Characteristics of seismic phase Pb and the Conrad interface beneath the Capital Circle region around Beijing

Seismic phase Pb is an important one in near events. Identifying this phase and summarizing its characteristics are important for enriching phase catalogs of regional networks, improving earthquake location accuracy and studying the Conrad interface. Due to many difficulties in identifying the phase, few studies have focused on this issue. This work employed waveform data from 369 M-L >= 2.5 local earthquakes between 2009 and 2015 recorded by the high density seismic network in the Capital Circle region around Beijing to identify 1153 Pb phases. We used a series of techniques to analyze these data, including time-frequency analysis, polynomial fitting, ray analysis, least square fitting, joint inversion, and theoretical travel time calculation and study the time-frequency features of the Pb phase and its propagating speed, and to infer the distribution of the Conrad interface. Combining with previous studies, the results show that in the study area, Pb arrivals that can be easily recognized generally have larger amplitudes and more high-frequency content than the first arrival Pn or Pg phases. There are also some cases in which the Pb phase has relatively weak amplitudes and no visible frequency changes, which can be attributed to differences in the focal mechanism, station azimuth, site response, and instrument type. Through time-frequency analysis, power spectral density and visual identification, we find that in the study area, the main energy of P-waves is concentrated in relatively low frequencies, including four main phases Pn, Pb, Pg and PmP (which are the only phases considered) share this common feature, with small variations due to different propagation paths. When Pg is the first arrival, the dominant low frequency content in the Pb phase is similar to that of the Pg phase (likely due to contamination of Pg phase), Pb, however, has relatively higher frequencies, and comes after Pg but before PmP. When Pn is the first arrival, its dominant low frequency content has broader bandwidth than the Pb, while Pb has higher frequencies and comes after Pn but before Pg. Given that the first appearance of Pb is highly influenced by source depth, and which has the least accuracy in source solutions, we suggest that for epicenter distances between 80 and 140 km, regional variations in the crust thickness, epicentral distance, source depth, and waveform characteristics must be combined to determine whether the Pb phase is the first arrival or not. The propagation speed of the Pb phase at the Conrad interface is about 7. 0 km . s (1), and the average depth of the Conrad interface is about 23 km. The distribution of the Pb ray paths suggests a continuous distribution of the Conrad interface in the study area. The theoretical travel times, calculated using the velocity model determined in a joint travel time inversion of Pg, Pb, Pn, and PmP phases, agree well with the actual observations, suggesting that our results are robust.