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ANNALS OF GEOPHYSICS, VOL. 47, N. 2/3, April/June 2004
Key words New Madrid earthquakes – intraplate –historic
1. Introduction
The earthquake sequence that struck the NewMadrid region of the
North American mid-conti-nent in 1811-1812 had remarkably
far-reachingeffects. By some accounts, they are among thelargest –
if not the largest – to have ever oc-curred in a so-called Stable
Continental Region(SCR) (Johnston, 1996). Ground motions fromthe
three principal events were felt in places as
far away as Canada, New England, and at a num-ber of locations
along the Atlantic coast(Mitchill, 1815; Bradbury, 1819; Fuller,
1912;Nuttli, 1973; Penick, 1981; Street, 1984; John-ston, 1996).
Contemporary accounts documentthree principal events: approximately
02:15 Lo-cal Time (LT) on 16 December 1811; around08:00 LT on 23
January 1812, and approximate-ly 03:45 LT on 7 February 1812
(henceforthNM1, NM2, and NM3, respectively; see fig. 1).All three
events were felt throughout much ofthe central and eastern United
States. Addition-ally, a large aftershock to NM1 (NM1-A) oc-curred
near dawn on 16 December 1811. Sub-stantial aftershock activity
following all eventswas also documented (Fuller, 1912;
Penick,1981).
Paleoseismic investigations suggest a repeattime of the order of
400-500 years for the New
Scientific overview and historical contextof the 1811-1812 New
Madrid
earthquake sequence
Susan E. HoughUS Geological Survey, Pasadena, California,
U.S.A.
AbstractThe central and eastern United States has experienced
only 5 historic earthquakes with Mw 7.0, four during theNew Madrid
sequence of 1811-1812: three principal mainshocks and the so-called
«dawn aftershock» followingthe first mainshock. Much of the
historic earthquake research done in the United States has focused
on the NewMadrid Seismic Zone (NMSZ), because the largest New
Madrid earthquakes may represent the archetype for themost damaging
earthquakes to be expected in intraplate regions. Published
magnitude values ranging from 7.0to 8.75 have generally been based
on macroseismic effects, which provide the most direct constraint
on sourcesize for the events. Critical to the interpretation of
these accounts is an understanding of their historic context.Early
settlments clustered along waterways, where substantial
amplification of seismic waves is expected. Ana-lyzing the New
Madrid intensity values with a consideration of these effects
yields preferred values of Mw 7.2-7.3, 7.0, and 7.4-7.5 for the
December, January, and February mainshocks, respectively, and of
7.0 for the «dawnaftershock». These values are consistent with
other lines of evidence, including scaling relationships. Finally,
Ishow that accounts from the New Madrid sequence reveal evidence
for remotely triggered earthquakes well out-side the NMSZ. Remotely
triggered earthquakes represent a potentially important new wrinkle
in historic earth-quake research, as their ground motions can
sometimes be confused with mainshock ground motions.
Mailing address: Dr. Susan E. Hough, US GeologicalSurvey, 525
South Wilson Avenue, Pasadena, CA 91106,U.S.A.; e-mail:
[email protected]
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Susan E. Hough
Madrid events; they also suggest that the NewMadrid seismic zone
tends to produce pro-longed sequences with multiple, distinct
main-shocks, the magnitudes of which are compara-ble to those of
the 1811-1812 events (e.g., Tut-tle and Schweig, 1996; Tuttle et
al., 2002).Thus, the magnitudes of the earthquakes be-come a
critical issue for the quantification of re-gional hazard in
central North America. A re-peat of the 1811-1812 sequence would
clearlyhave a tremendous impact. The New MadridSeismic Zone (NMSZ)
contributes a nontrivalcomponent of seismic hazard in relatively
dis-tant midwestern US cities such as St. Louis,Missouri (Frankel
et al., 1996).
