Crustal Attenuation beneath USArray

Abstract:

Seismic attenuation across the US is estimated using station ML magnitude data from the USArray. Station magnitudes are recalibrated back to amplitude and back projected in two-dimensional tomography. Data represent the amplitudes of the horizontal components of the Lg and Sg phases. Lg in western US shows regions of very high attenuation and contrasts with the lesser attenuation of the eastern US. Individual attenuation anomalies can be clearly tied to regional geology. Station gains show broad regional variations. Most of the high attenuation areas are regions of high geothermal activity; however, scattering from the San Andreas Fault is also imaged. Sediments of the Gulf Coast Plain, Willison Basin, and Michigan Basin do show up as intermediate attenuation while the Illinois Basin, Appalachian Basin, and other basins are not apparent. Sg amplitude imaging in the western US generally agrees with what’s found with Lg; albeit at lower resolution. However, in the east, Sg shows areas of very low Q suggesting that it is much more sensitive to sediments than Lg. In Alaska, Lg attenuation is generally less than the western US, but still much greater than the eastern US. In southeast Alaska, the Wrangell Volcanic Field causes a sizeable high attenuation zone. The volcanic Aleutian Mountains also have high attenuation. However, moderate to high attenuation also correlates with the tertiary sedimentary basins in Alaska. The North Slope Basin does not seem to attenuate. Thicker crust and mountain roots tend to show less attenuation, if anything, but this correspondence is most likely due to differences in temperature and seismic velocity. Heat, scattering, and young sedimentary basins create seismic attenuation in the continental crust.