Geoscience Reference
In-Depth Information
and at lower temperatures occurs at a lower pressure (shallower depth). Thus,
in the subducting plate this phase change is elevated (i.e., occurs closer to the
surface). For the models in Fig. 9.44 the equilibrium phase change is elevated by
over 50 km in the subducting plate. The increase in density associated with this
phase change is about 300 kg m
−
3
. The lower temperature of the subducting plate
means that the spinel-post-spinel phase change occurs there at a higher pressure
(greater depth) than it does in the mantle. This phase change is endothermic
(heat-absorbing). Thus, the downward force on the subducting plate has three
components: that due to the thermal contraction of the plate, that due to the
elevation of the olivine-spinel phase boundary and that due to the depression of
the spinel-post-spinel boundary. The third contribution is of opposite sign to the
first two, acting to impede the descent of the plate, but is smaller in magnitude.
Thermal contraction provides the greatest contribution to the overall driving force.
(See Section 8.2.4 for discussion of the driving forces of plate tectonics.)
(a)
OLIVINE
SPINEL
Pressure
(b)
POST-SPINEL
SPINEL
Pressure
Figure 9.45.
Equilibrium
pressure-temperature
curves (Clapeyron curves)
for the olivine ( and )to
spinel ( ) and the 670-km
spinel to perovskite and
magnesiow ustite (Pv +
Mw) phase changes.
Estimates of the slopes
are 2.0-3.0 MPa K
−1
9.6.3 Seismic activity at subduction zones
Subduction zones are delineated by earthquake activity (see Figs. 2.1 and 2.2)
extending from the surface down to depths of, in some cases, almost 700 km. The
volcanic and seismic
ring of fire
around the margins of the Pacific Ocean is due
to the subduction of oceanic plates (Fig. 2.2). It has been estimated that over 80%
of the total energy at present being released worldwide by earthquakes comes
from earthquakes located in this ring. The remaining 15% of the total energy
is released by earthquakes in the broader seismic belt which extends eastwards
from the Mediterranean and across Asia and includes the Alps, Turkey, Iran and
the Himalayas.
Table 9.6 shows the distribution of earthquake foci with depth for the major
subduction zones. These deeply dipping seismic zones are sometimes termed
Wadati-Benioff zones after the Japanese discoverer of deep earthquakes, Kiyoo
Wadati, and his American successor, Hugo Benioff. Fig. 9.46 shows the shal-
low geometry of the major subduction zones, and Fig. 9.47 shows the shape of
the Tonga-Kermadec subduction zone as defined by earthquake foci. This sub-
duction zone, which extends to over 600 km in depth, is S-shaped in plan view.
Figure 9.48 shows the focal distribution of microearthquakes along a cross sec-
tion perpendicular to the trench axis beneath northeastern Japan. A large amount
of very shallow seismicity is associated with volcanism and shallow deforma-
tion and thrusting. A cluster of low-frequency events (black circles) in the lower
crust/uppermost mantle beneath the volcanic arc seems to be caused by deep mag-
matic activity of mantle diapirs. The deeper foci, however, clearly delineate the
descending lithospheric plate. These foci apparently define two almost parallel
planes (which are also observed beneath the Aleutians): the upper plane is defined
by earthquakes with reverse faulting or down-dip compressional stresses and the
lower plane is defined by earthquakes with down-dip extensional stresses. The
for
the exothermic
olivine-spinel change
and -3
1 MPa K
−1
±
for
the endothermic
spinel-perovskite and
spinel-magnesiow ustite
change.