A second impetus to investigate the 1811-1812 sequence stems
from their implicationsfor general issues related to intraplate
earth-quake processes. The NMSZ is among the best-understood
intraplate source zones in the world,largely because it has been so
active throughoutthe historic and recent prehistoric past. This
rel-ative abundance of data affords the opportunityto explore
critical unanswered scientific ques-tions regarding large SCR
earthquakes, mostnotably the questions of why such events occurin
certain regions but (apparently) not in others.
Because an evaluation of the magnitudes ofthe 1811-1812 events
is so critical for severalreasons, tremendous effort has been
invested ingleaning quantitative information from the lim-ited
available data. Available data include i) pa-leoliquefaction
features preserved by the sedi-ments within the Mississippi
embayment (e.g.,Tuttle and Schweig, 1996); ii) the
present-daydistribution of seismicity in the NMSZ, whichis assumed
to illuminate the principal faultzones (e.g., Gomberg, 1993;
Johnston, 1996);iii) first-hand reports («felt reports») of
theshaking and/or damage caused by the eventsover the
central/eastern United States (e.g.,Nuttli, 1973; Street,
1984).
Determination of magnitudes for the 1811-1812 mainshocks hinges
exclusively on the feltreports and their interpretation for
ModifiedMercalli Intensity (MMI) values. Nuttli (1973)drew
isoseismal contours based on his compi-lation of approximately 40
felt reports. He de-termined mb = 7.2, 7.1, and 7.4 for NM1,
NM2,and NM3, respectively, based on a relationship
between ground motion and intensities fromsmaller and more
recent instrumentally record-ed 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
assignedthem intensity values. Street (1982, 1984) usedthese new
data and the same method used byNuttli (1973) to obtain mb = 7.1
and 7.3 forNM2 and NM3 and mb 7.0 for the 07:15 LT af-tershock of
16 December 1811. Street (1982)determined these values by assuming
the mbvalue for NM1 determined by Nuttli (1973)and comparing the
relative isoseismal areas ofthe other events.
Johnston (1996) carried out a comparison be-tween intensity
distribution and moment magni-tude Mw for large earthquakes in
stable continen-tal regions worldwide. He compared areas with-in
isoseismals of discrete intensities with instru-mentally measured
moment magnitudes. On thebasis of this calibration, he assigned 8.1
± 0.31,7.8 ± 0.33, and 8.0 ± 0.33 for NM1, NM2, andNM3,
respectively. In this calculation, Johnston(1996) used the only
published intensity con-tours; those determined by Nuttli
(1973).
Hough et al. (2000) revisited the magnitudedetermination for the
New Madrid mainshocksin two ways. First they reconsidered
intensityassignments for the reports compiled by Nuttli(1973) and
Street (1984). This reinterpretationfocused on effects that were
considered rela-tively objective, such as descriptions of damageto
structures.
The reinterpreted MMI values can be usedto define new isoseismal
contours using sub-jective as well as systematic approaches, andthe
isoseismal contours can then be used to ob-tain Mw estimates
following the procedure andcalibration established by Johnston
(1996).The results can then be interpreted with a con-sideration of
their historic context, most no-tably early American settlement
patterns. Thepopulation of the United States was≈ 7 000 000 in
1811, with sizable numbers inthe states of Tennessee, Kentucky, and
the re-gion including the present-day states of Mis-souri and
Louisiana. The 1810 Census givesthe population for several
districts for whichfelt reports are considered, including the
Dis-
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Scientific overview and historical context of the 1811-1812 New
Madrid earthquake sequence
Fig. 1. Map showing location of the New Madrid seismic zone as
illuminated by microseismicity between 1974and 1996. Locations are
from the New Madrid catalog (see Taylor et al., 1991), which are
reported only to twosignificant figures in decimal degrees.
Epicenters of the three principal 1811-1812 mainshocks are shown
withlarge open circles (after Johnston and Schweig, 1996). Solid
line shows inferred location of Reelfoot Fault (af-ter Odum et al.,
1998). Rupture scenarios for NM1, NM1-A, and NM3 are also
indicated. Scenario for NM2, in-dicated with dashed line, is
considered relatively uncertain.
