# Ground Motion of Virtual Large Earthquakes Using Ambient Seismic Field

We are studying how to use the information carried out by the ambient seismic field to predict strong ground motion for large earthquakes in southern California.

Over the past decade, seismologists have made tremendous progress through extracting Green's functions between two seismic stations using the ambient seismic field. The cross-correlation of the ambient seismic field recorded simultaneously at two seismic stations converges, with a great deal of time-averaging, to the surface-wave Green’s function. This capability frees seismologists from the need to rely on natural earthquake sources, and allows them to construct virtual sources in areas of interest. In the overwhelming majority of studies based on this capability, seismologists have used it to constrain the wavespeed structure of the Earth.

Our research is focused on recovering and interpreting not just the phase, but also the amplitude information from the ambient seismic field. We have used this to constrain models of anelastic structure. We are also using it to construct virtual earthquakes in order to predict earthquake strong ground motion. To model a double-couple earthquake source at depth, we need to correct for depth, and for radiation pattern. We developed a new technique to compute the excitation of surface-waves using spectral collocation. Our immediate future goal is to construct the response from a spatially and temporally extended source, and to use these empirical predictions of ground motion with simulations based on detailed wave propagation modeling through the Earth's crust.