Geodetic Constraints on Mantle Q at Periods from a Fortnight to 18.6 Years

Seismic observations have provided numerous constraints on the earth's spherically-averaged mantle anelasticity at periods of tens of minutes and shorter. Meanwhile, post-glacial rebound and other geodynamic studies provide information at periods of a few thousand years and longer. However, constraints at intermediate periods are scarce. Such constraints could be useful in trying to connect the seismic anelastic models with the longer-period visco-elastic behavior. Here we will describe constraints on anelasticity in this intermediate range of periods as obtained from earth tide and earth rotation observations. We discuss results from: (1) VLBI observations of the monthly and fortnightly tidal variations in rotation rate; (2) satellite laser ranging observations of the 18.6-year tidal variations in the earth's gravitational field; and (3) astrometric and geodetic observations of the 14-month Chandler Wobble period and damping. We find that these observations are consistent with a nearly frequency-independent mantle Q stretching from seismic periods all the way out to the 14-month Chandler Wobble period; but that Q appears to decrease significantly between 14-months and 18.6-years.