Abstract: The current and conventional fault-crossing short baseline measurement has a relatively high precision, but its measurement arrays usually fail to or cannot completely span major active fault zones due to the short length of the baselines, which are only tens to 100 meters. GNSS measurement has relatively low resolution on near-fault deformation and hence is not suitable for monitoring those faults with low motion and deformation rates, due to sparse stations and relatively low accuracy of the GNSS observation. We recently built up two experimental sites on the eastern boundary of the active Sichuan-Yunnan block, one crossing the Daqing section of the Zemuhe Fault and the other crossing the Longshu section of the Zhaotong Fault, aiming to test the measurement of near-fault motion and deformation by using fault-crossing arrays of one-kilometer-long baselines. In this paper, from a three-year-long data set we firstly introduce the selection of the sites and the methods of the measurement. We then calculate and analyze the near-field displacement and strain of the two sites by using three hypothetical models, the rigid body, elastic and composed models, proposed by previous researchers. In the rigid body model, we assume that an observed fault is located between two rigid blocks and the observed variances in baseline lengths result from the relative motion of the blocks. In the elastic model, we assume that a fault deforms uniformly within the fault zone over which a baseline array spans, and in the array baselines in different directions may play roles as strainmeters whose observations allow us to calculate three components of near-fault horizontal strain. In the composed model, we assume that both displacement and strain are accumulated within the fault zone that a baseline array spans, and both contribute to the observed variances in baseline lengths. Our results show that, from the rigid body model, variations in horizontal fault-parallel displacement component of the Zemuhe Fault at the Daqing site fluctuate within 3mm without obvious tendencies. The displacement variation in the fault-normal component keeps dropping in 2015 and 2016 with a cumulative decrease of 6mm, reflecting transverse horizontal compression, and it turns to rise slightly(suggesting extension)in 2017. From the elastic model, the variation in horizontal fault-normal strain component of the fault at Daqing shows mainly compression, with an annual variation close to 10^-5, and variations in the other two strain components are at the order of 10^-6. For the Longshu Fault, the rigid-body displacement of the fault varies totally within a few millimeters, but shows a dextral strike-slip tendency that is consistent with the fault motion known from geological investigation, and the observed dextral-slip rate is about 0.7mm/a on average. The fault-parallel strain component of the Longshu Fault is compressional within 2×10^-6, and the fault-normal strain component is mainly extensional. Restricted by the assumption of rigid-body model, we have to ignore homolateral deformation on either side of an observed fault and attribute such deformation to the fault displacement, resulting in an upper limit estimate of the fault displacement. The elastic model emphasizes more the deformation on an observed fault zone and may give us information about localizations of near-fault strain. The results of the two sites from the composed model suggest that it needs caution when using this model due to that big uncertainty would be introduced in solving relevant equations. Level surveying has also been carried out at the meantime at the two sites. The leveling series of the Daqing site fluctuates within 4mm and shows no tendency, meaning little vertical component of fault motion has been observed at this site; while, from the rigid-body model, the fault-normal motion shows transverse-horizontal compression of up to 6mm, indicating that the motion of the Zemuhe Fault at Daqing is dominantly horizontal. The leveling series of the Longshu site shows a variation with amplitude comparable with that observed from the baseline series here, suggesting a minor component of thrust faulting; while the baseline series of the same site do not present tendencies of fault-normal displacement. Since the steep-dip faults at the two sites are dominantly strike-slip in geological time scale, we ignore probable vertical movement temporarily. In addition, lengths of homolateral baselines on either side of the faults change somewhat over time, and this makes us consider the existence of minor faults on either side of the main faults. These probable minor faults may not reach to the surface and have not been identified through geological mapping; they might result in the observed variances in lengths of homolateral baselines, fortunately such variations are small relative to those in fault-crossing baselines. In summary, the fault-crossing measurement using arrays with one-kilometer-long baselines provides us information about near-fault movement and strain, and has a slightly higher resolution relative to current GNSS observation at similar time and space scales, and therefore this geodetic technology will be used until GNSS networks with dense near-fault stations are available in the future.

Key words: one kilometer-long baseline measurement, fault-crossing surveys, experimental observation, near-fault displacement, near-fault strain

摘要： 现行的基于数十至百m长的跨断层短基线的测量方法精度较高, 但往往不能有效跨越大型活动断裂带进行观测; 而GNSS目前受站点密度及观测精度所限, 对断层近场尤其是运动速率偏低的断层开展形变观测的分辨率较差。 基于上述现状, 在川滇块体东边界构造带新布设2个实验场地, 分别跨越则木河断裂大箐梁子段和昭通断裂的龙树分支进行测量。 实验利用km尺度跨距的基线测量活动断裂带的近场运动与变形, 获得3a的实验观测数据。 文中首先介绍了场地选建、 监测断裂、 基线测量方法以及实验观测结果, 然后利用测量资料, 基于刚体、 弹性和组合模型3种假设条件计算分析这2个场地的断层近场位移和应变。 则木河断裂大箐梁子段在刚体模型下2盘近场平行断裂走向的位移分量在±3mm内波动, 无明显趋势变化; 垂直断裂走向的位移分量在2015-2016年持续下降, 反映断裂呈横向水平压缩, 累计降幅达6mm, 但2017年出现近2mm的横向水平拉张; 弹性模型下该断裂段的横向水平应变分量ε_y以挤压为主, 年变化幅度接近1×10^-5, 另外2个应变分量均为10^-6 量级。 昭通断裂龙树分支2盘近场的相对位移虽然变化较小, 但表现出与该断裂地质活动一致的右旋走滑特征, 位移速率约0.7mm/a; 沿该分支断层走向的应变分量ε_x为挤压状态, 量值不超过2×10^-6; 垂直断层走向的应变分量ε_y则以拉张为主。 文中还讨论了应用组合模型的效果与问题。

关键词: km跨距基线测量, 跨断层测量, 实验观测, 断层近场位移, 断层近场应变