Martin Mai, KAUST

KAUST-IAMCS Workshop on Multiscale Modeling, Advanced Discretization Techniques, and Simulation of Wave Propagation

May 7-8, 2011

King Abdullah University of Science and Technology (KAUST)

Thuwal, Kingdom of Saudi Arabia

Near-Field Ground-Motion Simulation Including 3D Elastic Scattering

Authors: Walter Imperatori and P. Martin Mai

Wave scattering occurring in the Earth crust is an important physical process that strongly affects the observed seismic wavefield. Different empirical methods have been recently proposed to include such phenomenon in broadband ground motion calculations, considering scattering as a purely random stochastic process. In this work we take an alternative approach by solving the elastic wave equation considering realistic 3D random media into which we insert different earthquake source representations.

Our goal is to study scattering characteristics and its influence on the seismic wavefield at short and intermediate distances from the earthquake rupture, addressing in particular earthquake-engineering related parameters (PGA, PGV and SA). We investigate several scattering related phenomena, such as the loss of radiation pattern and the breakdown of directivity.

Our 3D media representations are generated using Von Karman correlation functions with correlation lengths of 2 – 15 km and standard deviation values of 2-20 %, based on recent findings on heterogeneities distribution in the upper crust. We then compute ground motions (0-10 Hz) for a double-couple point source characterized by an omega-squared spectrum model. Further complexities in the faulting process are added gradually, culminating in complex kinematic rupture models.

Results indicate that scattering acts globally as a pure energy-redistribution phenomenon, i.e. is no high-frequency energy is generated during the scattering process at the expense of the low-frequency energy. This has a profound implication for the dynamic rupture process: the faulting process is the only source of high-frequency seismic energy radiation.