Regional variations of Q in the crust and upper mantle

Brian J. Mitchell

Department of Earth & Atm. Sci., Saint Louis University, St. Louis, MO 63103


Shear-wave Q in the crust and upper mantle varies dramatically with depth and also from region to region. Q in the upper crust of most continental regions exhibits large regional variations (~50-1000 at 1 Hz and ~50-300 at 0.01- 0.05 Hz), with values in regions having undergone recent tectonic activity typically being much lower those in stable shields. Q in the uppermost portion of the lower crust varies less dramatically and is much higher in most regions than it is in the upper crust. At greater depths in the lower crust Q decreases with increasing depth and continues to decrease into the upper mantle. The rate of decrease is typically greater in tectonically active regions than in stable regions.

Exceptions to "typical" values occur in regions of continental collision where Q in the lower crust is similar to that in the upper crust, and in stable regions of major doming and uplift since the Mesozoic where Q is much lower than in other stable regions where those effects did not occur.

Shear-wave Q in the lithosphere and asthenosphere of oceanic regions increases with increasing lithospheric age. Lowest values (with large uncertainties) occur at oceanic ridges and highest values occur beneath old oceanic regions.

There is a clear relationship between upper crustal Q in continents and the time that has elapsed in any region since either the most recent compressional tectonic acitivity or major epeirogenic uplift. These relationships between Q and time since the occurrence of tectonic activity or uplift should allow us to make educated guesses about the variation of Q with depth in regions where no data for determining Q have as yet been obtained.

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