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

Far-reaching transient surface deformation following the 2001 Mw 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 × 1018 Pa s in the first 6 months to 2 × 1019 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 1020 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.