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Current Projects

Identifying and Modeling Complex Site Response Behavior
Sponsor: National Science Foundation
Primary Investigator: Laurie Baise

The near-surface properties of the earth (seismic velocities, density, and attenuation) modify seismic waves as they propagate from depth to the surface where they are recorded. 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 the 1989 Loma Prieta Earthquake and other earthquakes, the slower materials near the free surface influence damage patterns over short distances (Hanks and Brady, 1991; Borcherdt, 1970; Graves, 1993; Boatwright et al., 1991). Impedance contrasts within the local geologic materials at a site modify the incoming waves and create seismic resonances. Detailed information regarding the physical parameters and their spatial distributions is essential to accurate modeling of linear and nonlinear response. Ground motion records from multiple earthquakes and previously collected shear-wave velocity profiles at multiple locations across four KiK-Net seismic network in Japan provides an excellent opportunity to accurately characterize the three-dimensional spatial heterogeneity of the subsurface. With this dataset we will test the accuracy of the spatial and constitutive models for complex site response behavior. True predictive accuracy can only be addressed with a blind prospective test of the numerical modeling, which can only be provided by tests of future earthquakes. These sites have been carefully instrumented to provide excellent spatial observations of ground motions during an earthquake event in vertical arrays of instruments as active field site response tests.

The proposed work will enhance our understanding of complex site response behavior using multiple vertical seismometer arrays with multiple recordings of earthquakes. This research has the potential to transform the way site response is modeled because 3D heterogeneity is rarely considered in linear and nonlinear site response models. 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 linearelastic to hyperelastic-plastic.

The intellectual merit of the proposed work 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 three-dimensional effects independently. The proposed work 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, and the broad dissemination of four site response case sites with data through the Tufts Geohazards Database.