chen et al., 2010; Cheng et al., 2010; Liu and Huang, 2010), volcanoes (Brenguier et al., 2008a) and fault zones (Wegler et al., 2007; Brenguier et al., 2008b) by PII indicate that it can help understand the process of gestation, occurrence and recovery of quakes, recognize the active process in the volcano and even forecast its eruption with time and intensity.A transportable array with 297 broadband seismic stations was deployed in the western Sichuan(100°～105°E, 26°～32°N)in October of 2006 by the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, called Western Sichuan Seismic Array(WSSA). The WSSA covers the CD block, SG block and SC basin, 2/3 of the fault system activated during the Wenchuan earthquake. It worked for more than 2 years and provided unique continuous recordings before, during and after the Wenchuan earthquake. These data allow a intensive study on the Wenchuan earthquake.Three-component seismic ambient noise data continuously recorded at 137 stations of WSSA to the north of 29°N from January of 2007 to October of 2008 were used in this paper to research the crust velocity changes caused by the Wenchuan earthquake. Doublet processing was applied to the auto-correlation functions coda and cross-correlation function coda for each single station with three different components. Time shifts were measured in the frequency range of 0.33~1HZ between the 50 days moving averaged correlation functions coda and the long time averaged reference correlation functions coda, then the crust velocity changes with time were obtained. With the help of the high density of WSSA, the spatial distribution of co-seismic velocity changes was determined.The results can be summarized as follows:(1) Remarkable co-seismic velocity droops could be found in the Sichuan basin and around the Longmenshan fault along the direction of fracture extending. Meanwhile, in the opposite direction, there are week velocity changes, and so do the western Songpan block and Chuandian block.(2) Comparing with the results given by Chen et al. (2010) who used cross-correlation functions coda between each two stations in the same area of WSSA, the co-seismic velocity changes presented in this thesis are larger but with similar trend. The ratios of the co-seismic velocity changes to the velocity perturbations before the earthquake around the LMS fault are 2 and 2.1, respectively. So there is no essential difference between the two methods but with different depths of the sensitive zone.(3) Although the spatial distribution of co-seismic velocity changes around the LMS fault is similar to the volumetric strain changes during the Wenchuan earthquake, 2 to 3 orders higher of magnitude in velocity changes indicate that the latter may be not the primary cause to the former. To shallow crust, extent of porosity or fluid migration caused by co-seismic stress changes could be the main reason for co-seismic velocity changes.(4) With higher spatial resolution of the ambient noise auto-correlation method, a distinct co-seismic velocity increase was found at the area of the intersection of LMS fault and Xianshuihe fault which coincides with the Coulomb stress increase predicted by co-seismic Coulomb stress changes and the observations of surface deformation. It reveals that the stress increase continued for about 2 months after which it dropped back into a general decrease.(5) There are similar characteristics of velocity changes with time in different areas especially around the LMS fault and in the SC basin where velocity increased to different degrees about 200 days before the Wenchuan earthquake. It seems not related to season but possibly a reaction to stress increase. Unfortunately, observation before the earthquake only lasted about one and a half years, so it is difficult to give a verdict. Relations between stress changes and the Wenchuan earthquake should be further studied.更多还原