Using noise to map Earth’s deforming crust

Earth is awash in vibrations…literally. Interference between ocean waves near a coastline excites faint ambient noise throughout the planet. Thanks to a recent innovation in seismic imaging, long time sequences of the ambient noise can be used to reveal details about Earth’s interior hundreds of miles inland. The technique, ambient noise tomography, involves looking for correlations between the diffuse seismic surface waves recorded at pairs of closely spaced seismometers and then extracting the group and phase velocities from the correlated signals. By tapping high-frequency wave components, which are typically lost to scattering and attenuation in signals from distant earthquakes but strong in ambient noise, seismologists can probe the roughly 30-km-thick continental crust, whose rheology differs from that of the deeper mantle. Using the technique, Michael Ritzwoller and his colleagues at the University of Colorado, Boulder, have now found evidence for widespread crustal deformation in the western US, a region long thought to have suffered strain from extension ever since the Cenozoic era began 65 million years ago. Ritzwoller’s team processed waveforms captured between 2004 and 2007 by the USArray Transportable Array, which comprises some 400 broadband seismometers on a 70-km² grid, and extracted the radial anisotropy…the differences in speed of vertically and horizontally polarized surface waves, plotted here. The anisotropy, a proxy for strain in crustal rock, is consistent with the lattice orientation of minerals found in surface outcrops and constrains models of how continents are built. (M. P. Moschetti et al., Nature 464 , 885, 2010 http://dx.doi.org/doi:10.1038/nature08951 .)