Monday, April 29, 2013
Special Mechanical and Civil Engineering Seminar
Integrating Nonlinear Site Effects in Broadband Ground Motion Simulations
Dominiki Asimaki, Assistant Professor, School of Civil and Environmental Engineering, Georgia Institute of Technology
'End-to-end' ground motion simulations lie at the forefront of collaborative efforts between seismologists and engineers, and refer to large-scale simulations of earthquake scenarios that integrate fault rupture and wave propagation physics from source to surface to produce ground motion time histories for implementation in aseismic structural design. Near-surface soils lie at the 'interface' of seismological and structural models: highly inelastic, strongly heterogeneous, and characterized by irregular surface and subsurface geometry, they significantly modify the amplitude, frequency and duration of seismic waves. The integration of soil behavior in earthquake simulations, however, is associated with significant multi-scale challenges: seismic waves are modified in the top 100m of the crust, and the resolution and input parameters required to rigorously account for the so-called 'site effects' is prohibitively expensive for seismological models on the order of hundreds of thousands of cubic kilometers. In turn, site effects in seismological models are approximated to date by empirical amplification factors derived using data from a large number of sites within broadly defined categories (e.g., rock and soil), in favor of computational efficiency and cost-effectiveness. By representing a blended average site effect from numerous sites at individual grid points, however, these factors overshadow the significant advancements made to enable broadband physics-based earthquake predictions in practically the entire frequency range of interest [>1Hz] of earthquake engineering interest.
In this seminar, we first present how the reverberation, focusing, and scattering of seismic waves in the near-surface of the earth's crust alters the amplitude, duration and frequency content of ground shaking. We next derive site-specific and regional soil amplification models based on wave propagation principles that significantly reduce the uncertainty in predictions of site effects for seismological models, while retaining the numerical efficiency and cost-effectiveness of currently employed formulations. We draw examples from simulated ground motion scenarios in Southern California, centrifuge experiments on amplification and focusing of seismic waves in the near-surface, and recordings from recent catastrophic events such as the 3/11/2011 Tohoku M9.0 earthquake. We finally highlight the applicability of this work to hazard assessment and mitigation in high seismic risk U.S. regions such as California, the Pacific North-West and the Mid-America, to design provisions of critical infrastructure such as nuclear power plants and nuclear waste disposal facilities, and to the development of improved earthquake hazard maps and design code provisions in the US and abroad.