龙门山断裂带中段应力场的时空演化

发布时间：2018-03-03 12:52

本文选题：龙门山断裂带　切入点：震源机制　出处：《中国地震局地质研究所》2015年硕士论文　论文类型：学位论文

【摘要】：龙门山断裂带地处南北地震带中段,介于青藏高原东缘巴颜喀拉与华南地块四川盆地之间,是一条北东向断裂带。以映秀和北川附近为界,可将龙门山断裂带分为南、中、北3段(邓起东等,1994；陈国光等,2007；徐锡伟等,2008)。2008年5月12日在龙门山断裂带中段发生了Mw7.9级汶川特大地震,造成巨大人员伤亡和经济损失。主震震源破裂过程研究(王卫民等,2008；张勇等,2008)表明汶川地震是一次以逆冲为主兼具少量右旋走滑的事件。主要的动力来自巴颜喀拉块体朝SEE方向的运动受到华南地块阻挡而在龙门山断裂带的长期应力积累。龙门山断裂带能孕育出如此大的地震主要归结于三大特征(Burchfiel et al.,2008；张培震等,2008；李延兴等,2009；Zhang et al,2010),组成龙门山断裂带的是强度大的杂岩体,断裂带剖面为铲型的构造结构,及整体处于低活动速率的闭锁状态下。地壳中应力主要受板块运动、区域地质构造以及大地震触发的应力变化影响,其自身也是地震直接动力源。对应力场及其时空演化特征的研究可帮助我们更好地理解区域构造变形。测量应力场的方法有很多,利用震源机制解反演应力场是现今研究应力场主要思路之一汶川地震发生后,在龙门山断裂带上产生了大量的余震。丰富的余震资料以及大尺度破裂带的存在为研究龙门山断裂带动力学提供了重要数据基础和模型参数。许多学者及机构基于汶川中强余震震源机制解获得了应力场时空变化特征及蕴含的构造意义,这些结果提供了对龙门山断裂带构造应力场主要特征的认识,但由于现有研究主要针对4级以上的强余震,对局部应力场研究则稍显不足。中强地震所需的驱动力数量级远大于小震,这决定了其震源机制主要受区域构造应力的控制,对局部应力场变化不敏感。相对而言,小震不仅数量上占优,而且除受构造应力场影响外,还对大地震触发的库伦应力变化以及周边断层的影响敏感。小震的缺点在于信噪比低导致的有效观测数据少,随着近年来密集台网的架设及区域精细速度结构的研究,小震定位结果及震源参数求解质量有了显著提高。为了获得龙门山断裂带中段应力场更精细的时空变化,本文利用川西流动地震台阵记录到的2007年1月至2008年10月间发生在龙门山断裂带中段的小震,通过P波初动极性结合SH/P振幅比反演计算震源机制解(Hardebeck and Shearer,2002：2003),并应用阻尼线性反演方法(Hardebeck and Michael,2006)对龙门山断裂带中段应力场进行反演,以便更好的了解龙门山断裂带中段应力场调整变化和迁移过程。研究结果表明：(1)汶川震前龙门山中段震源机制多为逆冲型且性质相似,表明区域应力场主要受构造应力的作用。应力场反演结果显示S1近水平,方位为NWW-SEE,S3接近垂直,为逆冲型应力结构。滑动趋势研究表明汶川地震前区域应力场具有高度的一致性,区域已具备发生强震的动力条件。在汶川震中位置以南的宝兴地区小震震源机制的P轴方向变为沿龙门山走向,表明其应力场在汶川起始破裂区以南存在异常变化,这与汶川地震、芦山地震发生的关系尚需进一步研究。(2)对汶川小余震进行震源机制求解,发现震源机制以逆冲型为主,同时具有相当比例的正断型地震。各类型比重随时间变化明显,逆冲型随时间比例大幅上升,正断及其他型比例下降。震源机制投影发现余震性质空间分布与断层深部展布及区域速度结构具有相关性,时空形态揭示了局部应力场的复杂性。(3)应力张量反演结果显示,震后初期小鱼洞浅部地区受左旋走滑破裂影响,SH方位为NE-SW向,其他区域SH方位基本在EW和NWW-SEE范围内,表明震后初期应力场主要受主震破裂传播引起的动态应力影响。破裂传播引起的动态应力使小震破裂面走向与传播方向趋于一致,表现在SH方位趋向一致。随着时间推移,应力场趋于平衡的过程中,深部应力结构由逆冲、走滑混合型向纯逆冲型转变,表明深部区域通过走滑来平衡破裂传播造成的应力失稳,并向背景应力场转变。浅部区域的应力调整追随深部的变化特征,并有一定的滞后。(4)应力场反演揭示主震震中区深部SH方位模糊且为逆冲型应力结构,表明局部存在高强度的隆升凸起。先前发现的应力异常可能来自于20km深度左右彭灌杂岩的隆升,杂岩体内NE侧会出现由局部隆升导致的物质横向变化,小鱼洞-理县走滑断裂成因可能与之有关。本文对汶川地震前在整个龙门山地区、汶川地震后在龙门山中段发生的小震震源机制和应力场特征进行了研究,研究结果为汶川地震和芦山地震孕育、发生的动力过程研究提供了新的证据和约束。
[Abstract]:The Longmen mountain fault zone is located in the middle of the North South seismic belt, between the eastern margin of the Qinghai Tibet Plateau and the Southern China block between the Sichuan basin, is a North East fault zone. In the vicinity of Yingxiu and Beichuan border, the Longmen mountain fault zone is divided into South, north, 3 (Deng Qidong, 1994; Chen Guoguang so, 2007; Xu Xiwei, 2008).2008 year in May 12th in the Longmen mountain fault zone occurred in the middle grade Mw7.9 Wenchuan earthquake caused huge casualties and economic losses. The rupture process of the mainshock hypocenter (Wang Weimin, 2008; Zhang Yong, 2008) show that the Wenchuan earthquake is a thrust with a small amount of. Dextral strike slip events. The main motion power from the Bayan Har block toward SEE is blocked in the Southern China block, Longmen Shan fault zone long-term stress accumulation. The Longmen mountain fault zone creates such a large earthquake is mainly due to the three characteristics (Bu Rchfiel et al., 2008; Zhang Peizhen, 2008; Li Yanxing, 2009; Zhang et al, 2010), consisting of the Longmen Shan fault zone is the complex rock strength, structural fracture zone profile for shovel type, and the whole is closed when the low activity rate. Under stress to plate by the movement of the earth crust in effect, the regional geological structure and earthquake triggering stress changes, its seismic direct power source. The stress field and its temporal evolution characteristics can help us better understand the regional tectonic deformation. There are many methods of measurement should be stress field, using the focal mechanism solutions of stress field inversion is now on stress one of the main ideas of the Wenchuan earthquake field, in the Longmen mountain fault zone had a large number of aftershocks. Rupture of the aftershock data rich and large scale belt has provided important data base for the study of the Longmen Mountain Fault Zone dynamics The foundation and the parameters of the model. Many scholars and institutions of solutions for the stress field contains the spatial and temporal variation characteristics and tectonic significance of the focal mechanism of strong aftershocks in Wenchuan based on these results provide understanding of the fracture zone of Longmen mountain tectonic stress field characteristics, but because of the existing literatures mainly focus on strong aftershock of more than 4, the local the study of stress field is slightly insufficient. The driving force required strong earthquake magnitude greater than a small earthquake, which determines its focal