Bimodality of P-wave travel time residuals found in the CTBTO seismological data (REB-bulletins)

REB-bulletins of CTBTO from the period 2000-2023 were used for simple statistical evaluation of the travel time residuals of P-waves detected at the epicentral distance from 30° to 90° arrived from the seismic events of magnitude 5 or greater, with hypocentre no deeper than 50 km. While histograms of time residuals calculated for individual stations of the International Monitoring System CTBTO show expected unimodal (Gaussian) distribution, curves expressing distribution of the arithmetic means and medians calculated from data of these individual stations are bimodal. This bimodality can be explained by the sum of two Gaussian curves related to groups of “fast” and “slow” stations, with a modus difference of about 0.4 to 0.5 seconds. Although discussed bimodality is best visible on residuals related to phases detected at the epicentral distance 70° to 90°, it is evident that also curves related to the nearest evaluated phases (from 30° to 50°) are the sum of two Gaussian curves. So, the reason for bimodality should be sought in the upper mantle or crust, which is consistent with the assumption of “two kinds” of the continental lithosphere (Poupinet et al., 2003). The distribution of “fast” and “slow” stations on the map corresponds with published results of global seismic tomography studies.