The relationship between shear wave velocity, temperature, attenuation and viscosity in the shallow part of the mantle

• A VsVs model is obtained from Rayleigh waves from 3×1063×106 earthquakes.
• The model has resolutions of ∼300×∼30 km∼300×∼30 km horizontally and vertically.
• Temperature, viscosity and attenuation are calculated from VsVs.
• The activation energy and volume of the viscosity agree with laboratory values.
• Active Asiatic mountain belts have lithospheric thicknesses of 200 km or more.

Surface wave tomography, using the fundamental Rayleigh wave velocities and those of higher modes between 1 and 4 and periods between 50 and 160 s, is used to image structures with a horizontal resolution of ∼250 km and a vertical resolution of ∼50 km to depths of ∼300 km in the mantle. A new model, PM_v2_2012, obtained from 3×1063×106 seismograms, agrees well with earlier lower resolution models. It is combined with temperature estimates from oceanic plate models and with pressure and temperature estimates from the mineral compositions of garnet peridotite nodules to generate a number of estimates of SV(P,T)SV(P,T) based on geophysical and petrological observations alone. These are then used to estimate the unrelaxed shear modulus and its derivatives with respect to pressure and temperature, which agree reasonably with values from laboratory experiments. At high temperatures relaxation occurs, causing the shear wave velocity to depend on frequency. This behaviour is parameterised using a viscosity to obtain a Maxwell relaxation time. The relaxation behaviour is described using a dimensionless frequency, which depends on an activation energy E   and volume VaVa. The values of E   and VaVa obtained from the geophysical models agree with those from laboratory experiments on high temperature creep. The resulting expressions are then used to determine the lithospheric thickness from the shear wave velocity variations. The resolution is improved by about a factor of two with respect to earlier models, and clearly resolves the thick lithosphere beneath active intracontinental belts that are now being shortened. The same expressions allow the three dimensional variations of the shear wave attenuation and viscosity to be estimated.