SCATTER AND BIAS IN DIFFERENTIAL PKP TRAVEL-TIMES AND IMPLICATIONS FOR MANTLE AND CORE PHENOMENA

Differential travel times, commonly used to eliminate unwanted near-so urce and near-receiver heterogeneity from travel time studies, can be affected by slabs and hypocentral errors when phase pair slownesses di ffer greatly. In particular, D'' and core structure studies can be bia sed if differential PKP times are used. Here we present differential P KP travel time measurements having significant azimuthal travel time a nomalies consistent both in size and pattern with a near-source slab e ffect and explore other sources of error in differential times.