论文部分内容阅读
地震预测是地震学研究一个基本目标,而理解地震周期是地震预测的重要环节.遗憾的是,由于地震断层带不但结构复杂,大地震前后短时间内的震源过程更是随时间迅速变化,这些复杂因素使得常规研究方法不再适用.一类特殊的微震成群地发生在主震断层几乎同一地点上,震源系数几乎相同,近年来地震学界通称其为震群(earthquakemultiplet).本文论证如何将这类特殊的微震作为一组人工震源来阐述美国西部两次大地震(LomaPrieta和MorganHill)的震后弛豫过程.在加利福尼亚LomaPrieta(1989年9月18日,M7.0)及MorganHill(1984年4月24日,M6.2)附近的主震破裂带中就发生了此类震群.其中有两组各包含11个重复地震的震群,分别位于这两个破裂带中,其波形资料质量高,波形及震级变化极小,因而可与实验室条件下的人工震源相比.这些震群地震资料可直接用于计算P波沿波程衰减(即t)的时间性变化,其方法的简单性及结果的精确性都与实验室水平相当.我们的结果显示,每个震群周围分别存在一个异常体,在其各自主震之后的10个月中,该异常体中的P波衰减迅速上升并降落.衰减峰值发生在LomaPrieta地震后2～3周,其震群深度为10.2km.此峰值在MorganHill地震后5～6个月发生,其震群中心深度为2.6km.这两个事例中,t波动峰值都超过此前获得的加利福尼亚大致同一地区t绝对值的100%[1].此结果显示了因同震裂隙张开和震后弛豫过程(如流体移动、裂隙复原及岩石压缩),以及孔隙度和流体饱和度在主震源体部分区域发生的变化.衰减达峰值的时间之显著差异则提示我们,裂隙闭合及伴随的流体移动是负载敏感的,深度越大达峰值越快,反之亦然.如将此方法应用于主震前多重地震的资料,将有望更好地理解最终导致主震断层带大破裂的震前微破裂的物理过程.
Earthquake prediction is a basic goal of seismology research, and understanding the earthquake cycle is an important part of earthquake prediction.Regardless of the complexity of the seismic fault zone, the process of hypocenter in the short time before and after a large earthquake changes rapidly with time Complicated factors make conventional research methods no longer applicable.A special kind of microseism occurs in groups at almost the same location on main fault and its source coefficients are almost the same, and in recent years the seismological community has called it earthquake complex. These special microseisms serve as a set of artificial sources to illustrate the post-seismic relaxation process of the two large earthquakes in the western United States (Loma Prieta and Morgan Hill). In Loma Prieta, California (M7.0, September 18, 1989) and Morgan Hill, 1984 These groups of swarms occurred in the mainshock rupture near M6.2 on April 24, in which two groups of swaths, each containing 11 repeated earthquakes, were located in the two rupture zones, respectively, and their waveform data The high quality, minimal change in waveform and magnitude allows comparison with artificial sources under laboratory conditions, and these seismogenic seismic data can be directly used to calculate the temporal nature of P-wave attenuation (ie, t) The simplicity of the method and the accuracy of the results are comparable to those of the laboratory, and our results show that there is an anomaly around each swarm, respectively. Within 10 months after their respective mainstrings, the anomaly , The P wave decay rapidly rises and falls.The attenuation peak occurs 2 to 3 weeks after the LomaPrieta earthquake with a swarm depth of 10.2 km.The peak occurs 5 to 6 months after the MorganHill earthquake with a center depth of 2.6 km. In both cases, the peak value of t fluctuates more than 100% of the absolute value of t previously obtained in the same area of California [1]. This result shows that due to the onset of coseismic fissures and relaxation processes such as fluid movement , Fissure recovery and rock compression), as well as changes in porosity and fluid saturation in some regions of the main source body.The significant differences in the time to peak attenuation suggest that the fracture closure and accompanying fluid movement are load-sensitive, depth The larger the peak, the faster the peak, and vice versa.If this method is applied to multiple earthquakes before the main shock, it is expected to better understand the physical processes leading to pre-earthquake micro-fractures leading to a major rupture of the fault zone in the main shock.