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Figure 9.55.
Thermal
parameter plotted against
maximum depth for
subduction- zone
earthquakes. If the
occurrence of deep
earthquakes were directly
controlled by
temperature, their depth
should vary steadily with
the thermal parameter.
The subduction zones fall
into two groups, those
with
0
Next-deepest earthquake
Deepest earthquake
100
200
300
Izu-Bonin N
SE
Sumatra
400
500
Izu-Bonin S
5000 having
deep earthquakes and
those with
φ<
5000 not
having deep earthquakes.
This is consistent with an
abrupt phase change
causing the earthquakes.
(From Kirby
et al
.,
Metastable mantle phase
transformations and deep
earthquakes in subducting
lithosphere,
Rev.
Geophys.
,
34
, 261-306,
1996. Copyright 1996
American Geophysical
Union. Reprinted by
permission of American
Geophysical Union.)
φ>
S. America S
600
Tonga
S. America N
700
0 5000 10000 15000 20000
Thermal parameter
φ
(km)
Figure 9.44 shows the temperature structure and the predicted regions of insta-
bility for two subduction zones, one with a low thermal parameter for which a
metastable olivine wedge would not develop and the other with a high thermal
parameter in which a metastable olivine wedge would be expected to develop.
Table 9.6 and Fig. 9.55 confirm that subduction zones with a thermal parameter
less than 5000 km do not have deep seismicity whereas those with a thermal
parameter greater than 5000 km do. As the kinetics of the transformation of
metastable olivine to spinel is crucially dependent upon temperature, these trans-
formational earthquake focii should occur along the hotter outer edges of the
wedge. While very deep earthquakes in the Tonga and Izu-Bonin subduction
zones (the two subduction zones with the largest thermal parameters) seem to
occur along double seismic zones, other seismic evidence for a metastable olivine
wedge is hard to obtain. Since olivine is less dense and has a lower seismic veloc-
ity than spinel at the same temperature, a metastable olivine wedge would have a
low seismic velocity and so could be detected, although the imaging of the deep
structure of a subducting slab at the resolution of both velocity and depth required
is a particularly difficult task. The coincidence of low
v
p
and
v
s
velocities, deep
earthquakes and subducting lithosphere collecting just above the lower mantle
behind the Tonga arc (Fig. 9.47) provides strong support for their origin being
metastable olivine.
Some deep earthquakes take place in regions not associated with present-day
subduction or in deep remnants of slabs that are apparently detached from the
subducting plate. These earthquakes have been very difficult to explain. Trans-
formational faulting is, however, able to account for this isolated seismicity.
Remnants of subducted slabs containing regions of metastable peridotite will
continue to transform: a physical connection to the surface is unnecessary. Thus
