Earthquake Rupture Simulation: thermo-mechanical models and validation with strong motion data

San Diego State University Foundation, San Diego CA

Investigators

Abstract

Large earthquakes are rare, complex events and their sources are deep-seated and inaccessible to direct observation. Moreover, relatively few near-source strong motion recordings exist for events of very large magnitude, i.e., those earthquakes that pose the greatest potential threat to life safety and the built environment. Well-validated computer simulations provide a means to forecast strong ground motion from large earthquakes, based upon the best available earthquake physics and wave propagation models. Simulations can help fill the pressing need for site-appropriate ground-shaking estimates for use in performance-based engineering and structural analysis, a requirement that is especially acute in light of the construction boom in very tall buildings in western U.S. cities. Computer simulations are also essential for advancing our basic scientific understanding of earthquakes, as they reveal how large-scale effects emerge from smaller-scale interactions that are difficult to study experimentally or via traditional theoretical analysis. This project is developing numerical earthquake models that represent the principal mechanical and thermal processes of faulting, including the weakening of frictional resistance by the heating of rock contacts and the pressurization of pore fluids. These computer models also incorporate established statistical representations of fault roughness and of the stress state in the earth, as well as the strength limits of geologic materials (which place upper bounds on the amplitude of the stress wave disturbances excited by faulting). The models must run efficiently on large computer clusters (i.e., those with thousands of processor cores), so that they can combine (i) the small-scale resolution required to accurately simulate fault-zone processes with (ii) the large overall physical volume required to simulate large earthquake sources and compute surface ground motion at distances of engineering interest. The project engages both earth and computational scientists and trainees to address these modeling challenges. The project team is testing the computer models by comparing the ground motion predictions from large suites of simulations with comparable compilations from actual earthquake recordings. For example, the distributions (including median and statistical spread) of ground motion parameters such as spectral acceleration and peak ground velocity, as functions of event magnitude, site distance, and other variables, are among the targets of these observational tests. Those computer models that can be validated in the above sense are then used to (i) simulate earthquake shaking from future large earthquake scenarios, (ii) develop improvements to simplified modeling methods for ground shaking (the so-called kinematic methods), and (iii) improve the empirical ground motion models that are conventionally used in engineering design, by providing a physics-based extrapolation beyond the range in which they have significant support from data (i.e., to large magnitude and small source-to-site distances).