Structure of the Continental Lithosphere Beneath the United States Viewed Through the Lens of Seismic Attenuation

Abstract:

Over the last decade, we have gained invaluable insight into the architecture of the continental lithosphere beneath the United States thanks to EarthScope. Much of what we have learned has come from imaging of seismic velocity (through tomography of various kinds), or seismic velocity discontinuities (through receiver function analysis). Seismic attenuation provides complementary information but has not yet been explored to the same extent as other seismic properties. Seismic attenuation, the loss of energy during wave propagation due to anelasticity, may be especially relevant to questions of lithospheric strength. This is because the same mechanisms involved in the transient deformation that produces anelasticity are implicated in steady-state creep. We analyze waveforms from deep earthquakes recorded by the EarthScope Transportable Array using a time-domain waveform matching approach to measure differential attenuation across the United States. Although we measure a path-integrated effect, we argue our results are most sensitive to variations in the asthenosphere-lithosphere system, given the large contrast in quality factor between the lithosphere and asthenosphere. Our results are generally consistent with expectations in that the core of the continent shows lower attenuation than the edges, with attenuation being greater -to first order- west of the Rockies and east of the Appalachians than in the continental interior. However, significant smaller length scale variations are superimposed on that broad pattern. Most intriguingly, attenuation in the Superior Province is somewhat higher than average, when we would have expected it to be very low. Local lows in seismic attenuation are observed in the core of the Colorado Plateau, matching expectations, but also in eastern South Carolina and southern Florida. With regards to active deformation, the southern intermountain seismic belt coincides with a high gradient in the attenuation signal, but even larger gradients further inland do not show a similar association with seismicity. In this presentation we will explore the attenuation results, their robustness, how they compare to other geophysical observables and their relationship to intracontinental seismicity and deformation.