Geoscience Reference
In-Depth Information
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Figure 9.56. A map view of all well-located earthquakes (1980-1987) between 525
and 615 km depth (light grey), the 9 June 1994 deep Tonga earthquake (black) and its
best-located aftershocks (dark grey) shown as 95%-confidence ellipsoids. The
subducting Pacific plate is vertical here at 17-19 S (Fig. 9.47). The two linear bands
of grey ellipsoids suggest that there may be a double seismic zone, which would be
consistent with transformational faulting along the edges of a 30-40-km-wide
metastable olivine wedge. The aftershocks from the 1994 earthquake clearly cut
entirely across the seismic part of the slab into the surrounding aseismic region. The
main-event (black) rupture started in the cold core of the slab and terminated some
15-20 km outside the seismic zone, close to the outlying aftershock, where the
temperature is
200 C higher. This warmer region adjacent to the cold seismic core
of the slab may have a different faulting regime for deep earthquakes: rupture can
propagate here from the metastable core but aftershocks are rarely initiated in this
region. Colour version, Plate 23 (Reprinted with permission from Nature (Wiens
et al ., Nature , 372 , 540-3) Copyright 1994 Macmillan Magazines Ltd.)
The general feature of gravity profiles over convergent plate boundaries is a
parallel low-high pair of anomalies of total amplitude between 100 and 500 mgal
and separated by about 100-150 km. The low is situated over the trench; the high
is near to and on the ocean side of the volcanic arc.
Density models that can account for these gravity anomalies include the dip-
ping lithospheric plate and thick crust on the overriding plate. Details that also
have to be included in the modelling are the transformation of the basaltic oceanic
crust to eclogite with an increase in density of about 400 kg m 3 by about 30 km
depth.
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