Later earthquake-sourced PmP phases have the potential to significantly improve ray coverage and resolution of crustal tomography methods,as their trajectories are quite different from those of shallower P phases.This paper analyzes the characteristics of later PmP arrival times from earthquakes with different focal depths.The results show that PmP arrival time differences from earthquakes at a range of focal depths are gradually lowered with increasing offset.We found that where the first recorded P-wave phase was the intra-crustal refraction phase(Pg),the differences in arrival time between Pg and PmP phases decreased with increasing focal depth at an offset of less than 120 km.Where the first P-wave phase is the upper mantle refraction phase(Pn),the difference in arrival times between Pn and PmP phases became larger with an increase in focal depth at an offset of more than 150 km.A total of 394 PmP phases and 3356 first P phases were picked from seismograms in the active volcanic area of northeastern Japan,according to the characteristics of calculated arrival times,amplitudes and particle motions.These were used to investigate the role of PmP phases in crustal tomography beneath an active volcanic region.Results of the detailed resolution analysis show that the addition of PmP data can improve significantly the resolution of the lower crustal structure in tomographic images.After the PmP data were included in the tomographic inversion,the path of upwelling magma,along which a series of low-frequency microearthquakes is clearly distributed,was better imaged.These results suggest that the PmP phase has an important role in detailed crustal tomography.
