Crustal contributions in teleseismic P-wave travel times and their effect on Tomographic models below India and Tibet

Abstract:

The variations in crustal thickness and topography may affect the travel time anomalies and hence attribute significant contributions in the travel time delays into mantle. The delayed arrivals in Tibet and Himalaya are expected due to the thick crust and low average crustal velocities whereas the arrivals are early in the regions of Indian shield compared to IASP 91 and need significant crustal correction term to decipher an exact tomographic model of the whole region. As the near vertical arrivals of teleseismic P waves (distance range 30-90⁰) make it difficult to resolve crustal scale features for their inability to cross at crustal depths, station terms along with crustal corrections can be used to incorporate the effects of unresolved features of crust and upper mantle and to avoid the mapping of crustal travel time anomalies into the mantle. Keeping in view that these corrections using 3D models are as good as their resolution, we prefer CRUST1.0 over CRUST 2.0 to modify IASP91 for the crustal part and the modified velocity model has been used to obtain the travel time delays which may happen due to changes in velocity and thickness. The corrections are obtained for each event-station pair (ray or ray path) used in tomography and the times are subtracted from the input delay time vectors prior to inversion to prepare a modified tomographic model.