Surface wave analysis of 3D seismic velocity structure using dense seismographic network

提出了一种膜表面波合成方法。其主要特点是：(1)在将位移分离为垂直变化项（局部特征函数）和水平变化项（水平势函数）的情况下，可以通过求解二维势函数来评估三维非均质性对波形的影响；(2)不采用渐近近似，直接对非均质面波相速度分布的二维势函数进行数值计算；(3)评价三维层析核（偏导数）的能力。我们建立了一个初步的横向非均匀模型，用于日本群岛及其周围的结构。计算电位和层析核显示了明显的扭曲（弯曲路径和不对称模式），特别是在10s左右的短时间内。以2002年渤索半岛外地震为例，采用横向非均质模型对瑞利波进行的膜面波合成比采用平面层状模型对瑞利波进行的合成具有更强的频散效应。我们发现这种色散特征与观测到的波形特征相似，尽管目前的结构模型仍不足以完全预测观测结果。我们建议沉积层的重要性，特别是对洛夫波。

We developed a membrane surface wave synthetic method. Its main features are (1) the effect of the 3D heterogeneity on the waveform may be evaluated by solving 2D potential function under the ansatz of the separation of the displacement into term of vertical variation (local eigenfunction) and term of horizontal variation (horizontal potential function), (2) numerical (direct) evaluation of the 2D potential functions for heterogeneous surface wave phase velocity distribution without asymptotic approximations, and (3) capability in evaluating the 3D tomographic kernels (partial derivatives).We made a preliminary, laterally heterogeneous model for the structure in and around Japanese Islands. The computed potentials and tomographic kernels indicate significant distortions (curved path and asymmetric pattern), especially at short periods around 10s for the model structure.The membrane surface wave synthetic for Rayleigh waves from an example event (2002 Off Boso Peninsula) earthquake assuming the laterally heterogeneous model shows more intense dispersion effect than the synthetic for a flat layered model does. We find that this dispersive feature resembles the characteristics in the observed wave forms, although the current structural model is still not sufficient in fully predicting the observation. We suggest the importance of the sedimentary layer especially for Love waves.