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526
Susan E. Hough
trict of St. Louis (population 5667), Cincinnati(2540), New
Orleans (24 552), Louisville(1357), and New Madrid (2103).
Although present-day Missouri was relative-ly sparsely populated
in 1811, available contem-porary accounts (e.g., Brackenridge,
1817,Bradbury, 1819) provide a fairly thorough docu-mentation of
demographic and related informa-tion. These sources reveal that
some towns weremore than simple villages by 1811, with
solidlyconstructed houses appearing by the turn of thecentury. The
oldest brick building west of theMississippi was built in the town
of SainteGenevieve in 1804; this town is along the Mis-sissippi
River valley north of New Madrid. Thishouse and ≈ 50 others that
predate the NewMadrid sequence, are still standing today.
This paper summarizes and expands on theresults published by
Hough et al. (2000). Thereader is referred to this publication for
many ofthe details regarding the results summarizedhere.
Additionally, I summarize evidence thatthe 1811-1812 sequence
included a number ofsignificant, potentially damaging
earthquakes,that occurred well outside of the NMSZ.
2. Intensity reports
2.1. Original sources: general considerations
Hough et al. (2000) concluded that many ofthe original MMI
assignments by Nuttli (1973)were too high for two basic reasons: a
generalbias in the interpretation of reports whose dra-ma is belied
by low levels of actual damage re-ported, and, to a lesser extent,
a failure to takesite response issues into account. Many of
theoriginal accounts describe the effects of long-period shaking;
this kind of shaking can be dra-matic for large (Mw 7 and larger)
events at re-gional distances even when the overall effectsof the
shaking is low.
Overall, many of the accounts do not appearto support values as
high as those originally as-signed. In St. Louis, Missouri, Fuller
(1912) de-scribes reports (from the Louisiana Gazette, 21December
1811) of people having been wak-ened by NM1 and furniture and
windows hav-ing been rattled. He notes that «several chim-
neys were thrown down», and a few houses«split». To understand
such accounts one mustbe familar with the vernacular of the time;
inthis case, the word «split» seems to have beenused in a number of
accounts to mean«cracked» rather than destroyed. Consistently,as in
the above example, the phrase «throwndown» is used to describe
catastrophic damageto chimneys, walls, or houses. The
LouisianaGazette account goes on to note that «no liveshave been
lost, nor has the houses sustainedmuch injury». This observation
also suggeststhat the word «split» does not imply
substantialdamage. On the basis of these reports, a MMI ofVI-VII
appears to be more appropriate than thevalue of VII-VIII that
Nuttli (1973) assigns forNM1.
At many locations at regional distances(roughly 500-1000 km)
event NM1 is generallyreported as having been «distinctly» (often
theword «sensibly» is used) felt but with no reportsof damage.
Instead, reports describe the rattlingof washing stand pitchers,
glass, china, and fur-niture. Reports from these locations also
gener-ally indicate that «many» were awakened by theevent. Such
descriptions are consistent with anMMI value of IV-V, whereas
higher values wereassigned in the earlier study. In two
instances(Arkport, in Western New York, and Lexington,Kentucky) it
appears that Nuttli (1973) was sim-ply mistaken in either his
reading of the originalsources, or the MMI assignments.
The reinterpretation of Hough et al. (2000)thus represents both
a revision and a substantialexpansion of the original intensity
work byNuttli (1973).
2.2. Site response
In addition to the reassignments discussedabove, Hough et al.
(2000) also assigned – andinterpreted – MMI values with a
considerationof site response. Arguably, the key to under-standing
the effects of the New Madrid earth-quakes lies with an
appreciation for their his-toric context. As a first-order
observation, theintensity data are very sparse and
concentratedalong major river valleys and other bodies ofwater. The
latter observation reflects the distri-
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Scientific overview and historical context of the 1811-1812 New
Madrid earthquake sequence
1815). However, the potential magnitude of siteamplifications at
regional distances has perhapsnot been fully appreciated until it
was so dra-matically demonstrated in a number of destruc-tive
earthquakes in recent years (e.g., Singh etal., 1988). Recent
dramatic examples of site re-sponse have tended to involve lake
beds, valleysor basins, and coastal regions such as the
SanFrancisco Bay area, but significant site amplifi-cations along
river valleys have also been doc-umented (e.g., Stover and Von
Hake, 1982).
