Large 19th Century Earthquakes in Eastern/Central North America: A Comparative Analysis - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 340

1811; around 0900 LT on 23 January, 1812, and approximately 0345 LT on 7
February 1812 (henceforth NM1, NM2, and NM3, respectively). All three events
were felt throughout much of the central and eastern United States. Additionally,
a large aftershock to NM1 (NM1-A) occurred near dawn on 16 December 1811.
Available accounts also document substantial aftershock activity following all three
mainshocks (Drake, 1815; McMurtrie, 1819; Fuller, 1912; Penick, 1981).
The Charleston earthquake of 1 September 1886—9:50 p.m. LT on 31 August
1886—was the primary event in a more conventional earthquake sequence: a single
large mainshock preceded by a small number of foreshocks and followed by a con-
ventional, if perhaps widespread, aftershock sequence (Dutton, 1889; Seeber and
Armbruster, 1987).
Paleoseismic investigations suggest a repeat time of the order of 400-500 years
for both the New Madrid sequence and the Charleston earthquake (Talwani and
Schaeffer, 2001; Tuttle et al., 2002); they also suggest that the New Madrid seis-
mic zone tends to produce prolonged sequences with multiple, distinct mainshocks,
the magnitudes of which are comparable to those of the 1811-1812 events (e.g.,
Tuttle and Schweig, 1996; Tuttle et al., 2002). Thus, the magnitudes of these earth-
quakes are a critical issue for the quantification of regional hazard in central North
America. A repeat of the 1811-1812 sequence would clearly have a tremendous
impact. Because of low regional attenuation, the New Madrid seismic zone (NMSZ)
contributes a nontrival component of seismic hazard in relatively distant, large mid-
western U.S. cities such as St. Louis, Missouri (Frankel et al., 1996). The Charleston
seismic zone also contributes significantly to regional as well as local hazard.
A second impetus to investigate the 1811-1812 sequence stems from its impli-
cations for general issues related to intraplate earthquake processes. The NMSZ is
among the best-understood intraplate source zones in the world, largely because it
has been so active throughout the historic and recent prehistoric past. This relative
abundance of data affords the opportunity to explore critical unanswered scientific
questions regarding large SCR earthquakes, most notably the questions of why such
events occur in certain regions but (apparently) not in others, why and to what ex-
tent large earthquakes are clustered, and the nature (i.e., scaling) of large intraplate
earthquakes.
In a sense, the importance of the New Madrid earthquakes—both scientifically
and for hazard-correlates with their magnitudes, yet these values remain grossly
uncertain. Considerable effort has been invested in gleaning quantitative informa-
tion from the limited available data. Available data include (1) paleoliquefaction
features preserved by the sediments within the Mississippi embayment (e.g., Tuttle
and Schweig, 1996); (2) the present-day distribution of seismicity in the NMSZ,
which is generally assumed to be a long-lived aftershock sequence that illuminates
the principal fault zones (e.g., Gomberg, 1993; Johnston, 1996; Mueller et al., 2004);
(3) first-hand reports (“felt reports”) of the shaking and/or damage caused by the
events over the central/eastern United States (e.g., Nuttli, 1973; Street, 1984).
While the size of both inferred mainshock ruptures and liquefaction features
provides some constraint on magnitude, such estimates are invariably less well-
constrained than those based on macroseismic effects. Determination of magnitudes