Geoscience Reference
In-Depth Information
Example: seismic velocity and width of a low-velocity zone
The velocity of the lower crust in a model of the East Pacific Rise at 12
◦
Nis
7kms
−
1
. Calculate the seismic P-wave velocity of a lower-crustal magma chamber
if ray paths crossing the axis are delayed by 0.1 s and the width of the magma
chamber is assumed to be (a) 1 km, (b) 3 km and (c) 6 km.
If
w
is the width of the chamber, the travel time in normal lower-crustal material
is
w
/
7.0 s. Ray paths crossing the axis are 0.1 s slower than this, so their travel time
is 0.1
+
w
/
7.0 s. If the seismic velocity in the magma chamber is
a
km s
−
1
, then
w
α
w
7
.
0
Rearranging this equation gives the seismic velocity in the chamber as
=
0
.
1
+
7
w
w
+
0
.
7
α
=
Therefore, (a) for
w
=
1 km,
a
=
4.12 km s
−
1
; (b) for
w
=
3 km,
a
=
5.68 km s
−
1
;
and (c) for
w
=
6 km,
a
=
6.27 km s
−
1
.
(9
◦
N, 1.3-1.6 km; Valu Fa Ridge,
3 km; Reykjanes Ridge, 2-3 km). It seems
that the depth and accumulation of melt within the crust is controlled by a balance
between the magmatic heat input and the loss of heat by hydrothermal cooling:
the melt lens is likely to be shallow on fast-spreading ridges and deep on slow-
spreading ridges. At 9
◦
N and 13
◦
S the Moho reflection extends laterally to within
2-3 km of the ridge axis, but determining exactly what happens beneath these
low-velocity axial bodies is very difficult.
Several thousands of kilometres of common-depth-point seismic-reflection
data have been obtained from the axial region of the East Pacific Rise in the
search for the elusive crustal magma chamber. Data from a reflection line that
crossed the ridge axis are shown in Fig. 9.26(a). A large-amplitude reflector about
3km wide and centred on the ridge axis can be seen at about 0.6 s beneath the
seabed. This reflector, which is continuous beneath the ridge axis for many tens
of kilometres (Fig. 9.26(b)), has negative polarity, indicating that it arises at an
interface with a strong negative velocity contrast. All these data are consistent
with this arrival being a reflection from the lid of a magma chamber. The magma-
chamber reflection was observed on about 60% of the 3500 km of reflection profile
of this particular survey. The depth to the top of the magma chamber varies
between about 1.2 and 2.4 km beneath the seabed and can be correlated with the
depth of the ridge axis. Where the ridge axis is shallowest, the magma-chamber
lid is shallowest; where it is deepest, the magma chamber is discontinuous or
nonexistent, which shows the effect of magma supply on the axial topography.
The presence of a magma chamber is also indicated by three expanding-spread
reflection profiles, which were shot parallel to the East Pacific Rise at 13
◦
N
(Fig. 9.27). The profile shot along the ridge axis (ESP 9) shows a pronounced
∼
