Identifying and Modeling Complex Site Response Behavior

当地震波从深处传播到地表时，地球的近地表特性改变了它们，在那里它们被感觉到并影响到社会。这一过程通常被称为场地反应，是影响特定地点地震危险性的重要因素。正如在过去的地震中观察到的那样，靠近自由表面的速度较慢的物质会在短距离内影响破坏模式。场地响应是近地表材料的物理性质和这些性质的空间分布的函数。不幸的是，在土壤行为的线性和非线性范围内的盲目预测实验一致表明，预测的放大很少与观测的放大相匹配。我们假设，现有的场地响应模型的性能差是由于标准假设不能充分代表在许多情况下的场地响应行为的复杂性。大多数场地响应模型依赖于横波通过横向均匀介质垂直传播的假设（SH1D）。在这项研究中，我们着手评估多个站点的响应，这些站点既测量了弱运动，也测量了强运动，并且存在三维（3D）土壤信息。所选站点提供了从简单到复杂的站点响应行为的顺序。场地响应模型将包括三维波在三维空间变量和非线性介质中的传播。这项研究将测试一个更复杂的站点响应模型是否可以解释在这些站点中观察到的行为。此外，我们将概述一种在需要时识别和模拟复杂站点响应的方法。这项研究将集中在四个KiK-net网站。这些站点满足两个标准：(1)这些站点记录了2003年M8.3 tokachio - oki地震的大加速度（最大加速度从0.40到0.51 g）；(2)这些站点包括那些对弱地面运动的SH1D响应具有最佳、中等和最差拟合特征的站点（即从简单到复杂的站点响应行为）。有了这个数据集，我们将测试空间和本构模型预测复杂场地响应行为的准确性。简约原则要求数值模型只与数据所需的一样复杂。因此，我们将量化在不同复杂程度下可以实现的准确性，以便从业者可以对特定项目所需的空间数据范围和本构模型的复杂性做出明智的决策。我们将考虑从线弹性到超弹塑性的一系列本构模型。该研究的智力价值在于挑战了横波通过横向恒定介质的一维垂直传播的标准假设，并为更复杂和更精确的场地响应模型奠定了基础。可用的计算能力在不断增加，并且正在接近模拟从震源到地点的波传播的能力；然而，目前最常见的方法是独立模拟非线性效果和三维效果。这项研究是将建模的这两个重要方面结合起来的第一步。这项工作的广泛影响包括对项目团队内外的指导，特别关注塔夫茨大学的本科工程人群，以及通过在线地质灾害数据库联盟@塔夫茨大学广泛传播四个不同复杂性的现场响应站点的现场数据。

The near-surface properties of the earth modify seismic waves as they propagate from depth to the surface where they are felt and effect society. This process is often called site response, and is an important factor that contributes to the seismic hazard at a specific location. As observed in past earthquakes, the slower materials near the free surface influence damage patterns over short distances. Site response is a function of both the physical properties of near surface materials and the spatial distribution of those properties. Unfortunately, blind prediction experiments both in the linear and nonlinear range for soil behavior have consistently shown that predicted amplifications rarely match the observed amplifications. We hypothesize that the poor performance of existing site response models is that the standard assumptions do not adequately represent the complexity of site response behavior in many cases. The majority of site response models rely on the assumption of vertically propagating S-waves through laterally homogeneous media (SH1D). In this research, we set out to evaluate site response at multiple sites where both weak and strong motions have been measured and three-dimensional (3D) soil information exists. The selected sites provide a sequence from simple to complex site response behavior. Site response models will include 3D wave propagation through a 3D spatially variable and nonlinear medium. This reserach will test whether or not a more complex site response model can explain the behavior that is observed at some of these sites. Further, we will outline a method to identify and model complex site response when needed.This research will focus on four KiK-net sites. The sites fulfill two criteria: (1) the sites recorded large accelerations from the 2003 M8.3 Tokachi-Oki earthquake (with maximum accelerations from 0.40 to 0.51 g), and (2) the suite of sites include those that are characteristic of the best, intermediate, and worst fit to the SH1D response for weak ground motions (i.e., simple to complex site response behavior). With this dataset we will test the accuracy of both spatial and constitutive models for predicting complex site response behavior. The principle of parsimony demands that numerical models be only as complex as the data require. Thus, we will quantify the accuracy that can be achieved at various levels of complexity so that practitioners can make informed decisions about the extent of spatial data and complexity of the constitutive model needed for a particular project. We will consider a sequence of constitutive models from linear-elastic to hyperelastic-plastic. The intellectual merit of the research is to challenge the standard assumptions of one-dimensional vertical propagation of S-waves through a laterally constant medium and lay the groundwork for more complex and more accurate site response models. The available computational power is continually increasing and is approaching the ability to model wave propagation from source to site; however, the most common approaches currently model nonlinear effects and 3D effects independently. This research is a first step toward combining these two important aspects of modeling. The broader impacts of this work include mentoring within and beyond the project team with specific focus on the undergraduate engineering population at Tufts, and the broad dissemination of in situ data for four site response sites of varying complexity with through the online Geohazards Database Consortium @ Tufts.