【作者】 张彬；

【导师】 杨选辉； 陆远忠；

【作者基本信息】 中国地震局地壳应力研究所 ， 固体地球物理学， 2007， 硕士

【摘要】 地震应力触发指一个大震产生的应力场变化对邻近和远程地震活动的影响。地震应力触发包括静态应力触发和动态应力触发。对静态应力触发研究较早,并且取得很多研究成果。而对动态应力触发研究开展较晚,真正的动态应力触发研究是1992年美国兰德斯地震后,由于其在解释远程触发中具有很大的优势,所以近年来动态应力触发研究发展迅速。我国位于环太平洋地震带和亚欧地震带之间,中国大陆及周边强震频发,研究地震动态应力在中国的触发作用是非常必需的。本文旨在以研究中国南北地震带动态应力触发作用为例,探讨强震动态应力触发问题。文中较系统总结了国内外关于地震动态应力触发的研究动态,包括目前动态应力触发存在的证据、研究现状、触发机理以及存在的问题等。从1992年兰德斯地震后,地震学家在全球范围内寻找动态应力触发证据,目前存在的主要证据包括:从地震频度图中探寻被触发震群和独立地震、数字滤波后新发现的被触发地震和形变仪器记录的同震变化和应力阶跃等。目前,提出的触发机理包括库仑破裂模型、速率-状态模型和孔隙压力扩展等。国内对动态应力触发问题研究的较少,很大程度上可能是因为地震动态应力计算复杂。2004年苏门答腊8.7级地震后,动态应力触发研究升温,我国部分地震学者也开始进行动态应力触发研究方面的探讨。计算地震动态应力主要用到的方法是反射率法和离散波数分解法。计算方法涉及到的数学函数较多:贝塞尔函数、格林函数、反射率函数和Bouchon坡度函数等;涉及到的物理过程也非常复杂:破裂时间的离散、格林函数褶积和频率域内的微分等。我们应用前人一些公式导出了地震动态应力计算公式,改写了Cotton提供的部分FORTRAN程序。为了计算被触发断层上的库仑应力,建立了和MATLAB的连接,同时实现了MATLAB程序可视化界面计算和绘图。计算地震动态应力分为两步:第一步利用FORTRAN程序计算应力张量的六个分量;第二步计算被触发断层上的动态应力,并绘制动态应力变化曲线。我们应用上述地震动态应力计算方法和程序研究了部分强震在中国大陆的触发情况。南北地震带是中国大陆一条代表性地震活动带,近几年来地震活跃,资料丰富,所以文中选用南北地震带为研究区域,并主要从两种思路开展了研究。第一,被触发地区应力张量和库仑应力计算。包括2000年苏门答腊M_S8.0地震对甘肃景泰M_S5.8地震和缅甸M_S6.5地震的触发作用; 2003年中、俄、蒙交界M_S7.9地震对甘肃民乐-山丹M_S6.1地震和岷县M_S5.2地震的触发作用以及2004年苏门答腊M_S8.7地震对云南双柏M_S5.1地震和思茅M_S5.1地震的触发作用。结果表明,这几次强震在被触发地震上产生的动态应力峰值都超过地震触发阈值。第二,被触发地区小震突然增强。我们利用小地震频度变化探讨了1990年以来部分地震在南北地震带上的触发作用,并以2001年昆仑山口西M_S8.1地震对四川、甘肃等地地震活动的触发作用为例进行了分析。总的看来,南北地震带易于被触发地震活动,但是不同地区的地震对南北地震带的触发作用不同,这可能与地质构造有关系。地震动态应力研究之所以复杂,因为它还受到很多因素影响:主震参数和被触发地区构造,同时还有时间延迟问题。在计算主震在被触发点的应力张量时,主震的参数影响非常大。改变断层三要素(走向、倾角、滑动角)、破裂时间以及破裂时的上升时间对最终计算的动态应力大小和变化趋势都有很大影响;改变主震地震矩和发震断层的大小只是影响了计算结果的大小,其趋势没有改变。大部分主震参数的变化都会影响到最后的计算结果,所以在计算地震产生的动态应力变化时,应该获得更准确的主震参数。构造区对地震动态触发影响也非常大,研究结果表明,大部分被触发的地震活动发生在地热或火山区及扩张性块体上,同时也在挤压性块体上发生动态触发的少量证据。动态应力对走滑和正断层地震触发更明显。强震面波到达后被触发地区不久可能触发当地地震活动,但是还是很多被触发的地震具有时间延迟,延迟时间从几天到几个星期不等。总之,我通过硕士论文研究,改进了动态应力触发计算方法和程序,计算了南北地震带几次被触发地震的动态库仑应力变化,同时深入探讨了主震参数、触发地区构造特征对地震动态应力触发的影响,还初步研究了地震动态触发时间延迟问题,在研究过程中也发现了一些值得今后进一步研究的问题。更多还原

