基于天然地震波形的活动断层三维结构地震偏移成像研究

There is a high potential of earthquake hazard in the seismically active fault zones. By constructing a high-resolution three-dimensional (3D) structural model of active faults, it is helpful for building the relationship between fault structure variations along the strike and earthquake rupture segmentations. It also provides a reliable model for earthquake hazard assessment. These will also contribute to the further development of earthquake sciences. Currently, seismic methods used to characterize fault zone structure generally include seismic travel time tomography and ambient noise tomography. However, they can only give smooth component of the fault zone structure. For characterize fine-scale structures of the fault zone, active seismic survey needs to be used but it is move expensive. This project plans to combine earthquake waveform data and seismic migration methods to develop a passive seismic imaging algorithm independent of the source information to image 3D fault zone structures. We will further propose a new imaging condition that is applicable to the case of irregular and sparse seismic station distributions. On this basis, the newly developed algorithm will be applied to earthquake datasets around Parkfield, California and the Southern California plate boundary region. Fine structures of San Andreas Fault zone and San Jacinto Fault zone will be obtained, respectively. Finally, we can analyze the relationship between fault zone structures and the background seismicity distribution as well as large earthquake rupture distribution, which will be very important to improve our understandings of how structures control earthquake behaviors.

活动断层地区地震频发，面临很高的地震灾害风险。通过构建高分辨率的活动断层三维结构模型，有助于建立活动断裂几何结构与地震破裂分段之间的成因联系，为地震灾害评估提供可靠模型，推动地震科学发展。目前刻画断层结构的地震类方法中，天然地震走时成像和噪声面波成像只能给出地下介质的平滑结构，而精细结构的刻画通常需要较为昂贵的人工地震勘探。本项目计划结合天然地震观测波形数据和人工地震勘探中常用的地震偏移成像方法，发展不依赖震源信息的天然地震偏移成像算法，并提出新的分组成像条件以适应天然地震不规则和较为稀疏的观测系统，得到地下活动断层三维的空间形态。在此基础上，将应用到美国加州Parkfield地区和南加州板块边界地区较密集台站记录的天然地震数据，分别得到圣安德烈斯活动断层和圣哈辛托活动断层的三维精细结构，进而研究断层的三维结构特征和小震分布以及较大地震破裂分布之间的关系。

目前刻画断层结构的地震类方法中，人工地震勘探可以刻画精断层的精细结构，然而较为昂贵；天然地震走时成像和背景噪声成像被广泛使用，但是只能给出地下介质的平滑结构。如果获取了天然地震震源信息，可以将天然地震作为"伪主动源"地震进行成像，但震源信息误差会影响成像质量。本项目结合天然地震观测波形数据和人工地震勘探中常用的地震偏移成像方法，发展了不依赖震源信息的天然地震偏移成像算法，得到地下活动断层三维的空间形态。在此基础上，将该算法应用到美国加州Parkfield地区与其他地区记录的天然地震数据，得到断层的三维精细结构，从而研究断层的地下三维结构特征。在成像结果中，断层结构与地表断层对应良好，与“伪主动源”地震成像结果一致性高，验证了算法的可靠性。高分辨率的活动断层三维结构模型有助于建立活动断裂几何结构与地震破裂分段之间的成因联系，为地震灾害评估提供可靠模型。