The lateral variation of Pn velocity gradient under Eurasia

Mantle lid P wave velocity gradient, or Pn velocity gradient, reflects the depth and lateral variations of thermal and rheological state of the uppermost mantle. Mapping the Pn velocity gradient and its lateral variation helps us gain insight into the temperature, composition, and dynamics of the uppermost mantle. In addition, because Pn velocity gradient has profound influence on Pn propagation behavior, an accurate mapping of Pn velocity gradient also improves the modeling and prediction of Pn travel times and amplitudes. In this study, I used measured Pn travel times to derive path-specific Pn velocity gradients. I then inverted these velocity gradients for two-dimensional (2-D) Pn velocity-gradient models for Eurasia based on the assumption that a path-specific Pn velocity gradient is the mean of laterally varying Pn velocity gradients along the Pn path. Result from a Monte Carlo simulation indicates that the assumption is appropriate. The 2-D velocity-gradient models show that most of Eurasia has positive velocity gradients. High velocity gradients exist mainly in tectonically active regions. Most tectonically stable regions show low and more uniform velocity gradients. Strong velocity-gradient variations occur largely along convergent plate boundaries, particularly under overriding plates.