PKP phases observed in Japan show strong precursors with clear onsets for earthquakes that occurred in south America. The onset times of the PKP precursors can all be explained by existence of seismic scatterers in the lowermost 100 km of the mantle west of Mexico, within an area of 200 km¿300 km. Forward modeling of PKP precursor amplitudes indicates that a P-wave velocity variation of at least 6% is required to explain the large amplitudes of the observed PKP precursors, suggesting that the core-mantle boundary region beneath west of Mexico is a highly anomalous region characterized by strong seismic scatterers. ¿ 2001 American Geophysical Union