Geoscience Reference
In-Depth Information
(a)
(c)
D
C
b
G
E
B
A
v
1
a
F
v
2
v
1
v
1
<
v
2
v
2
v
1
v
2
>
(b)
(d)
A
F
B
C
E
b
D
a
G
Distance
Distance
Figure 8.2.
The 'shadow zone' resulting from a low-velocity zone. As an example,
consider a two-layered sphere for which the seismic velocity increases gradually
with depth in each layer. The seismic velocity immediately above the discontinuity in
the upper layer is
V
1
and that immediately below the discontinuity is
V
2
. The ray
paths for the case
V
2
>V
1
(the velocity increases at the discontinuity) are shown in
(a). If
V
2
< V
1
(the velocity decreases at the discontinuity, resulting in a low-velocity
zone at the top of the second layer), then the ray refracted into the inner layer bends
towards the normal (Snell's law), yielding the ray paths shown in (c). The
travel-time-distance curves for (a) and (c) are shown in (b) and (d), respectively.
When
V
2
>
V
1
, there is a
distance interval over the shadow zone. The angular extent of the shadow zone (b to
B) and the corresponding delay in travel time (b to B) are dependent on the depth
and extent of the low-velocity zone and on the reduction of velocity in the
low-velocity zone. (After Gutenberg (1959).)
V
1
, arrivals are recorded at all distances, but when
V
2
<
These increases have been verified independently by computing synthetic seis-
mograms to match earthquake and nuclear-explosion amplitudes and waveforms.
Earthquake activity in subduction zones ceases at about 670 km depth, and this
depth is also commonly taken as the boundary between upper and lower mantle.
Global maps of topography on this discontinuity between upper and lower mantle
reveal variations of up to 30 km. The depressions in this discontinuity seem to
be correlated with subduction zones, suggesting that it provides some impedi-
ment to the continuation of subduction into the lower mantle. The entire region
between 400 and 670 km depth is often called the
mantle transition zone
. The
