La « discontinuità 20° »

Abstract

In reference to some circumstances ascertained in a
recent study of the seismic novement that occurred in Sicily on the 23rd
December 1959, two a t t e m p t s have been carried out in order to determine
the depth of the discontinuity surface t h a t we suppose to be the cause of
anomalous behaviour observed in t h e travel-time curve of P-waves near 20°
I n the first part some possible q u a n t i t a t i v e consequences deriving from
t h e observed disagreement with Jeffreys-Bullen travel-time curves have been
characterized, on the assumption that the velocity of P-waves resumes
ordinary behaviour just below the Moliorovicic discontinuity. On this
assumption, a probable velocity law has been determined t h a t presents a
remarkable discontinuity at a depth of 586 km.
I n the second part it has been demonstrated t h a t , assuming the numerical
values of velocity proposed by Gutenberg in the range 40 <= h <= 80 kin, the deduced equations of the two branches of P-waves travel-time
curve give a distribution of velocity values as function of depth t h a t presents
a marked discontinuity at the depth of 536 km, immediately followed
by a marked increase in the rate of increase of velocity with increase of
depth.