Intraplate seismic hazard: Evidence for distributed strain and implications for seismic hazard - Intraplate Earthquakes - page 310

the 1811-1812 sequence were in the range 0.2-0.46 g. Holzer et al .( 2011 ) state that
these moderate-to-high values are consistent with M w

7.5 values and inconsistent with
magnitudes smaller than 7. However, predicted accelerations at deep-sediment sites for
earthquakes of M w 7.0 and larger are uncertain in intraplate regions owing to a lack of
direct observation. For example, the preliminary results of Ramirez-Guzman et al .( 2011a ) ,
based on ground motion simulations, reveal perhaps surprising insensitivity of predicted
intensities within the embayment, at distances over which significant liquefaction occurred,
to magnitude values ranging from 7.0 to 7.7. That is, over this range of magnitudes, pre-
dicted intensities do not vary enough to be useful as a discriminant given the uncertainties
in observed intensity values. Noting this uncertainty, Ramirez-Guzman et al .( 2011b ) sug-
gest that predicted shaking intensities within the Mississippi Embayment will not be useful
to constrain historical earthquake magnitudes within the range 7.0 to 7.7, and conclude
that additional constraints, “such as those provided by paleoliquefaction analyses,” will
be needed to reduce uncertainties in magnitude estimates. However, while the distribution
and size of liquefaction features might conceivably shed further light on magnitudes, the
method developed by Holzer et al .( 2011 ) uses paleoliquefaction analysis to derive ground
motions, the interpretation of which will be plagued by the same uncertainties documented
by Ramirez-Guzman et al . (2011a, b).
In light of these uncertainties, determination of magnitudes for the 1811-1812 main-
shocks has thus hinged critically on the felt reports and the interpretation of associated
modified Mercalli intensity (MMI) values by various investigations. Magnitude estimates
have varied enormously, from (Nuttli, 1979 ) .

Values close to M w 7 have also been suggested based on constraints on the overall present-
day strain rate (e.g., Newman et al ., 1999 ) , although inference of magnitude from strain
rate is highly model-dependent (e.g., Kenner and Segall, 2000 ) . While different analyses of
macroseismic data from the New Madrid sequence have yielded widely varying magnitude
estimates, underscoring the inherent difficulty in assessing historical earthquake records,
in all studies the four principal events are found to have roughly comparable magnitudes,
with a range of at most 1 magnitude unit. For the maps released by the U.S. Geological Sur-
vey National Seismic Hazard Mapping Project (NSHMP) in 2002, values of 7.3-8.0 were
considered using a logic-tree approach, with highest weight assigned to 7.7 (Frankel et al .,
2002 ; Petersen et al ., 2008 ) .
Investigation of the Charleston earthquake dates back to the immediate post-earthquake
investigations led by Clarence Dutton, an Army officer detailed to the U.S. Geological
Survey. This effort culminated in the publication of one of the earliest comprehensive,
scientific reports of a large earthquake (Dutton, 1889 ) . The so-called Dutton Report includes
thorough and consistent compilations of near-field geological effects of the earthquake and
accounts of far-field macroseismic effects. Whereas about 100 or fewer intensity values
are available for each of the New Madrid mainshocks, the Dutton Report provides the
basis for assignment of over 1,000 intensity values. In a comprehensive interpretation of
these accounts, Bollinger ( 1977 ) assigned almost 800 intensity values based on the 1,337
intensity reports tallied by Dutton ( 1889 ) . Bollinger ( 1977 ) estimated an m b value of 6.8-7.1

7 (Hough and Page, 2011 ) toashighas8 ¾