Test of the upper mantle low velocity layer in Siberia with surface waves

The existence of the upper mantle low velocity layer (LVL) below 100 km depth in cratonic areas is tested with surface waves dispersion curves. Given the ambient noise we find that a pronounced LVL (80 km thick and 2% velocity reduction or 40 km thick and 5% velocity reduction) can be distinguished from a constant velocity model by comparison of the fundamental mode group velocities, whereas a thin LVL (less than 40 km thick) with small velocity contrast (less than 2%) cannot be resolved. The fundamental modes of Love and Rayleigh waves have similar properties and, in general, the phase velocity differences are smaller than the standard error. Phase velocity alone cannot discriminate between the models, and the group velocity is in general more sensitive to the velocity structure than the phase velocity. The higher modes at short periods could potentially determine a LVL but in reality it is difficult to obtain sufficiently accurate measurements. We invert the synthetic dispersion curves by the non-linear Hedgehog inversion method. A pronounced LVL (more than 40 km thick and with a strong velocity contrast of about 5%) is detectable by the non-linear inversion but for a thin LVL with a strong velocity contrast it is not possible to resolve both velocity and thickness. In the inversions all solutions include a LVL for models with a pronounced LVL, whereas the solution space includes models with and without a LVL for models with a zero or positive gradient velocity-depth structure. We invert also real data with travel path across the Siberian craton with the Hedgehog method. Almost all solutions include a LVL in the depth range of 80-150 km with a velocity contrast up to 2% to the surrounding intervals. Hence, the LVL appears to be a common feature of the Siberian upper mantle, although a constant velocity at the same depth range cannot be totally excluded. Despite low resolution at large depth, a pronounced asthenospheric LVL below a depth of about 225 km is a constant characteristic of the set of solutions.