Geoscience Reference
In-Depth Information
12
Intraplate seismic hazard: Evidence for distributed
strain and implications for seismic hazard
SUSAN E. HOUGH
Abstract
In contrast to active plate boundaries, the lack of a fundamental scientific
framework to understand seismogenesis combines with the lower rates of earth-
quakes to make probabilistic seismic hazard assessment (PSHA) challenging
in intraplate regions. Seismic hazard maps for intraplate regions tend to indi-
cate hazard dominated by source zones that have produced large earthquakes
during historical times. The level of hazard can also depend critically on the
magnitudes of pre-instrumental earthquakes, which are clearly far less cer-
tain than earthquakes during the instrumental era. In this brief chapter I first
summarize recently published results that address the long-standing paradox
of low strain accrual and high moment release in the Central/Eastern United
States (CEUS); I then summarize recent published studies from the CEUS
and elsewhere suggesting that strain release is more spatially distributed over
timescales of millennia to tens of millennia than over the past few centuries. I
explore a simple quantitative model that has been discussed in numerous past
studies, namely that strain is everywhere low, with weak modulation in the
CEUS due to glacial isostatic adjustment and concentration within failed rifts
that represent large-scale zones of weakness in the crust. Within this paradigm,
one can consider an overall moment-release budget associated with low but
spatially distributed strain. I show that even a conservative estimate of strain
rate (0.5-1.0
10
−
9
/yr) can generate the rate of moment release observed
in historical times, if the strain is distributed and large historical earthquakes
were close to M
w
7.0. Basic considerations from both geological observations
and statistical seismology support the assumption that, over decadal to century
timescales, activity is likely to remain concentrated in areas that have seen
elevated activity in the recent geological past. Significant rate changes are pos-
sible, however, over short timescales, and may be likely over millennial scales -
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