青藏高原及周边区域两次大地震前震序列特征及机理研究

Foreshock sequence is an important seismological phenomenon, associated with earthquake preparation, which has been discussed for many years. The characteristics of the waveforms of the sequence and quantitative relationship among the foreshock, asperity and the mainshock need to be profoundly discussed. With the increase of the seismic station density in China, it makes it possible to detect the strength and distribution of the asperity related to the nucleation process of the large earthquakes from the recorded waveforms. And it affords a good chance to study the mechanism of the foreshock sequences because two strong earthquakes preceding foreshock sequences occurred in Tibetan Plateau and the region around it. They are the 2010 Ms 7.1 Yushu, Qinghai, earthquake which occurred in Yushu-Ganzi fault and the 2014 Ms 7.3 Yutian, Xinjiang, earthquake which occurred in Altyn Tagh fault. We can contrast the consistency of the S-minus-P travel times (Ts-Tp) and the first motion of P wave between the foreshock sequences before these two strong earthquakes and the other sequences which are not the foreshock sequences. We will recognize the size and location of the asperity based on the precise relocation hypocenters obtained from joint relocation methods and get the spatial relative location between the foreshock sequences and the main shock sequences by calculating the similarity of the waveforms by cross-correlating. The similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms so that we can obtain extremely accurate relative locations. Then we can use these results to identify small-scale fault zone structures that could influence nucleation and understand the mechanism of earthquake. These results can play the important role in the seismic hazard assessment for some earthquakes of this region.

前震序列是与主震孕震过程有直接关联的一种重要地震学现象，是国内外地震学家关注的一个热点科学问题。主震孕震成核过程中前震序列活动与凹凸体（Asperity）强度和分布密切相关。2010年发生于玉树-甘孜断裂带上的青海玉树7.1级和2014年发生于阿尔金断裂带上的新疆于田7.3级地震均具有明显前震序列，2009年以来新布设好的中国地震台网很好地记录了这两个主震和前震序列，为开展典型地区典型地震的前震震源性质、定位断层附近凹凸体分布和强度，进而用以研究强震地震成核过程提供了必要的资料保障。本研究拟以这两个地震及其前震序列为研究对象，重点通过分析前震序列P波初动极性和S-P到时差的一致性、震源精定位、波形相似度等特征，识别凹凸体大小和位置，对断层的精细结构和介质分布进行详细探测和研究。该工作对板内大地震成核尺度、孕震机理和青藏高原及周边区域未来前震-主震型大地震短临风险的评估具有重要科学意义。

本研究收集整理了中国地震台网多年来积累的震相报告资料，编写了可以对震相报告信息进行灵活提取和分析的软件。基于本研究提出的方法和程序，首先分析了玉树地震、于田地震震源区附近地震台站地震前后Pg和Sg波到时信息。特别是对玉树地震震源区3个台站到时资料分析表明，玉树地震前后视速度发生了显著变化，特别是纵横波波速比地震之后明显增大，该结果对于理解地震孕育和同震过程中应力的变化和调整具有重要意义。进一步用我们发展的方法，处理了龙门山断裂带长期积累的震相报告，对汶川地震、芦山地震前后视速度变化进行了深入分析，结果中显示了龙门山断裂带两次地震前后视速度的显著变化，最后通过不同时段层析成像方式，对速度变化的具体空间分布特征进行了深入研究。.在本项目执行过程中，课题组收集到了青藏高原东北缘甘东南地区150多个台站组成的密集台阵资料，对于该台阵记录到的小震进行了识别和整理。该区域2017年发生了九寨沟7.0级地震，我们对密集台阵所在的甘东南地区两年的小地震进行了重新定位，进而对九寨沟地震深部结构特征进行了研究。结果表明九寨沟地震震源区位于高低速相间的区域，该成果对于探究青藏高原东北缘大震的深部构造背景具有重要科学意义。.此外，本项目基于青藏高原东北缘密集台阵资料接收函数上Moho面产生的转换波和多次波频率特征，研究了青藏高原东北缘Moho性质，进而结合区域构造背景，对该区域壳幔物质运移机制进行了探讨。利用体波波形频率特征进行Moho面性质研究工作，在该区域较为鲜见，本研究提出的方法为Moho面性质研究提供了一个新的可行性途径。