Three patches of low-velocity zone above high-velocity D'' layer

We analyze shear velocity structure of the lowermost mantle above the D'' layer globally based on waveform modeling. While the waveforms of the seismic waves appear normal for most of the sampling paths, anomalously broadened waveforms are clearly observed at the epicentral distances ranging from 90° to 100° from the seismic waves sampling the lowermost mantle beneath Kamchatka, north of Iceland and west of Hawaii. Those anomalous waveforms are observed for the seismic events occurring in Mindanao of Philippines, Hindu Kush region and Solomon Islands and recorded in Alaska and Canada, Europe and North America. Waveform modeling analysis suggests that the anomalous waveforms can be explained by a 460 - 670 km thick low shear velocity zone with a velocity reduction of about -2% situated above a high-velocity D'' layer. The bottom high-velocity D'' layer has a shear velocity jump of 1.5-2.5% at 180 -240 km above the core-mantle boundary. The low-velocity zone could be geographically offset with the bottom D'' layer, while still consistent with the seismic data. We will further explore the relationship between these thick low shear velocity zones, the bottom D'' layer and the African and Pacific Anomalies.