斜入射情形场地基本周期的计算

Calculation of site fundamental period in the case of oblique incidence

场地基本周期是场地效应模型的重要变量,是场地分类的重要指标。已有基本周期的计算假定基岩地震波为垂直入射,常用方法包括利用垂直入射情形下场地内SH波竖向一维传播模型建立的模拟与经验方法,其中逐层单自由度法这一经验方法因计算简易、结果与模拟方法相当而被日本抗震设计规范所采纳。然而垂直入射假定大多与实际不符,为此给出可考虑基岩地震波斜入射的场地基本周期计算方法。首先利用Snell定律将基岩斜入射地震波作用下场地内的SH波二维传播表征为一维竖向视传播建立了斜入射情形场地内SH波传播模型。其次利用这一模型给出了斜入射情形基本周期计算的模拟和修正逐层单自由度法并应用于KiK-net台站场地,结果表明基本周期随入射角的增加而变小,就统计所得平均入射角57°而言,基本周期将缩减11%;修正逐层单自由度法可准确考虑斜入射的影响且计算简易度和原方法相当。最后,讨论了基岩面选取对基本周期计算的影响,结果表明,若依据土体首次达到给定剪切波速以确定基岩面,当剪切波速大于700m/s时,不同基岩面所对应的基本周期具有高度线性相关性。

The site fundamental period is a crucial variable for site effect models and serves as a significant index for seismic site classification.The existing methods for obtaining site fundamental period assume that the angle of incident seismic wave at the site base are vertical.These commonly used methods include simulation and empirical method based on the vertical SH wave propagation in seismic site.The empirical method of layer by layer single degree of freedom method is adopted in the Japanese seismic design code for its simple calculation and the results are equivalent to the simulation method.Nonetheless,the assumption of vertical incidence does not align with the actual incidence angle of ground motion at the site base.This paper addresses this discrepancy by focusing on establishing a method for the calculation of the basic period under obliquely incident seismic wave.First,the 2D SH wave propagation in seismic site under the action of oblique incidence wave is characterized as vertical 1D apparent propagation to establish the SH wave propagation model based on Snell law.Building upon this foundation,a wave propagation simulation model is applied to establish a numerical method and modified layer by layer single degree of freedom method for the calculation of the basic period and then applied to KiK-net stations.The findings indicate that the fundamental period decreases with an increasing incident angle.For the statistical average result of the base incident wave angle at 57°,the average deviation can be 11%.The modified method accurately considers the influence of the seismic wave incident angle,while maintaining calculation simplicity equivalent to the original method.Finally,the paper discusses the influence of selecting the bedrock surface burial depth on the calculated site fundamental period.The results demonstrate a high correlation between the basic period of the site and different depths if the top of the soil layer,with a shear wave velocity larger than 700 m/s,is defined as the bedrock surface.