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relative high or low activity that extend for several decades (Hardebeck et al ., 2008 ) . Using
simulated ETAS catalogs, Page et al .( 2010 ) show that, for any region, reliable estimation
of a long-term a -value requires a catalog that is several times longer than the recurrence
time of the M max event in the region. Even in California, where strain rate is relatively high,
the long-term a -value remains uncertain by 20% or more (e.g., WGCEP, 2013 ) . Relative
to California, the historical record in the CEUS is at most a factor of 2 longer, but the
strain rate is a factor of 1,000 or more lower. Even without detailed statistical calculations,
it is clear that a 100- or 300-year snapshot of seismicity cannot capture all potentially
active source zones or provide a reliable estimate of long-term rates on timescales of
millennia.
12.5 Discussion and conclusions
Quantification of seismic hazard in the CEUS, and by extension any similar low strain rate
region, emerges as a daunting prospect. Probabilistic seismic hazard assessment remains
challenging in a well-studied, high strain rate region such as California (e.g., WGCEP,
2013 ) , where geological and geodetic constraints are each uncertain, but presumably allow
hazard to be characterized to first order. In the low strain rate CEUS region, where the
seismic catalog is at best a factor of 2-10 longer than that in California, earthquake rates
are a factor of 1,000 or more lower, geological and geodetic constraints are limited or
non-existent, and hazard is likely to be dominated by low-probability events. There is little
question that the inputs, for example given published magnitude estimates of characteristic
earthquakes that vary by nearly a full magnitude unit, to PSHA remain highly uncertain in a
low strain rate region. And, again, to quote fromClark et al .( 2012 ) , “This apparent bimodal
recurrence behavior poses challenges for probabilistic seismic hazard assessment.”
In the absence of adequate constraint on long-term slip rates, deformation, and seismic
rates, an alternative to the use of smoothed seismicity models involves consideration of
tectonic zonation. Since 2005 the seismic hazard map of Canada has been determined
in part based on an interpretation of tectonic zonation (e.g., Adams and Halchuk, 2003 ) .
Whereas the maps prior to 2005 had conspicuous “bull's-eyes” around the largest known
late Holocene earthquakes, high hazard in the new maps is more distributed throughout
broader zones, with less pronounced bull's-eyes corresponding to regions of historical
activity.
As paleoseismologists continue to study the Central and Eastern United States, it also
appears likely that geological investigations will identify other source zones that have pro-
duced large Holocene earthquakes. Inclusion of rates based on observed prehistoric earth-
quakes in other areas will likely serve to distribute hazard over broader zones. A reduction
of the estimated magnitudes of the NewMadrid and perhaps Charleston mainshocks would
further serve to lessen the estimated hazard in proximity to these zones.
A fundamental challenge, however, will be to account for uncertainties associated with
long-term rate estimates, not only in the NMSZ and CHSZ regions, but also in regions
where no large earthquake has occurred during historical times. In the NMSZ and CHSZ, as
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