Exploration of the low-velocity layer within the lithosphere beneath the eastern Qinghai-Xizang Plateau using the P wave shadow zone

This paper analyzed the first arrivals of vertical P waveforms of the Pu'er M_S6.1 earthquake in Yunnan Province on October 7, 2014 recorded by 35 broadband digital seismic instruments that belong to seismic networks of Xizang (Tibet) and its surrounding areas. The result shows that amplitudes of P wave onsets are relatively weak in the epicentral distance of 5°-18°, which is consis-tent with the characteristics of shadow zone. Therefore, it is deduced that the low-velocity layer within lithosphere beneath the Qinghai-Xizang Plateau might be the most possible reason for the shadow zone. Based on the numerical tests of trial and error method and comparison of the observed amplitudes with synthetic ones of P wave onsets predicted by the reflectivity method, the model with a low-velocity layer of 24 km thick at the depth about 78 km can well explain the observations, and the velocity gradient within the low-velocity layer is -0.05/s.