P wave travel time residuals for local intermediate and deep focus earthquakes are studied to infer a velocity model of the asthenosphere beneath the southern Okhotsk Sea and the western Japan Sea. Tectonically, both regions belong to plate subduction zones. Large positive travel time anomalies are observed at stations above the deepest earthquakes. Consideration of source, path, and receiver effects leads to a conclusion that within the asthenosphere there is a lower velocity region below about 300 km depth with velocities about 5-10% below the Jeffreys-Bullen velocity model.