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要旨:
P,•
phases
are
observed
along
many
refraction
seismic
profiles
and
are
common
in earthquake
records.
Their
velocities
usually
range
from
7.8
to 8.2
km
s-t.
Classical
ray
theory
used
to
interpret
these
observations
implies
a positive
upper
mantle
velocity
gradient.
However,
a wide
spread
positive
velocity
gradient
in
the
lithospheric
mantle
is not
expected
from
petrological
and
petrophysical
data.
Laboratory
velocity
measurements
a.t
elevated
temparatures
and
pressures
suggest
positive
velocity
gradients
only
for
very
low
heat
flow
values
(_< 40 mW
m-2).
Higher
heat
flow
causes
negative
gradients.
Consequently,
petrological
models
of
the
upper
mantle
would
restrict
Pn
observations
to
Precambrian
shields
and
old
platforms,
contrary
to
observations.
We
overcome
this
contradiction
by
considering
media
that
contain
random
velocity
fluctuations
superimposed
on
positive
or
negative
velocity
gradients.
In
both
cases,
these
structures
generate
Pn
phases
by
wide-angle
scattered
waves.
Short-wavelength
random
velocity
fluctuations
of
only
0.5-1%
superimposed
on
negative
velocity
gradients
are
sufficient
for
generating
Pn
phases.
Consequently,
this
implies
that
an
observed
Pn
wave
does
not
necessitate
a positive
upper
mantle
velocity
gradient.
For
a peridotitic
upper
mantle,
fluctuations
of
this
size
can
be
explained
by
slightly
varying
the
relative
proportions
of
its
minere[logical
constituents.
Anisotropy
is likely
to
contribute
to
the
inferred
fluctuations.
