Shallow three-dimensional structure of the San Jacinto fault zone revealed from ambient noise imaging with a dense seismic array

We use 1 month of continuous seismic waveforms from a very dense seismic network to image with unprecedented resolution the shallow damage structure of the San Jacinto fault zone across the Clark fault strand. After calculating noise correlations, high apparent velocity arrivals coming from below the array are removed using a frequency-wavenumber filter. This is followed by a double-beamforming analysis on multiple pairs of subarrays to extract phase and group velocity information across the study area. The phase and group velocity dispersion curves are regionalized into phase and group velocity maps at different frequencies, and these maps are inverted using a neighbourhood algorithm to build a 3-D shear wave velocity model around the Clark fault down to approximate to 500m depth. The model reveals strong lateral variations across the fault strike with pronounced low-velocity zones corresponding to a local sedimentary basin and a fault zone trapping structure. The results complement previous earthquake- and seismic noise-based imaging of the fault zone at greater depth and clarify properties of structural features near the surface.

我们利用来自一个非常密集的地震台网的1个月连续地震波形，以前所未有的分辨率对横跨克拉克断层带的圣哈辛托断层带的浅层损伤结构进行成像。在计算噪声相关性之后，使用频率 - 波数滤波器去除来自台阵下方的高视速度波至。随后对多对子台阵进行双波束形成分析，以提取研究区域内的相速度和群速度信息。将相速度和群速度频散曲线区域化为不同频率下的相速度和群速度图，然后使用邻域算法对这些图进行反演，以构建克拉克断层周围直至约500米深度的三维剪切波速度模型。该模型揭示了沿断层走向的强烈横向变化，有明显的低速区，对应于一个局部沉积盆地和一个断层带圈闭结构。这些结果补充了之前基于地震和地震噪声对更深处断层带的成像，并阐明了近地表结构特征的属性。