基于重复地震定量研究龙门山断裂带强震前后的深部变形

Repeating earthquakes with magnitude range from 0.5 to 4.0 that rupture the same fault patch, namely, repeating microearthquakes, are a useful and natural tool for investigating fault deformation process at seismogenic depths. The proposed project aims at the in-situ measurement of deep deformation before and after strong earthquakes occurred along Longmenshan fault zone using repeating earthquakes. According to available seismic station distribution, we will employ different approaches,together with the cross-correlation technique and the multiplet relative relocation method, to search for all the members of repeating earthquake families using continuous waveforms, and then relocate repeating earthquakes using a latest version of the double-difference method in a 3D velocity model. In order to make up for poor constraints on deep deformation inverted from surface measurements, we further analyze the effect of assumed parameters (e. g., stress drop) on assessment of deep slip rates, and track the spatio-temporal evolution of deep slip rates from detailed analysis of repeating earthquakes for the quantitative description of fault deep deformation before and after strong earthquake. Moreover, we will monitor subtle changes of subsurface medium properties at high spatial and temporal resolution in and around the source region of strong earthquakes along Longmen Shan fault zone using moving window cross correlation technique and Code Wave Interferometry method，and then analyze the velocity change characteristics in time and space and the possible factors affecting velocity change, which can be viewed as reflecting causative mecnanism of deep deformation associated with strong earthquakes. This study may provide new quantitative basis for better understanding of earthquake hazard assessment and recurrence interval of strong earthquakes occurred along Longmenshan fault zone.

发生在同一断层位置上0.5-4.0级的重复地震(或称重复微震)是研究断裂带深部变形的天然（有力）工具。本项目在波形互相关和多事件定位等方法基础上，应用不同台站布局条件下识别重复地震的方法，分析连续波形资料来完整辨识龙门山断裂带重复地震,采用最新版双差定位法进行重复地震的三维定位，并基于重复地震"原位"观测强震前后的深部变形。深入探讨应力降等参数选择对断层深部活动速率估算的影响，以期基于重复地震研究给出龙门山断裂带深部活动速率时空演化过程来定量描述强震前后的深部变形，以弥补浅表观测资料反演深部变形的局限性；并采用滑动窗互相关方法和尾波干涉技术检测震源区地下介质的高时空分辨率的微小变化，分析波速时空变化特征以及引起变化的可能因素，研究与强震相关的深部变形机制。希望通过强震前后的深部变形研究为进一步认识龙门山断裂带地震危险性和强震的复发周期提供新的地震学定量依据。

了解强震前后的变形信息难，获取深部变形信息更难，板内强震更是不易。滑动速率是断裂带深部变形的定量描述，重复地震是天然的“地下蠕变计”，具有原位观测深部滑动速率的优势。地下介质的物性变化是强震震后深部变形的动力学机制之一，而重复地震是动态监测地下介质波速变化的有力工具。.汶川巨震和芦山强震相继发生在龙门山断裂上，积累了海量的地震波形资料，提供了一次难得利用重复地震深入剖析龙门山断裂带强震前后深部变形的机会。. 本项目基于年四川台网和水库台网的波形资料，在龙门山断裂带上完整识别了15组重复地震，发现了震前主震破裂区周缘分布的重复地震，展示利用重复地震分布勾画潜在闭锁区的可能；并基于重复地震首次获得了汶川强震前不同深度滑动速率的差异分布图像，高滑动速率位于断裂带深部13～18 km，揭示了汶川强震前孕震闭锁区构造活动的深浅差异，并发现了震前不同程度的深部滑动短期加速变化的迹象。断裂带南段高滑动速率值显示了汶川震后该地区深部断层运动的活跃性。由重复地震获取的深部滑动速率弥补了浅表观测资料的不足，为龙门山断裂带强震孕震机理研究提供了难得的深部变形约束资料。. 本项目分析了四川台网、水库台网、川西台阵和震后临时架设流动台站的海量波形资料，首次基于源于同一破裂位置的重复地震的尾波研究发现了汶川和芦山强震破裂区震后波速随时间的微小变化。结果展示了位于西南段起始破裂区汶川地震震后波速恢复（增加）约0.1%～0.3%，而位于东北段的北川破裂区波速没有明显变化，揭示了汶川震后不同破裂段断层愈合过程的明显差异。由重复地震尾波变化监测到了芦山地震震后短时内约0.1%～0.2% 波速的增加，反映了震后初期破裂断层较快的愈合过程，揭示了孕震深处传播路径上深部介质动态变化的信息。. 在龙门山断裂发现的重复地震以及获取的深部变形信息对于认识龙门山断裂带强震复发和孕震机理以及震后断层愈合具有重要的意义。