New Madrid Seismic Zone: Overview of Earthquake
Hazard and Magnitude Assessment Based on Fragility of Historic
Structures (May 2003, 85 p.)
The assessment of earthquake hazard has been a long-standing
concern in areas known to be prone to earthquakes. While housing
construction in the United States is generally considered
to be earthquake-resistant in comparison to many forms of
construction found worldwide, the assessment of seismic hazard
has significant implications with regard to the balance of
housing affordability and safety. Seismic hazard assessments
affect building code design requirements (i.e., mapped design
ground motions), construction guidelines, building costs,
insurance rates, expected consequences of future earthquake
activity, and regional economies as a whole. The seismic design
provisions of the International Building Code (IBC-2000) and
the International Residential Code (IRC-2000) are both currently
being considered for adoption by local political jurisdictions
across the U.S., and have generated much concern and controversy
as to the accuracy and validity of the new seismic provisions
in the Central and Eastern United States, and particularly
in the New Madrid Seismic Zone (NMSZ).
This study provides an overview of the seismic hazard characterization
procedures used in the NMSZ and implemented in the IBC-2000
and IRC-2000. Furthermore, a series of structural fragility
evaluations of historic accounts of building damage are conducted
to provide additional and independent constraints on the magnitude
estimates of the 1811-1812 earthquakes. This approach to magnitude
assessment is particularly appealing given that the magnitude
estimate is ultimately used for regulation of building construction
through the use of seismic hazard maps that are integral with
seismic design provisions in modern building codes. The study
aims to communicate the level of uncertainties involved in
the seismic hazard assessment procedures and their impact
on the assignment of seismic design ground motions and categories
and to provide recommendations regarding implications associated
with adoption or modification of newer seismic hazard provisions
found in the IBC-2000 and IRC-2000.
Results of this study further confirm the high level of seismic
hazard in the NMSZ and the need for continued attention to
and consideration of adequate mitigation measures. This high
level of seismic hazard is evidenced by large earthquakes
that have repeatedly occurred in the past reaching destructive
magnitudes. The study makes several concludes that deterministic
capping of ground motions in the immediate vicinity of the
modeled faults in the NMSZ should be considered for establishing
design level ground motions, and hazard de-aggregation represents
a useful and practical tool for communicating concepts and
sources of seismic hazard to the stakeholders in a more transparent
manner. Additionally, recent studies of paleoseismology and
magnitude estimation have improved the understanding of the
past behavior of the NMSZ, whereas questions remain open as
to the future behavior of the NMSZ. The study makes recommendations
for further research and implementation, including methods
to perform future post-earthquake damage assessments and building
evaluations.
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