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occur in the high-wave-speed anomalies that mark cold slabs,
anomalies that have been traced to depths as great as 2000 km
(1220 mi) or more.
Curiously, earthquakes occur no deeper than 650–700 km
(397–427 mi), far shallower than the maximum depths to which
slabs descend. This abrupt shutoff and the gradual onset of
the deep earthquake population at 300–350 km (183–214 mi)
bracket approximately the transition zone of the mantle where
seismic wave speeds abruptly increase (Fig. 5a). High-pressure
experiments indicate that the mineralogy of the mantle
changes at those depths and pressures from upper-mantle
mineralogy (dominantly olivine and pyroxenes) to the minerals
spinel, ilmenite, and majorite in the transition zone and, in turn,
to the lower-mantle perovskite and oxide minerals. Slab mantle
penetrating through the transition zone is expected to trans-
form to these denser minerals.
Most deep earthquakes occur in the depth interval of the
transition zone where upper-mantle slab minerals are recon-
structed to their denser structural forms. Attention has there-
fore been drawn to the possibility that deep earthquakes are
somehow caused by the mineralogical transformation of slabs
as they descend into and through this region. Early speculation
was that deep earthquakes represent rapid implosions that
might occur when slab minerals transform suddenly to their
denser, high-pressure forms. The patterns of seismic waves that
radiate from deep earthquake sources indicate, however, that
such disturbances represent slip on a fault, as do shallow earth-
quakes. If a connection exists between deep earthquakes and
mantle phase changes, the underlying process must facilitate
failure by faulting.
A clue to the nature of this possible connection comes from the
observation that deep earthquakes do not occur in all slabs,
only in those that are very cold because they are descend-
ing at very fast rates. Low slab temperatures are important
because such conditions favor the metastable persistence of
upper-mantle minerals in the coldest interiors of slabs as they
descend into the transition zone (Fig. 5b). Laboratory studies
show that some minerals deformed under metastable condi-
tions will rupture by an unusual shear instability in which the
mineral is transformed to denser minerals in the shear zone.
This shear instability, called transformational faulting, is not in-
hibited by high-pressures and hence is an attractive candidate
for the faulting mechanism of deep earthquakes. According to
this theory, deep earthquakes do not occur in the lower mantle
because low-density upper-mantle slab rocks are too buoyant
to sink into the lower mantle.
Earthquake (continued)
