Identifying and Modeling Complex Site Response Behavior

Tufts University, Medford MA

Investigators

Abstract

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.