mechanism is mainly controlled by regional tectonic stress, is not sensitive to the changes of local stress field. In contrast, the number of small earthquakes is not only the dominant, but also affected by tectonic stress field effects, also on the earthquake triggered Kulun effect of stress changes and faults. The disadvantage is that the small earthquake sensitive effective observation data with low signal-to-noise ratio leads to less, with the erection and fine velocity structure of network intensive in recent years. The research results of small earthquake location and source parameters for quality has been significantly improved. In order to stress the finer temporal and spatial variation of middle Longmen mountain fault belt has received, the Sichuan Mobile seismic array recorded from January 2007 to October 2008 occurred in the small earthquake belt in the section of the Longmen mountain fault, the first motion of P wave polar ratio inversion calculation of focal mechanism solutions combined with the amplitude of SH/P (Hardebeck and Shearer, 2002:2003), and the application of damped linear inversion method (Hardebeck and Michael, 2006) of the Longmen Mountain Fault Zone Middle stress field inversion, in order to better understand the Longmen Mountain Fault Zone of stress field change and migration process. The results of the study show that: (1) Wen Chuanzhen before the middle section of Longmen mountain is the focal mechanism of thrust type and properties similar to that of regional stress field is mainly affected by tectonic stress. Stress field inversion results show that the S1 horizontal side For NWW-SEE S3, close to the vertical stress, structure of thrust type. Study on sliding trend shows that before the Wenchuan earthquake the regional stress field has a high degree of consistency, area has dynamic conditions of strong earthquake epicenter in Wenchuan. Position in the P axis direction of focal mechanism of small earthquakes in Baoxing area south of the direction along the Longmen that mountain, the stress field in the Wenchuan area to the south of the initial rupture abnormal changes, the Wenchuan earthquake and the Lushan earthquake, the relationship is in need of further research. (2) the focal mechanism solution of small aftershocks were found in Wenchuan, the focal mechanism of thrust type earthquakes are broken, and has a considerable proportion. The proportion of each type changed over time, thrust type with a substantial increase in the proportion of normal fault and other types decreased. The focal mechanism that aftershock spatial distribution and projection properties of deep faults and regional distribution of velocity structure has a correlation, when The empty form revealed the complexity of stress field. (3) stress tensor inversion results show that the Xiaoyudong shallow areas after the earthquake early by sinistral strike slip rupture, SH range is NE-SW to other areas of SH in EW and NWW-SEE basic range range, showed that the initial period after the earthquake stress field to dynamic shock rupture propagation caused by stress effects. The dynamic stress caused by the rupture of small earthquakes rupture direction and propagation direction are consistent, consistent performance in SH range. With the passage of time, the process of stress field tends to balance in the deep stress structure composed of thrust, strike slip hybrid to pure thrust the transformation that through deep regional strike slip rupture propagation to balance the stress caused by the instability, and the stress field change to the background of force adjustment. Follow the deep shallow region, and there is a certain lag. (4) the stress field inversion reveal the main deep earthquake epicenter area SH range and fuzzy thrust type stress structure, show that the local existence of high strength uplift raised. Previously discovered abnormal stress may come from a depth of about 20km the PENGGUAN complex uplift, complex NE in the rock side will appear by the local uplift and lateral variation of material lead, Xiaoyudong Lixian the strike slip fault causes may be related to this paper. Before the Wenchuan earthquake in the Longmen mountain area, Wenchuan after the earthquake in the Longmen mountains of small earthquake focal mechanism and stress field characteristics were studied, the results for the Wenchuan earthquake and the Lushan earthquake, the dynamic force provides new evidence and constraints study.

【学位授予单位】：中国地震局地质研究所
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P315.2

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