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Upper mantle velocities determined from local observations of deep earthquakes

SEISMOLOGICAL OBSERVATORY GEOPHYSICS DIVISION DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH, P.O. BOX 8005, WELLINGTON, New Zealand

Abstract

A method for determining upper mantle velocities is presented. Observations of deep earthquakes at small epicentral distances enable standard travel-time tables to be modified for regional studies. This is done by calculating the ratio of the actual travel time to that given in the tables, for a range of focal depths. Local velocity models may then be determined.

The Jeffreys-Bullen travel times have been modified to suit the New Zealand region. Strong evidence is presented for velocities higher than the Jeffreys-Bullen values at depths less than about 160 km, and below that a low-velocity zone extending to a depth of at least 300 km.

Footnotes

^* Present address: the Seismographic Station, University of California, Berkeley, California, 94720.