Abstract
Abstract Slowness data from earthquakes in the Mindanao and Philippine regions recorded at the Warramunga array indicate a small, but abrupt, decrease in d T /d Δ at a distance of 29.5°. There is evidence also of a triplication in the P travel-time curve at about this distance. These data strongly suggest the presence of a rapid or discontinuous velocity increase of about 2% in P-wave velocity at a depth of about 770 km. Such a velocity increase is consistent with the occurrence of more than one phase change between 600 and 900 km, as predicted by the pyrolite model of Ringwood. Previous observations of increasing d T /d Δ with distance may have resulted from the predominance of the 650-km branch as it approaches its cusp. If so, then it is not necessary to invoke a decrease in velocity with depth near 800 km to explain the increase in mdT /d Δ observed between 32–34°.
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