A close reading of original sources revealsthat the role of site
response in controllingground motions from the New Madrid events
isdocumented in several contemporary accountsof the events. For
example, Fuller (1912) quotesan account by Daniel Drake of
Cincinnati,Ohio: «(Event NM1) was so violent as to agi-tate the
loose furniture of our rooms, open par-tition doors that were
fastened with fallinglatches, and throw off the tops of a few
chim-neys in the vicinity of the town». It was this ac-count that
apparently prompted Nuttli (1973) toassign a MMI value of VI-VII
for Cincinnati forNM1, yet Drake goes on to say that, on the
«el-evated ridges» in Kentucky, less than 20 milesfrom the river,
many people were not awakenedby the event. This account (in
particular the factthat many people away from the river
sleptthrough the event) suggests a MMI value ofperhaps IV,
certainly not as high as V. Consid-ering reported effects from the
river valley andthose from higher ground, one obtains a MMIrange of
IV-VI for Cincinnati, or an average ofV. Equivalently, this
approach corresponds toseparate assignments for river valley and
hillsites at Cincinnati. Of the felt reports from theNew Madrid
sequence, site response is explicit-ly documented at six different
locations.
The town of Sainte Genevieve, which hadbeen moved onto a
hard-rock site, provides akey example. No account of the
earthquakesfrom this town was included in the compilationof Street
(1982). A brief account was discov-ered by the author following a
focused archivalsearch. The account states that the earthquakeswere
felt in Sainte Genevieve but caused nodamage (Rozier, 1890). The
pristine, originalappearance of brick and other masonry homesin the
town also testifies to the absence of dam-
bution of the overall population in the moresparsely populated
parts of the central andsoutheastern United States in the early
1800s.Because the New Madrid sequence predates theconstruction of
railroad lines into the midconti-nent, settlements tended to remain
clustered inproximity to waterways. Westward expansionfollowed the
major rivers, and virtually all early1800s settlements in Missouri
(the extent of thewestern frontier at that time) were within a
fewmiles of the Mississippi River. In addition to theinflux of
settlers from the east, settlers of Frenchdescent also arrived in
the area from Quebec tothe north, primarily along the Wabash
River.
By the early 1820s, early settlers had begunto recognize the
pitfalls associated with life onthe immediate river banks, which
included poordrainage, floods, and disease (Missouri Histor-ical
Review, 1911). However, the very earliestsettlements of the late
1700s and very early1800s often were on fluvial sites,
immediatelyadjacent to rivers. New Madrid was built soclose to the
river bank that even before theearthquakes, parts of the town
regularly gaveway under the continued assault of river cur-rents
(Penick, 1981). One of the other sizableMissouri settlements of the
time, SainteGenevieve, had been moved to higher groundapproximately
a mile from the river after aflood in the late 1700s resulted in
substantialerosion of the river bank upon which the townhad
originally been built (Brackenridge, 1817).This town, which is 160
km north of the townof New Madrid and 75 km south of St.
Louis,provides a unique hard-rock sample point, as Iwill discuss
later.
