On crustal structure deduced from seismic time-distance curves

Abstract

The numerical computation of the time-distance curves for several quite different velocity-depth functions shows that first arrivals are not sufficient to determine the velocity-depth function. Minor second-order fluctuations in velocity superimposed on a general velocity gradient in the crust give a travel-time curve with multiple arrivals separated by fractions of a second. These initial bursts of multiple arrivals are observed in explosion seismology for shot-point ranges out to 450 km. With observed seismic data the position and amplitudes of the late arrivals and especially cusp points determine the general details of the crustal structure.

Gradients of travel‐time curves from deep‐focus earthquakes

“…The method has been successfully used by a number of seismologists includingDowling and Nuttli [1965].An alternative mathematical procedure is to adopt a particular law of velocity variation of simple type in each shell. Such methods have been developed in geophysical prospecting Green and Steinhart [1962]. have described acom-purer program which assigns a linear velocity gradient in each shell.Bullen [1963, p. 112] introduces the parameter a -2dlog r/dlog 7.…”