Constraints on the viscosity of the continental crust and mantle from GPS measurements and postseismic deformation models in Western Mongolia

M. Vergnolle , F.F. Pollitz , E. Calais (1)

1 GEOAZUR 6526 - Géoazur

We use GPS measurements and models of postseismic deformation caused by seven M6.8 to 8.4 earthquakes that occurred in the past 100 years in Mongolia to assess the viscosity of the lower crust and upper mantle. We find an upper mantle viscosity between 1 Â 10 18 and 4 Â 10 18 Pa s. The presence of such a weak mantle is consistent with results from independent seismological and petrological studies that show an abnormally hot upper mantle beneath Mongolia. The viscosity of the lower crust is less well constrained, but a weak lower crust (3 Â 10 16 to 2 Â 10 17 Pa s) is preferred by the data. Using our best fit upper mantle and lower crust viscosities, we find that the postseismic effects of viscoelastic relaxation on present-day horizontal GPS velocities are small (<2 mm yr À1) but still persist 100 years after the 1905, M8.4, Bolnay earthquake. This study shows that the GPS velocity field in the Baikal-Mongolia area can be modeled as the sum of (1) a rigid translation and rotation of the whole network, (2) a 3-5 mm yr À1 simple shear velocity gradient between the Siberian platform to the north and northern China to the south, and (3) the contribution of postseismic deformation, mostly caused by the 1905 Bolnay-Tsetserleg sequence and by the smaller, but more recent, 1957 Bogd earthquake.

Mots clés: seismotectonics; postseismic; viscoelastic; GPS; crust-lithosphere rheology; Mongolia

Domaines: Planète et Univers [physics]

M. Vergnolle, F.F. Pollitz, E. Calais. Constraints on the viscosity of the continental crust and mantle from GPS measurements and postseismic deformation models in Western Mongolia. Journal of Geophysical Research, 2003, 108 (B10), pp.2502. ⟨10.1029/2002JB002374⟩. ⟨hal-00407056⟩