Notwithstanding a handful of exceptions, atthe time of the
1811-1812 sequence, the popu-lation of the US was clustered in
proximity towaterways, especially throughout the sparselypopulated
mid-continent. Intensity data fromriver bank and other coastal
regions will almostcertainly reflect a significant site response
re-sulting from the amplification of seismic wavesin unconsolidated
(and often water-saturated)sediments. The importance of site
response incontrolling earthquake ground motion has beenunderstood
for over a century (Milne, 1898),and even correctly inferred by one
astute wit-ness to the New Madrid sequence (Drake,
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Susan E. Hough
age. This illustrates an important general pointabout poorly
sampled intensity data from his-toric earthquakes: people are more
likely todocument their observations (by writing letters,etc) if
their experiences were dramatic, than if afelt earthquake had
little real impact. It alsoprovides prima facie evidence that the
hard-rock ground motions from the New Madridearthquakes were not
damaging (even to vul-nerable structures) at a distance of ≈ 160
km.
For those cases where shaking and/or damageis reported to have
been worse within a valley oralong a riverbank than on adjacent
higher ground,one can assign distinct MMI values for
river-bank/valley sites and «hard rock» sites away fromthe
waterways. Where the reports do not explicit-ly document relatively
higher shaking alongshorelines, Hough et al. (2000) do not attempt
tocorrect the MMI values for site response. Howev-er, in some
cases, it appears that high intensityvalues were assigned based
solely on riverbankeffects which may have been the result of
agita-tion of the river itself; some of these values
weredowngraded. Clearly, differentiating between theeffects of
river disturbances along the Mississippiand ground shaking is
difficult, if not impossible.
In the final analysis, some level of bias willinevitably remain
in any set of interpreted MMIvalues. However, in some cases the
availabledata are sufficient to assign a more representa-tive
regional MMI level based not on the maxi-mum effects reported at
soft-sediment sites buton a full consideration of all available
reports.
Overall, Hough et al. (2000) assigned sig-nificantly more MMI
IV-V values and signifi-cantly fewer VI-VII ones compared to the
earli-er studies, although in a few instances theirMMI assessments
for a given location werehigher than those of Street (1982).
Clearly,however, the reinterpreted values were lower ingeneral than
those assigned in the earlier stud-ies, which implies that the
differences are dueto systematic differences in interpretation
ratherthan random differences in interpretation ofambiguous
accounts. A final map of reinterpret-ed MMI values for event NM1 is
shown in fig.2. These results include MMI values based ondata from
the following sources: Mitchill(1815), Fuller (1912), Nuttli
(1973), and Street(1984), as well as a small number of
additional
sources, including a single point west of theMississippi River,
at the location of Fort Osage.
3. Isoseismal areas
Considering the data shown in fig. 2, it isclear that isoseismal
contours are not well-con-strained. To obtain magnitude estimates
usingthe equations derived by Johnston (1996), how-ever, one must
estimate isoseismal areas. Houghet al. (2000) employed three
different approach-es to contour the data from NM1: one
subjec-tive, one based on the least-squares minimiza-tion schemes
presented by Seeber and Arm-bruster (1987) (see also Armbruster and
Seeber,1987), and one in which the MMI values aretreated as Boolean
data. If a data point fallswithin the appropriate isoseismal area
(e.g., avalue of IV that falls between the MMI IV andV contours)
the residual is zero. If a data pointis outside the appropriate
contour, the residual isequal to the (whole number) difference
betweenthe observed and calculated values. This ap-proach was
designed to reproduce the usual con-ventions applied when intensity
data are con-toured subjectively. That is, isoseismals are
gen-erally drawn to outline areas of equal intensity.
In both regressions, the starting model forthe falloff of
intensity with distance is derivedfrom the empirical equations of
Johnston(1996). The inversion schemes then allow for it-eration
away from this model based on the dis-tance decay of the data.
Using the regression approaches, the treat-ment of the «not
felt» (NF) reports becomes acritical issue. Following Street
(1984), Houghet al. (2000) assign a NF value to those loca-tions
where a local newspaper is known to havenot mentioned an earthquake
as having beenfelt in that location. Because NM1 and NM3occurred at
times when people can be assumedto have been asleep, a NF report is
taken to in-dicate a bound of MMI < IV (the shaking levelat
which «some» people are awakened (Stoverand Coffman, 1993). For
NM2, which occurredaround 08:00 a.m. LT, a NF report is taken
toimply a bound of MMI < III.
