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

Gregory Beroza, Stanford University
Seismic Ground Motion Prediction Using Virtual Earthquakes


Earthquake strong ground motion estimation is traditionally based on empirical attenuation relations. To overcome the scarcity of large event ground motion records, seismologists increasingly turn to physics-based ground motion simulations. To reproduce earthquake ground motions accurately, these simulations must take into account source complexity, elastic and anelastic structure of the crust as it affects wave propagation and local site effects. Incomplete knowledge of the subsurface translates into uncertainty in the amplification, so it is important to validate ground motion predictions against data.

For many areas, we lack earthquake records to test simulations. In these cases the ambient seismic field can be used for validation because it is subject to the same elastic and anelastic propagation effects as earthquakes. We have shown that to first order, for a period range of 4-15s, station-to-station impulse responses from ambient noise show very similar amplification effects to earthquake records for moderate earthquakes in southern California. To achieve more accurate ground motion predictions, we have to correct for the difference in wave excitation for earthquakes, which have double-couple focal mechanisms at depth over a finite source, and ambient noise impulse responses, which are for a point source at the Earth's surface. We compute the depth-dependence of the fundamental mode surface waves by calculating the eigenmodes using Chebyshev spectral collocation.

Virtual earthquakes can also be used as data for reconstruction of the seismic wavespeed model, using a scattering integral or adjoint approach, and they have been used to recover anelastic properties of the crust and upper mantle as well.