Lowermost Mantle Velocity Estimations Beneath the Central North Atlantic Area from Pdif Observed at Balkan, East Mediterranean, and American Stations
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Lowermost mantle velocity in the area 15°S–70°N latitude/60°W–5° W longitude is estimated using two groups of observations, complementary to each other. There are 894 Pdif observations at stations in the Balkan and Eastern Mediterranean areas from 15 major earthquakes in Central and South America. Another 218 Pdif observations are associated with four earthquakes in Greece/Turkey and one event in Africa, recorded by American stations. A Pdif slowness tomographic approach of the structures immediately above the core-to-mantle boundary (CMB) is used, incorporating corrections for ellipticity, station elevation and velocity perturbations along the ray path. A low-velocity zone above CMB with a large geographical extent, approximately in the area (35–65°N) × (40–20°W), appears to have the velocity perturbations exceeding the value actually assumed by some global models. Most likely, it is extended beneath western Africa. A high-velocity area is observed west of the low-velocity zone. The results suggest that both Cape Verde and Azorean islands are located near transition areas from low-to-high velocity values in the lowermost mantle.
KeywordsPdif slowness tomography Cape Verde Azores Super-plume root D″ layer Central North Atlantic area
The authors express their deep gratitude to people from IRIS, GEOFON, and ORFEUS DMC for their support in acquiring the input data for this study. GMT software (Wessel and Smith 1996) was used to draw figures. Professor Zhao kindly provided the 3-D global velocity model in a digital format. We thank Satoshi Kaneshima, Pawel Wiejacz, and two anonymous reviewer for valuable comments. Distances have been evaluated with the routine of Brown (1984) incorporated in Herrmann’s Computer Programs in Seismology. Ellipticity corrections have been evaluated with a routine available at RSES, ANU.
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