2015年Mw6.4 皮山地震震后变形及其对岩石圈流变结构的制约

Postseismic deformation plays an important role during the seismic cycle. The long-term postseismic deformation is one of the characterizations of the viscoelastic relaxation behavior of the lithosphere. The viscosity coefficient, which controls the viscoelastic relaxation, is an important physical quantity for understanding the deformation and dynamic mechanism of the lithosphere. The 2015 Mw6.4 Pishan earthquake, which occurred in the hidden faults in the transition zone between the Qinghai-Tibet Plateau and the Tarim Basin，is an ideal experimental case to study the mechanism of blind reverse fault-type earthquake and the rheological structure of the lithosphere in this area. However, there is little research on the continuous and extensive postseismic deformation of the 2015 Mw6.4 Pishan earthquake, especially on the inversion of near-field rheological structure of the lithosphere by using the postseismic deformation of the main earthquake. This project intends to use the InSAR time series analysis technology to obtain five years’ time series of the surface deformation in the source area since the Pishan earthquake. Then, we use a 3D viscoelastic finite element model with Burgers constitutive to calculate the time series of surface deformation caused by the viscoelastic relaxation of the lithosphere after the main shock. Constraining the viscous parameters (size, distribution) of the viscoelastic model and obtaining the rheological structure of the lithosphere in this area after comparison between the observed and modelled deformation fields. It is expected to further understand the lateral heterogeneity of the three-dimensional rheological structure between the Qinghai-Tibet plateau and the Tarim basin, and provide a new perspective for the dynamic control of the crust postseismic deformation by the lithospheric rheological behavior.

震后变形是地震活动周期的重要阶段。长时间尺度的震后变形是地球介质粘弹性松弛行为的表征之一，而控制粘弹性松弛的粘滞系数则是认识岩石圈变形及其动力学机制的重要物理量。2015年发生在青藏高原与塔里木盆地过渡带内隐伏断层上的Mw6.4皮山地震，是研究盲逆断层型地震孕育机制及岩石圈流变行为的理想实验场。然而，当前皮山地震连续、覆盖面广的震后变形研究较少，特别是利用震后变形反演近场岩石圈粘滞系数的研究更是匮乏。本项目拟利用InSAR时序分析技术获取皮山地震震后五年内地表变形时间序列；再采用有限元数值模拟方法，依据Burgers体本构关系计算震后岩石圈粘性松弛导致的地表变形，将其与InSAR获取的观测值进行拟合对比，约束流变模型的粘滞参数（大小、分布）进而反演该区岩石圈的流变结构。以期为深入理解青藏高原与塔里木盆地之间三维流变结构横向非均一性以及岩石圈流变学行为对震后地壳变形的动力学控制提供新的视角。

近年来，随着InSAR观测能力的提升，研究者对强地震震后变形的研究也更加深入和全面，震后变形不仅可以揭示区域孕震机理，还为研究震源区岩石圈的流变结构提供了切入点。本项目获取2015年皮山Mw6.4地震长时间周期的InSAR震后变形时间序列，以较精确的观测数据来约束青藏高原和塔里木盆地西南部交接处岩石圈的粘滞系数，主要开展了以下工作：1、深入研究InSAR时间序列的提取方法，对影像干涉对质量逐个挑选，最终获得皮山地震五年内清晰、稳定的震后形变时间序列图；2、探索震后变形机制，定量分析震后余滑、粘弹性松弛行为在不同时间段内对地表震后变形的贡献，同时认识震源区岩石圈流变结构的横向不均一性对震后变形行为的影响。获得的认识和结论主要有：1、皮山地震震后地表变形区域与同震形变场重叠，非线性衰减较快，震后5个月左右，震源区地表震后变形达到10 mm（雷达视线向），震后三年达到约32 mm，此后震后变形变化比较微弱；2、发现单独利用下地壳、上地幔的粘弹性松弛无法很好地解释地表变形特征，基于震后余滑-粘弹性松弛耦合模型模拟计算，当青藏高原岩石圈粘滞系数为5e18 Pas，塔西南粘滞系数为1.5e19 Pas 时，震后形变理论值与各时期InSAR时间序列观测值总体特征较为符合，震后余滑对震源区地表的早期震后变形起主要贡献，同时发现震后余滑主要发生在断层面上部，即近地表处未产生破裂的脆性层。本项目对认识盲逆冲型地震的震后变形机制和震后浅部地壳的地震危险性判定有实际指导意义。

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