Velocity filtering of seismic core phases

Recent studies have proposed complexities in the velocity-depth function for the region surrounding the inner core which require additional branches in the travel time curve for PKP in the epicentral range of 125° to 160°. The proposed PKP arrivals can be separated on the basis of their apparent velocities, which range from 24 km/sec to 100 km/sec. Using the Tonto Forest array in Arizona coupled with adjoining LRSM stations in the western United States, an effective linear array of 400 km in size is attained. Data from several events in the distance range from 130° to 160° recorded on this array have been velocity filtered and show some evidence of two precursors to PKP in the distance range from 135° to 143° and at least one intermediate branch between PKP1 and PKP2 at distances greater than 143°. The results appear to support the velocity solution for the core proposed by Adams and Randall, although more data are required before a conclusive discrimination can be made between competing velocity models.