Abstract: We collected 269 Pb seismic phases from 114 earthquakes recorded by Liaoning seismic network from 2012 to 2019. The results show that: The period of Pb is less than that of Pn and the amplitude is between the ones of Pn and Pg, and Pb seismic phase shows jagged shape and much burr. Using the modified velocity model of the Liaoning area to calculate the theoretical travel time of P-wave, we found that: when the focal depth is deeper than 11 km and the range of the epicenter distance is between 80 and 120 km, the Pb can be found as first phase. When the focal depth is less than 11 km, the Pb gradually arrives as the subsequent seismic phase. The band of Pb is wider than the one of Pn, and higher frequence energy is stronger than the one of Pn. The bandwidth and higher frequence energy of Pb and Pg are similar to Pg. The velocity of Pb is 6.9082 km/s by polynomial fitting, the average depth of Conrad interface is 23 km by using the travel-time equation inversion. The distribution of Pb ray path proves that the Conrad interface is continuously distributed in the northern part of Bohai Bay and Liaoning province area.

Key words: Pb seismic phase, Conrad interface, Travel time equation, Theoretical travel time, Time-frequency analysis