【Abstract】 Seismic stress triggering points to the influence induced by one earthquake to the nearby and remote seismic activity, including static stress triggering and dynamic stress triggering. Scientists have studied static stress triggering for a long time and have got lots of achievement. On the other hand, researching on dynamic stress triggering was scarce. The time we indeed researched on seismic dynamic stress triggering was after the Landers earthquake of 1992, USA. As dynamic stress triggering has superiority in explaining remote triggering, so it is developed rapidly in recent years. China locates between the Pacific Ocean seismic zone and the Asia-Europe seismic zone, so Chinese mainland and its periphery has more strong shocks. Although Chinese seismologists study seismic dynamic stress triggering later, it is necessary to study seismic dynamic stress triggering in China. In order to explore Chinese seismic dynamic stress triggering, we take Chinese North-South seismic belt as an example in this article.First, we summarized the development of seismic dynamic stress triggering at home and abroad systematically, including the existed evidence of dynamic stress triggering, current situation, triggering mechanism and so on. From Landers earthquake in 1992, seismologists began to seek evidence of dynamic triggering on a global scale. The main existent evidence at present includes: seek triggered earthquake swarm and single earthquake in seismic frequency map, find new triggered earthquake by digital filter, co-seism change and stress step recorded by strain instrument. The triggering mechanism includes coulomb failure model, rate-state model and pore pressure expansion etc. The reasons why the study on dynamic stress are so less in China mainly maybe rely on the complexity of the computation. After Sumatra 8.7 magnitude earthquake in 2004, some seismologists start to study dynamic stress triggering in China.The main methods of computing dynamic are reflectivity method and the discrete wavenumber decomposition method. It involves lots of mathematics functions, including Bessel function, Green function, Reflectivity function and Bouchon slope function etc. The physical process related is also complicated: break time scattering, Green function convolution and different in frequency area etc. According to some predecessors’formulae, we deduced seismic dynamic stress calculating formula and rewritted part FORTRAN procedure that Cotton provided. In order to calculate coulomb stress on triggered fault, we connected FORTRAN procedure with MATLAB. At the same time, we realized the MATLAB procedure visualization interface for calculating and drawing. The calculation of Seismic dynamic stress comprised two steps: first, calculate six component of stress with FROTRAN procedure; second, calculate dynamic stress on triggered fault and its curve.With the mentioned method and procedure of calculating seismic dynamic stress above, we researched the triggered situation by some strong earthquakes in Chinese mainland. As the North-South seismic belt is a representative one in Chinese mainland, so we take it as the researching zone, and study mainly from two aspects. First, calculate stress tensor and coulomb stress in triggered area, including M_S8.0 earthquake of 2000 in Sumatra triggered M_S5.8 earthquake of Jingtai in Gansu and M_S6.5 earthquake of Burma, M_S7.9 earthquake of 2003 in border of China、Russia and Mongolia triggered M_S6.1 earthquake of Minle-Shandan and M_S5.2 earthquake of Minxian in Gansu, M_S8.7 earthquake of 2004 in Sumatra triggered M_S5.1 earthquake of Shuangbai and M_S5.1 earthquake of Simao in Yunnan. The results show that the dynamic stress peak value on triggered fault produced by the several strong shocks all exceeds to triggering threshold value. Second, the small earthquakes were strengthened abruptly in triggered area. We discussed the triggering effect on the North-South seismic belt by using the change of some earthquakes frequency since 1990. At the same time, we take west Kunlun mountains pass M_S8.1earthquake as an example to analyse triggering effect in Sichuan and Gansu provinces. All in all, the earthquake activity is triggered easily in the North-South seismic belt, but the earthquakes in different area have different triggering effect in the North-South seismic belt, it is influenced geologic structure.The research on seismic dynamic stress is therefore complicated, because it is affected by so many factors: the main earthquake parameters and structure in triggered area, there still have the time delay problem at the same time. The main earthquake parameters have big effect when we calculate stress tensor in the triggered point. Change fault three key elements(rake, dip angle and slide angle)、breaking time and rising time have great effect on the final result and trend of the dynamic stress. The change of the main seismic moment and the size of the seismic fault only affect calculated result, but whose trend does not change. The change of most main earthquakes’parameters can affect the final result, so we must get much more accurate parameters when calculating the dynamic stress changing produced by earthquakes. Besides, tectonic setting also has great effect on the earthquake dynamic triggering. Studying indicates that most triggered earthquakes happened in geothermal or volcanic area and extension blocks. We also find a little of triggered earthquakes in nobbing blocks. Strike-silp and normal fault earthquakes are more easily triggerred dynamic stress. Seismic activity will be triggered not long before strong shocks surface wave arrived triggered area, but lots of triggered earthquakes have time delay, the time may change from several days to several weeks.In short, in my thesis, I have improved the calculating method and procedure of dynamic stress, calculated dynamic coulomb stress change, which triggered by some earthquakes in the North-South seismic belt. At the same time, we studied effect of earthquake dynamic stress triggering induced by main earthquakes’parameters and tectonic characteristic of triggered area, and also,we discussed the problem of dynamic triggering time delay . At last, we found several problems that are worth to study in future.更多还原