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for the 1811-1812 mainshocks thus hinges on the felt reports and their interpretation
for modified Mercalli intensity (MMI) values. In a seminal investigation, Nuttli
(1973) drew isoseismal contours based on his compilation and interpretation of ap-
proximately 40 archival accounts. He determined body-wave magnitude, m
b
, values
of 7.2, 7.1, and 7.4 for NM1, NM2, and NM3, respectively, based on a relationship
between ground motion and intensities from smaller and more recent instrumen-
tally recorded earthquakes in the central United States. With an exhaustive archival
search, Street (1984) greatly expanded the number of reports (to approximately 100
for NM1) and assigned his own intensity values. Street (1982, 1984) used these new
data and the same method used by Nuttli, (1973) to obtain m
b
of 7.1 and 7.3 for NM2
and NM3 and 7.0 for the 0715 LT aftershock of December 16, 1811. Street (1982)
determined these values by assuming the m
b
value for NM1 determined by Nuttli
(1973) and comparing the relative isoseismal areas of the other events.
Following the introduction of the moment-magnitude scale in 1979 (Hanks and
Kanamori, 1979), attempts were made to convert earlier m
b
values to moment-
magnitude, M
w
. It was at this time that the magnitude estimates grew to very large
values, with estimates as high as 8.75 (Nuttli, 1979). Even as these estimates were
made, it was recognized that they were based on extrapolations of data from smaller
earthquakes and thus were highly uncertain. The lack of true calibration events
from central/eastern North America led Johnston (1996) to undertake a comparison
between intensity distributions and moment magnitudes M
w
for large earthquakes
in stable continental regions worldwide. He compared areas within isoseismals of
discrete intensities with instrumentally measured moment magnitudes. On the ba-
sis of this calibration, he assigned Mw values of 8
.
1
+−
0
.
31, 7
.
8
+
/
−
0
.
33, and
8
33 for NM1, NM2, and NM3, respectively.
Hough et al. (2000) reinterpreted the accounts compiled by Nuttli (1973) and
Street (1984), identifying a small number of outright transcription errors in the study
of Nuttli (1973) and a larger number of inappropriately high intensity values that had
apparently been assigned based on subjective perceptions of shaking. This study
also addressed the bias due to early American settlement patterns, namely the fact
that observers of the earthquakes were concentrated along major river valleys where
substantial sediment-induced amplification is expected (e.g., Singh et al., 1988), and
was in fact documented (e.g., Drake, 1815).
Hough et al. (2000) did not correct MMI values for site-response. Rather, the
MMI values were assigned based on a careful consideration of the overall macro-
seismic effects as described by available archival accounts. In their interpretation,
Hough et al. (2000) considered site response biases, in effect not allowing biased
values to inappropriately control inferred isoseismal areas. Using the method of
Johnston (1996), Hough et al. estimated Mw values of 7.2-7.3, 7.1, and 7.4-7.5 for
NM1, NM2, and NM3, respectively.
The method of Johnston (1996) was developed using MMI values for a set of
instrumentally recorded calibration earthquakes in so-called Stable Continental Re-
gions (SCR) world-wide. If there are biases in the MMI values for the calibration
earthquakes, or if other SCR regions are not perfect analogs for central/eastern
North America, then the application of the Johnston (1996) method will introduce
.
0
+
/
−
0
.
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