Nature of travel‐time anomalies at Lasa

The 1968 P-velocity distribution and a representative core model are used to compute relative Lasa subarray travel-time anomalies for over 600 short-period phases processed by the IBM Seismic Array Analysis Center. Observed phase velocities and computed anomalies for core waves are consistent with a core model in which PKP arrivals following the DEF branch between 145° and 160° are identified with an extension of the BC branch of the caustic curve rather than the Bolt GH branch. Reflected and refracted core phases and diffracted P waves incident to Lasa along common ray paths are found to have nearly identical travel-time anomalies. This phenomenon suggests that the anomalies are not significantly affected by lateral variations that may exist in the lower mantle, core, and source region. Hence the source of observed anomalies must lie almost entirely in the crust and upper mantle beneath the array.