风化河谷场地SH波传播特性与散射规律研究

Due to a long-term process of weathering, inhomogeneous weathered zones occur in valleys and canyons. The shear wave velocity of the media in the weathered zones usually varies in space. Weathered valleys and canyons can significantly affect the propagation of seismic waves. As a result, the amplification and spatial variation of ground motions across the weathered valleys and canyons are created, which is named topographic and site effects. The amplification and spatial variation of transverse waves, especially SH waves, may aggravate the seismic response of long-span structures such as bridges and dams built in valleys and canyons. With the aid of wavefunction expansion method, this study is to investigate the scattering of SH waves induced by semi-circular, V-shaped and U-shaped weathered valleys and canyons analytically or semi-analytically. The influences of the topography geometry, thickness and weathering degree (wave velocity model and ratio) of the weathering zones and the frequency and angle of the incident waves on the earthquake wave progagation and scattering will be investigated in a systematic way. The characteristics of spatial variation, amplitudes and spectra of the ground motions on the three types of weathered sites will be revealed. These results, acting as theoretical foundation, can contribute to the proper selection of ground motions and the improvement of earthquake design of structures in weathered valleys and canyons.

裸露的河谷长期受到各种类型的风化作用，在河谷表面会形成非均质风化层，其剪切波速具有空间变化的特性。这种风化河谷会对地震波的传播产生影响，造成地面运动的放大及空间变化，引起地形及场地效应。地震波中的横波尤其是SH波的放大及空间变化对大型跨河工程如桥梁和大坝的抗震极为不利。本项目基于波函数展开方法，提出常见的风化半圆形河谷、风化V形河谷和风化U形河谷对SH波散射的解析解和半解析解，研究河谷形状、风化层厚度、风化程度（波速模型和波速比）、入射波频率和角度等因素对地震波的传播与散射的影响规律，旨在探究河谷表面非均质风化层的地震放大效应机理，揭示三类风化河谷场地地震动的空间分布特征、幅值特性与频谱特性，为河谷区工程建设提供地震动参数的设计依据，为大型跨河工程的多点地震动输入提供理论基础。

我国山区占国土面积67%，河谷众多且风化强烈，风化河谷场地上兴建了大量土石坝工程，然而，河谷场地地形地质条件复杂、地震频发、震害严重，例如汶川地震造成1713座土石坝震损。震害调查和实测地震动记录表明：风化河谷场地的地震放大效应是工程震害严重的重要原因。如何模拟地震波从震源传播到风化河谷场地的地形效应成为学术界和工程界面临的重大挑战。本研究提出了变系数波动控制方程，耦合了波函数展开与传递矩阵方法，建立了径向非均质河谷地形SH波传播的理论模型，包括单层幂函数波速模型和多层任意波速模型，突破了传统的波函数展开法无法模拟非均质岩土体波动问题的局限。探明了风化河谷波速变化对地震波传播特性的影响，发现地面运动放大的最大值往往发生在非均质层范围内，且二维风化层放大效应强于地形效应。建立了风化河谷-土石坝耦合波动模型，发现并解释了地震SH波在土石坝坝体中的聚焦现象，探明了风化河谷地震放大效应对土石坝的影响规律。揭示了风化层几何形状、波速模型和波速比等对地震动时程的影响规律，提出了考虑河谷风化层地震放大效应的地震动输入方法，为河谷区工程建设提供了地震动参数。