Weak mantle in NW India probed by geodetic measurements following the 2001 Bhuj earthquake

Far-reaching transient surface deformation following the 2001 M-w 7.6 Bhuj intraplate earthquake in NW India reveals visco-elastic flow in the mantle with only modest contributions from crustal relaxation processes. The relatively rapid decay of GPS-measured deformation rates indicates increasing effective viscosities of the mantle from 3 x 10(18) Pa s in the first 6 months to 2 x 10(19) Pa s during the 6-year observation period, consistent with a time and stress-dependent rheology, such as power-law flow by dislocation creep. The observed data do not require relaxation of the lower crust over these time scales and indicate a lower bound of 10(20) Pa s on its effective viscosity. The unusually low viscous strength of the mantle below the earthquake epicentral region may be the long-lasting result of thermal weakening by the late Cretaceous Deccan plume and may be responsible for the unusually active intraplate seismicity in the region. (C) 2009 Elsevier B.V. All rights reserved.