Anomalously broadened P waveforms of deep focus events were observed at some regional stations after sampling flattened high velocity zones. The source processes of these events are short and the P waveforms at other stations as well as the corresponding SH waves do not show such anomaly. The rays of the anomalous waves propagated in the vicinity of those whose waveforms can be synthesized with a relatively simple model M3.11 or M2.0 for a flattened cold slab in the transition zone. We suggest that the P waveform broadening is caused by SV converted waves at the heterogeneities within a narrow zone near the "660 km" discontinuity or slab boundary. The creation of such heterogeneities may involve in the complex structure of phase transformation with water and cold temperature anomaly associated with stagnant slab.