南北构造带深部变形的重复地震研究

Repeating earthquakes (repeating microearthquakes, i.e., sequences of seismic events with magnitude range from 0.5 to 5.0 that represent a repeated rupture of the same fault area), may act as not only subsurface creepmeters for providing a useful and natural tool to gauge fault deformation at depths, but also indicators of areas that delimit the locking of the segment in the seismogenic zone. The proposed project aims at the identification repeating earthquakes and the in-situ measurement of deep slip rates derived from repeating earthquakes for the quantitative description of the deformation at depths. According to available seismic station distribution, we 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. We also determine the focal mechanisms of repeating earthquakes and infer possible rupture directivity, which can help understand the slip direction of repeating earthquakes. In this way, we compare and disscuss in great detail the deep deformation behaviors and features along the North-South Belt including Haiyuan fault, Honghe fault and Longmenshan fault and so on. Combined with surficial geological and geodetic slip-rate measurements, seismicity and active tectonic setting, a comprehensive analysis of the identified repeating earthquakes and estimated slip rates within the seismogenic depth range may provide new quantitative basis for better understanding of earthquake hazard assessment and strong earthquake cycle.

滑动速率是断裂带变形的定量描述，而发生在同一断层位置上0.5-5.0级的重复地震(或称重复微震)作为天然的地下蠕变计，是探测深部构造变形的有力途径，也是揭示构造变形闭锁状态的有效约束。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南北构造带的重复地震, 采用最新版双差定位法进行重复地震的三维定位，并基于重复地震的“原位”信息来探测断裂带深部滑动速率。深入探讨应力降等参数选择对断层深部滑动速率估算的影响，结合重复地震的震源机制解对比分析南北构造带主要的海原断裂带、红河断裂带和龙门山断裂带等的深部变形特征，以弥补浅表观测资料反演深部变形的局限性；并结合浅表观测、活动构造背景和地震活动性分析，力求通过断裂带深部变形的定量研究为进一步认识南北构造带地震危险性和强震复发周期提供新的地震学定量依据。

揭示断层的深部变形是一个极具挑战性的问题。重复地震(也称重复微震)作为天然的地下蠕变计，是探测深部构造变形的强有力途径。南北构造带作为我国最重要的地震监测区和强震危险地带，其主要断裂带深部变形特征的客观探测亟待具有“原位观测”优势的重复地震研究的深入分析。. 本项目系统汇集了川滇地区主要断裂带识别出的76组重复地震研究结果，构建了川滇地区重复地震的时空分布图像和断裂带深部变形时空演化特征，结果表明：丽江-宁蒗断裂带在脆韧转换带约23 km深处的滑动速率为4.3～5.4 mm/a，小江断裂带3.0～12.3 km深处的滑动速率为1.6～10.1 mm/a，红河断裂带北段在6.0～13.4 km深处的滑动速率为2.3～10.0 mm/a，鲜水河断裂带南段3.0~18.7 km深处的滑动速率为3.0～10.2 mm/a，龙门山断裂带在4.0～17.3 km的汶川8.0级地震孕育深处的滑动速率为3.5～9.6 mm/a，龙门山断裂带南端3.6～18.7 km处滑动速率为5.8～10.2 mm/a。综合分析认为：川滇地区主要边界断裂带的深部滑动速率较为一致，揭示了川滇地块和巴颜喀拉地块整体协同变形的特征。由重复地震与深部滑动速率变化构建了孕震深处的变形模式，即重复地震与断裂带局部闭锁段在空间位置上存在密切的关联性，强震前孕震闭锁区存在明显的深浅部构造形变差异，震前存在的深部加速变形过程可能是断层亚失稳阶段的具体表征。. 本项目分析了2001～2019年甘肃、宁夏和青海区域固定台网以及喜马拉雅台阵二期大量的波形资料，基于祁连-海原断裂带及其周缘发现的71组重复地震（至少包含4个地震）获取了地下4.3～26.1 km深处的滑动速率为0.4～15.1 mm/a，滑动速率的分布图像显示了断裂带深部滑动的不均性和复杂性。长时间持续缓慢复发的重复地震揭示了老虎山断裂可能存在长期蠕滑现象。. 由重复地震原位量测的深部滑动速率填补了浅表观测资料难以获取深部变形的不足。汶川8.0级地震前出现的深部滑动速率加速的现象为断层亚失稳实验研究提供了难得的观测证据。老虎山断裂可能存在长期缓慢的蠕滑现象为研究该断裂带复杂的深部滑动模式提供了新的地震学资料。