Abstract
The lower boundary of the lithospheric plates has remained as an enigmatic
boundary for seismologists, since it is relatively poorly observed
by seismic means. There is traditionally a broad consensus that the
asthenosphere is observable as a low velocity zone by seismic surface
waves. Seismic techniques which use shorter period P-to-S or S-to-P
converted body waves are now far enough developed to be successful
in observing such a low velocity zone with a higher resolution. The
principle of this technique (the so-called receiver function technique)
is that a strong teleseismic mother phase (e.g. P, S, PP or SKS)
incident from below on any seismic discontinuity beneath a station
produces a converted phase (Ps or Sp) which indicates its depth and
properties. We discuss details of this technique. A sufficient number
of such observations exist already to indicate that the top of the
low velocity zone is a globally observable discontinuity and it is
sharper than previously thought. An intriguing observation is that
in some cratons the new seismic data indicate that the low velocity
zone exists already at shallower depths than obtained from surface
waves. This confirms earlier results from controlled source observations
(Thybo and Perchuc, 1997). We discuss possible interpretations of
this shallow low velocity zone in cratonic regions.
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