Hough et al. (2000) do not attempt a subjec-tive contouring of
the data for events NM2 and
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Scientific overview and historical context of the 1811-1812 New
Madrid earthquake sequence
Fig. 2. MMI values based on a reinterpretation of original felt
reports from towns as documented by Nuttli(1973) and Street (1984).
Interpolations are done using a standard mathematical algorithm
(for details see:Hough and Martin, 2002); black circles indicate
locations where MMI values are available, while outlined
graycircles indicate locations where Hough et al. (2000) assigned a
«not felt» value.
< .17 .17-1.4 1.4-3.9 3.9-9.2 9.2-18 18-34 34-65 65-124
>124
< 0.1 0.1-1.1 1.1-3.4 3.4-8.1 8.1-16 16-31 31-60 60-116
>116
None None None Very light Light Moderate Moderate/Heavy Heavy
Very heavy
Not felt Weak Light Moderate Strong Very strong Severe Violent
Extreme
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Boolean approach would be preferred given asufficiently complete
set of felt reports, it isyielding overestimates of isoseismal
areas forthe New Madrid events because of the biasedsampling of
site conditions. The least squaresregressions, on the other hand,
result in contoursthat are closer to what one would draw
subjec-tively based on an assessment of site response.
The least squares results for events NM2 andNM3 yield the
preferred magnitude estimates.For event NM1 the preferred estimate
is the oneresulting from the subjective contouring. Al-though it is
not possible to quantify the uncer-tainties precisely, the
bootstrap results do pro-vide a good general indication of the
appropriateerror bars. The final, preferred estimates for thethree
events are Mw 7.2, Mw ≈ 7.0, and Mw 7.4,respectively, with
uncertainties of ≈ 0.3 units ineach case. Hough and Martin (2002)
estimateMw 7.0 for the dawn aftershock, and concludethat this event
most likely occurred on a south-eastern segment of the Reelfoot
thrust fault.
5. Remotely triggered earthquakes
In his compilation of accounts of the NewMadrid sequence, Street
(1982) compiled alist of all events for which there are
multipleaccounts, identifying a number of «large after-shocks» that
were widely felt. At the time ofthis earlier study the
seismological communi-ty did not yet generally appreciate the fact
thatlarge earthquakes are capable of triggeringevents at distances
far greater than those asso-ciated with classic aftershocks. Since
the 28June 1992, Landers, California, mainshock,however, numerous
studies have documentedthe reality of so-called «remotely
triggeredearthquakes» (e.g., Hill et al., 1992). Trigger-ing
appears to be associated with dynamicstrain associated with the
surface wave(Gomberg and Davis, 1996). Although thephysical
mechanism where by strains causedistant earthquakes remains
unclear, remotelytriggered earthquakes are generally assumedto be
earthquakes that would have happened atsome point, but that were
«nudged along» bythe triggering event. It is possible, however,that
remotely triggered earthquakes represent
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Susan E. Hough
NM3. Given the sparsity of the data for theseevents, both the
ellipticity and the shape of thedistance decay are fixed to match
that determinedfor NM1. The decision to fix ellipticity is a
prag-matic one; allowing another free parameter withthe sparse data
results in unstable solutions.
Once the isoseismal contours are deter-mined, Mw values can be
estimated from eachindividual isoseismal contour using the
equa-tions derived by Johnston (1996). Johnston(1996) derives
western correction factors forextrapolation of isoseismals from the
NewMadrid sequence to the west, using the 1843Marked Tree, Arkansas
earthquake to derivecorrection factors for NM1 and the
1895Charleston, Missouri, earthquake to derive adifferent set of
factors for NM2 and NM3.Hough et al. (2000) used the same
factors.
The method of Johnston (1996) yields inde-pendent estimates of
Mw from each isoseismalarea (MMI 4-8) from each event. To obtain
anaverage Mw for each event, one can estimateseismic moment, Mo
using the standard formu-la, log (Mo) = 1.5 Mw + 16.05, and compute
anaverage Mo value that we then translate it to Mw.
