用近场大地测量强化昆仑山地震破裂及区域流变结构研究

The Ms8.1 Kunlun Shan earthquake of 14 November 2001 ruptured a 400 km-long and more stretch of the sinistral strike-slip fault along the eastern Kunlun Shan located at the northern flank of the Tibetan Plateau Ranking as the largest strike-slip event ever documented instrumentally in Asia. It has exerted a significant impact on the regional seismicity in years to come. Earthquake rupture is a fundmental process in which elastic stresses localized on seismogenic faults give rise to irreversilbe tectonic deformation by abrupt faulting. Our understanding of this rare rupture requires a refined slip model that sheds detailed insights into how the tectonic stresses are accumulated and released on the seismogenic faults Unfortunately, previous works of slip models for the Kokoxili earthquake were hampered obviously by the facts that near-field data were insufficient and inaccuracy. We are proposing that such a drawback can be greatly overcome by reoccupying conventional geodetic network across the Kunlun Shan fault. Our slip model will be well constrained with the addition of denser near-field geodetic data and may help，with an unprecedented resolution, to understand why the rupture front propogated across neighbouring segments to grow into a great event for hundreds of kilometers. .In this study, we focus on slip behavior below the seismogenic layer, which will help to understand the mechanism of large earthquakes and may shed insights into the self similartiy of earthquake. The comparative study on the 2008 Wenchuan (Mw7.9) and 2001 Kokoxili (Mw7.8) earthquakes are pursued in order to explore the seismicity of large intraplate earthquakes and mitigate seismic hazards in China. We also investigate lithosphere rheology by modelling observed postseismic relaxations induced by the 2011 event. Repeated GPS surveys of a geodetic profile across the eastern segment of the surface ruptute will be extended for next five years in our project. The elongated time series derived by postseimic GPS measurements together witn the InSAR images definitely offer a better constraint on a numerical modelling for the rheological structure under the northern part of the Tibetan Plateau.

2001年昆仑山八级地震是大陆地区观测到破裂最长的特大型走滑地震，并对此后十年大陆地震活动性产生深远影响。地震破裂是地壳弹性应变转化为永久性构造变形的重要环节，断层破裂模型是认识地震动力过程和震源机制的基础，震后地壳松弛变形是分析岩石圈流变结构的重要途径。然而，十年来基础资料仍比较欠缺，制约了破裂模型和流变结构研究的进一步发展。本项目将GPS和InSAR和传统测量结合，通过复测昆仑山地区三角控制网，以密集近场数据约束地震同震位移场, 建立精细滑动分布模型，研究昆仑山断层应力状态, 深部破裂特征及其断层地表几何结构的关系。 本项目还将延续震后十年GPS震后观测，并利用现有InSAR震后监测资料, 通过多种数值模拟算法, 约束昆仑山地区下部壳幔流变结构参数，研究藏北地区块体运动方式和岩石圈变形机制。研究成果将推进昆仑山与汶川地震的比较研究，深化对区域大震活动性的认识。

2001年昆仑山Ms8.1级地震是近半个世纪以来在我国大陆发生的震级最大的地震事件，对大陆的地震活动以及岩石圈的地壳变形有着深远影响。由于该地震处于青藏可可西里无人区，近场资料的匮乏制约着对该地震的破裂模型和流变结构的研究。本项目通过对2001年昆仑山Ms8.1级地震近场（主要是同震破裂最大区，库塞湖区域）三角点的复测，新增了33个近场三角点的同震位移，填补了该地震长期以来在近场区域缺少大地测量观测资料的空白，改善了GPS同震位移场的空间分布。利用早期的震间速度场以及同震模型结果，扣除了震间变形和震后变形，获得了厘米级精度的GPS同震位移场，其中近场三角点的同震位移量级较大，最大可达2.3m左右。以该同震位移场为约束，基于可变滑移模型反演了昆仑山断层库塞湖段的同震滑移分布。反演结果表明，在库塞湖段分布有两个较为明显的凹凸体，西边的凹凸体滑移相对较小，但深度可达30km，超出孕震层的最大深度，而右边的凹凸体滑移量大，最大滑移约为8m，但较浅，主要分布在15km以内的深度。另外，利用GPS同震位移反演出的同震滑移结果显示在断层的东端还存在<2m的滑移。通过本项目的研究，不仅加深了对昆仑山地震的认识，而且丰富了大地震的破裂机制研究。