Geoscience Reference
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Figure 9.50. A map of the
Tonga-Kermadec Arc
showing the location of all
the shallow, main shock
epicentres (aftershocks
are not shown) with m b
greater than 4.9 which
occurred between 1963
and 1981. Regions with
high and low seismicity
are marked A (active) and
Q (quiet), respectively.
The very-active zone
marked A1 is associated
with the change in strike
of the plate boundary
from northerly to
westerly. Bathymetric
contours are in
kilometres. (From
Haberman et al .(1986).)
about 500 yr have occurred along this subduction zone. A damaging tsunami that
affected Japan on 27 and 28 January 1700 is thought to have been caused by a great
Cascadia earthquake (probable magnitude
9), which would have occurred at
9pm on 26 January. The present pattern of deformation along the subduction zone
indicates that most accumulated strain is stored elastically and so will ultimately
be released in earthquakes. While some of the strain has resulted in permanent
deformation of the North America plate, there is no evidence that any aseismic slip
is currently occurring: the subduction zone is locked. The amount of accumulated
strain may be sufficient to generate a great Cascadia earthquake. The seismic
moment accumulated there over 500 yr would be released by one magnitude-9
earthquake, or about thirty magnitude-8 earthquakes (Section 4.2.3). Potential
implications for the coastal cities of British Columbia, Washington and Oregon
are severe. Evidence suggests that the 1700 earthquake may have ruptured almost
the whole length of the subduction zone. Far to the south the Rivera plate is being
subducted beneath North America at 2.0 cm yr 1 , slower than the 5.2 cm yr 1
of the adjacent Cocos plate, which complicates the subduction processes. The
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