Evidence for a sharp increase in P-wave velocity at about 770 km depth |
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| Authors: | Ram Datt KJ Muirhead |
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| Institution: | Research School of Earth Sciences, Australian National University, Canberra, A.C.T. Australia |
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| Abstract: | Slowness data from earthquakes in the Mindanao and Philippine regions recorded at the Warramunga array indicate a small, but abrupt, decrease in dT/dΔ 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 dT/dΔ 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/dΔ observed between 32–34°. |
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