New Tomographic Methods
The purpose of seismic tomography is to image the interior of the Earth by using
data provided by seismic waves. Traditionally, most researchers have focused on
using earthquake travel times to determine Earth properties. While such travel-time
tomography has been extremely successful, travel times are primarily only sensitive to
seismic wave speeds (Vp and Vs) and are not very sensitive to variables like density
We have recently developed new methods of surface-wave tomography to explicitly make use of the
wave amplitudes. In one development, we have taken the Helmholtz tomography method, a method
which empirically accounts for wave scattering (without needing to use sensitivity kernels),
and extended it to measure amplitude variations across a seismic array. This methodology allows
us to robustly determine the local amplification of waves, and also make attenuation measurements
(Lin et al. 2012a). Since local amplification depends on both velocity and density structure,
this method has the potential to determine density variations in the crust and upper mantle.
In a second development, we have looked at horizontal-to-vertical (H/V) surface-wave amplitudes. These
H/V measurements turn out to be strongly sensitive to much shallower velocity structure than typical
phase-velocity measurements at the same periods, and also have independent sensitivity to density.
We are therefore able to improve not only shallow estimates of shear-wave velocity (Vs), but we
obtain high-resolution maps of shallow density structure (Lin et al. 2012b). This shallow (0-3km)
density structure that we find across the western US (using USArray) is shown at right. See publications
section for more details.