基于密集台阵地震背景噪声和近震波形的沉积层S波速度与衰减结构研究

Sediment is characterized as extremely low S wave velocity and strong attenuation, which also is one the most important part of the velocity model for seismology. Accurate S wave velocity and attenuation structures of sediment are essentially important for earthquake disaster assessment, strong ground motion simulation, determination of earthquake magnitude and location, tomography of crust and upper mantle, et al. However, for current methods, it’s still very difficult to obtain reliable S wave velocity and attenuation structures of sediment with several kilometer thickness. An effective method with low-cost, deeper-depth, high-resolution as well as independent of earthquakes is needed. The proposal will implemented the investigation of the S wave velocity and attenuation structures of thick sediments in a relative small area by surface wave dispersion and attenuation information derived from ambient noise recorded by dense seismic array. Dense seismic array with 100 short-period seismographs will be deployed in the two borehole seismic stations located in the capital region (Beijing) with thick sediment and strong seismicity. By synthetic simulations and field observations, we will study the effects of array scale, insterstation distance, array geometry, observation period et al, on the obtained sedimentary structures. With the reflected and converted waves recorded by seismic stations in the borehole and on the surface, S wave velocity and attenuation structures of sediments will be studied independently and compared with the results from dense seismic array. The observation, data processing and inversion method with dense seismic array and ambient noise method will be optimized and improved for investigating the S wave velocity and attenuation structures of thick sediments.

沉积层S波速度低、衰减强，是地震学重要的模型参数之一。准确的沉积层S波速度和衰减结构对于地震灾害评估、强地面运动模拟、地震震级和位置测定、壳幔结构成像等研究具有重要意义。然而，对于数千米较厚沉积层，目前尚缺乏一种低成本、大深度、高分辨、不依赖地震事件的S波速度和衰减结构被动源研究方法。本申请拟利用密集台阵地震背景噪声方法，提取面波频散和衰减曲线，实现对小区域、大厚度沉积层结构的有效探测。拟选择沉积厚度大、地震活动性高的首都圈两个井下摆开展多期次密集台阵观测（100台地震仪），基于理论数值模拟和实际观测，研究布设方式、台阵尺度、台站间距、观测周期等因素的影响；布设地表地震仪与井下摆共同记录近震波形，基于反射波、转换波等方法开展沉积层结构探测，并与密集台阵方法对比验证。优化密集台阵地震背景噪声方法的观测、数据处理和反演方法，改善数千米厚度沉积层S波速度和衰减结构的探测效果。

沉积层S波速度低、衰减强，是地震学重要的模型参数之一。准确的沉积层S波速度和衰减结构对于地震灾害评估、强地面运动模拟、地震震级和位置测定、壳幔结构成像等研究具有重要意义。然而，对于数千米较厚沉积层，目前尚缺乏一种低成本、大深度、高分辨、不依赖地震事件的S波速度和衰减结构被动源研究方法。在本项目中，我们提出了基于井下地震仪和近震波形数据的沉积层S波衰减结构研究方法，获得了首都圈地区以及中国大陆沉积层S波衰减（Qs）结构，进一步统计分析获得了Qs与频率的经验关系。同时，我们利用固定地震台的P波和S波震相报告，基于球坐标系有限差分层析成像方法，完成了唐山地震区的走时层析成像研究，获得了三维高分辨P波、S波和波速比结构模型。结果发现了唐山断裂表现为近直立的深大断裂，并且可能错断了莫霍面达2~3千米。这一工作与前人利用深地震测深剖面给出的唐山断裂在莫霍面的错断特征类似。我们在唐山地震区布设了密集台阵，记录了地震事件和地震背景噪声数据，基于密集台阵地震背景噪声和近震事件波形的沉积层S 波速度研究方法，获得了唐山地震区沉积盆地地区井的沉积层厚度和S 波速度结构信息。在方法上，我们优化了密集台阵地震背景噪声方法的观测、数据处理和反演方法，实现了对小区域、大厚度沉积层结构的有效探测。上述工作为沉积盆地地区地震波模拟、强地面运动计算提供了可靠的模型，为研究唐山地震的深部孕震机理提供了重要参考。所发展的密集台阵成像方法，为研究地壳浅层和深部结构提供了重要支持。本项研究工作在GJI、BSSA、PEPI、SRL、地球物理学报等国内外学术期刊共发表SCI论文13篇，培养硕士研究生3名，博士研究生1名，完成了项目制定的研究目标。