Frederick Pollitz 2019 <p><span>In tectonically active regions, post-earthquake motions are generally shaped by a combination of continued fault slippage (afterslip) on a timescale of days to months and viscoelastic relaxation of the&nbsp;lower crust&nbsp;and&nbsp;upper mantle&nbsp;on a timescale of days to years. Transient crustal motions have been observed following numerous magnitude &gt;~7 earthquakes in various&nbsp;tectonic settings: continental&nbsp;rift zones&nbsp;(Basin and Range), continental&nbsp;plate boundary&nbsp;zones (San Andreas fault corridor; Alaska; Turkey),&nbsp;subduction zones&nbsp;(Japan, Chile, Sumatra), ongoing&nbsp;continental collision&nbsp;zones (Arabia; Tibet), and mid-ocean rifting zones (Iceland). When afterslip can be discriminated from viscoelastic relaxation and when temporal coverage of the postseismic measurements is broad (i.e.,&nbsp;geodetic surveys&nbsp;of at least several years duration are available), a wide spectrum of relaxation timescales are usually identified. Current temporal resolution and modeling approaches (e.g., Burgers body analog) allow identification of transient (Kelvin) and steady-state (Maxwell) viscosities that are operable in the short-term and long-term, respectively. I compile results from 40 studies of post-earthquake motions, augmented by ten studies of contemporary surface loading or unloading, that illuminate current estimates of transient and steady-state viscosity of the lower crust and/or uppermost mantle. Lower crust viscosity estimates range from&nbsp;</span><i>~</i><span>10</span>¹⁸<span>&nbsp;to 10</span>²¹<span>&nbsp;Pa s, with most estimates near the upper end except in areas of overthickened crust. Mantle lithosphere and&nbsp;asthenosphere&nbsp;viscosity estimates are particularly abundant and yield a picture of transient viscosity ranging from ~10</span>¹⁶<span>&nbsp;to 10</span>¹⁹<span>&nbsp;Pa s and steady-state viscosity ranging from ~10</span>¹⁸<span>&nbsp;to 10</span>²¹<span> Pa s. To first order, both transient and steady-state viscosities are well correlated with regional heat flow, and steady-state viscosities are comparable with temperature and strain-rate dependent rock viscosities from laboratory-based flow laws.</span></p> application/pdf 10.1016/j.pepi.2019.106271 en Elsevier Lithosphere and shallow asthenosphere rheology from observations of post-earthquake relaxation article