Ground Motion Prediction in Los Angeles and Tokyo Using Ambient Seismic Noise

Earthquake strong ground motion estimation is traditionally based on empirical prediction equations. To overcome the scarcity of large event ground motion records, seismologists increasingly turn to physics-based ground motion simulations. To accurately reproduce earthquake ground motions, 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 simulated 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. We also investigated the response of Kanto basin in Japan using impulse responses for the dense seismic network MeSO-net. 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 methods. We take advantage of the 3D wavefield and simulate the radiation pattern of double couple. Finally, we show the potential of this method to represent an extended source using a temporary deployment on the San Andreas Fault.