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seismological and geodetic measurements, it may become possible to routinely detect
potential locations of moderate to large intraplate earthquakes.
As intraplate earthquakes jump from one fault to another and there is an absence of a
stationary source, we need to revise our strategies for studying them and for estimating the
attendant seismic hazard. A new set of questions need to be addressed. Do we abandon
the probabilistic seismic hazard analysis approach and are deterministic site-specific inves-
tigations the correct approach in estimating seismic hazards in intraplate regions? Do we
examine the entire rift structure and look for local stress concentrators, or do we need to
focus our efforts on areas of past large historical earthquakes?
Mooney et al . ( 2012 ) showed that in addition to old rift structures, many intraplate earth-
quakes are located near craton boundaries. Dedicated field studies are needed to identify
the responsible seismic sources. A possible starting point may be based on Ziegler's (1987)
observation that, in mid-plate regions, deep faults that extended to lower crustal depths are
likely to be reactivated by the stresses originating at plate boundaries. Identification of such
faults by geological and/or geophysical studies would be a good starting point for further
investigations of the potential seismic source zones associated with craton boundaries.
Future theoretical studies could address the reasons why large earthquakes do not recur
on the same fault but instead jump around, suggesting that after an earthquake most of the
built-up seismic energy is expended, while at other locations it continues to build until the
next earthquake. The observation of “roaming” or “wandering” earthquakes also suggests
the presence of several potential stress concentrators that are sequentially reactivated.
Theoretical studies are needed to estimate the magnitude of the stresses that accumulate
at different stress concentrators and mechanisms to explain their temporal and spatial
behavior.
The maximum magnitudes (M max ) of intraplate earthquakes vary from region to region.
For example, in Western Europe it is rare to observe an earthquake with M > 5, or in
Brazil with M > 6. Several earthquakes with M max > 6, however, have occurred in Aus-
tralia and Eastern Canada. Considerably larger earthquakes have been observed in northern
China, the New Madrid seismic zone and in the Kutch Rift Basin. What controls M max in
intraplate regions? Along plate boundaries, M max is related to the fault length, a correla-
tion not seen in intraplate regions. Modeling suggests that M max is related to the amount
of stress that can build up on a stress concentrator, which in turn depends on the kind
of stress concentrator present. Historically, shallow plutons have been associated with
M max < 5 and rift pillows with M max <
5.5. As mentioned earlier, for the larger earth-
quakes in the New Madrid seismic zone, Kutch Rift Basin, and in the source zone for
the 1886 Charleston earthquake, M max appears to be related to the width of the compres-
sional step between the en echelon faults associated with the source zone. This speculative
association needs to be confirmed with data from other source zones and by numerical
modeling.
The diverse studies described in this topic represent a step forward in our understanding
of intraplate earthquakes. While they answer some questions about our understanding, they
also raise many others that should be the focus of future research.
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