Geoscience Reference
In-Depth Information
Figure 9.49.
A cross
section perpendicular to
the Tonga Trench,
showing the location of
the subducting plate
relative to the zones of
high and low seismic
attenuation (
Q
). (Based on
Roth
et al
.(1999).)
a future large earthquake. Variations in the level of seismicity along the length
of a subduction zone may also be controlled by factors such as geological or
bathymetric structures in either of the two plates. Figure 9.50 shows the seismicity
along the Tonga-Kermadec Arc. The Tonga-Kermadec subduction zone has very
high seismicity; approximately one earthquake with
m
b
greater than 4.9 occurred
every 2 km along the arc during the eighteen years of recording shown. There are
clear variations in the level of the seismicity on a length scale ranging from tens
to hundreds of kilometres.
One particular 250-km-long seismic gap in the Aleutian Arc is the Shumagin
gap (Figs. 9.51 and 9.52). Analysis of background seismicity along the Aleutian
Arc shows that activity there is low, although very large earthquakes have occurred
along this arc in the past. The amount of stress built up along the Shumagin gap
since the last great rupture there would be sufficient to result in an
M
w
=
8.3-8.4
earthquake within the next few decades, unless aseismic slip is occurring (i.e.,
the plates are moving smoothly past each other). An
M
S
6.9 earthquake in
1993 ruptured a small part of the gap but did not significantly relieve the build-up
of strain. The available strain, tiltmeter and GPS measurements are unable to
determine strain accumulation: aseismic slip could but might not be occurring.
Unfortunately for those who live in the vicinity, only time may provide the answer
to the question of the origin of the Shumagin gap.
Two other subduction zones where large earthquakes have long repeat times are
the Cascadia subduction zone where the Juan de Fuca plate is descending beneath
North America and the Rivera subduction zone where the tiny Rivera plate is being
thrust beneath the Jalisco region of Mexico. Small earthquakes occur frequently
along the 1400-km-long Cascadia subduction zone but there has been no large
earthquake on the subduction thrust fault since record keeping began. Coastal
geological evidence indicates that very large earthquakes with repeat times of
=