To investigate the uncertainties associatedwith each regression,
we apply a bootstrap analy-sis in which isoseismals are fit using
50 random-ly resampled sets of data points. For each intensi-ty,
the five most extreme results are discarded andbounds are estimated
from the remaining 45 sets.The uncertainty ranges resulting from
the boot-strap analysis are approximately ± 0.1-0.2 unitsfor NM1
and ± 0.2-0.4 for NM2 and NM3. Thereader is referred to Hough et
al. (2000) for amore thorough discussion of these results.
4. Interpretation
For NM1 the range from both regressions is0.3 units. For NM2 and
NM3, ranges of 0.35-0.7are inferred. However, considering the range
ofresults from both the Boolean and least squaresapproaches for
each event, one obtains uncer-tainties of approximately a full
magnitude unitfor all three events. Hough et al. (2000) con-cluded,
however, that the magnitudes of eachevent are constrained to better
than ± 0.5 magni-tude units. They concluded that while the
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Scientific overview and historical context of the 1811-1812 New
Madrid earthquake sequence
ly strengthening to «tremendous», which is thedescriptor Brooks
reserved for the most severelevels of shaking (McMurtrie, 1819).
Accordingto Brooks, the strongest shaking from NM3-Blasted only a
few seconds, suggestive of an eventin the midwest rather than the
New Madrid region.Daniel Drake also described the ground
motionsfrom NM3-A and NM3-B as having been qualita-tively different
from those caused by other events.Evaluating the distribution of
intensities with theJohnston (1996) regressions, one obtains
magni-tudes of ≈ 4.5 and 5.0-5.5 for NM3-A and NM3-B, respectively,
and locations well outside theNMSZ (fig. 3).
Event NM2-A, which followed NM2 by 4days, was apparently smaller
than NM3-A andNM3-B; Hough (2001) does not determine amagnitude for
this event. Additionally, a hand-ful of accounts describe the NM2
mainshock ashaving comprised multiple episodes of shakingwithin a
few minutes. While not definitive, thissuggests that remote
earthquakes of substantialsize could have been triggered in the
immediatewake of the S/surface wave arrivals generatedby NM2. Such
immediate triggering is oftenobserved following modern earthquakes
(e.g.,Hough and Kanamori, 2002).
In addition to the inferred remotely trig-gered earthquakes
discussed above, Hough andMartin (2002) analyzed accounts from a
large
events that would not have occurred other-wise.
Hough (2001) reexamined three of the large«aftershocks» of the
New Madrid sequence,events occurring at approximately 08:45
a.m.(LT) on 27 January 1812; 08:30 p.m. (LT) on 7February 1812; and
10:40 p.m. (LT) on 7 Feb-ruary 1812 (hereinafter, NM2-A, NM3-A,
andNM3-B, respectively; table I). The first of theseevents followed
NM2 by approximately fourdays; the second and third events occurred
thenight following the NM3 mainshock (whichoccurred at
approximately 03:45 a.m., LT).
Many of the accounts describe the shakingfrom NM3-B as «severe»
or «violent». The NM3-A event is generally described as less severe
thanNM3-B, but still strong. Two individuals experi-enced the New
Madrid sequence and endeavoredto not only document every event they
felt, but al-so to rank the events by severity of shaking:Daniel
Drake of Cincinnati, Ohio, and JaredBrooks, of Louisville, Kentucky
(see, McMurtrie,1819; Fuller, 1912). Brooks describes NM3-A
ashaving been, «violent in the first degree, but of tooshort
duration to do much injury». (He presum-ably means short in
relation to the shaking fromthe New Madrid mainshocks, which are
typicallydescribed as lasting for 2-4 min. in the Ohio-Ken-tucky
region). Brooks describes the shaking fromNM3-B as «violent in the
second degree», quick-
Table I. New Madrid sequence: mainshocks, principal aftershock,
and triggered events.
Event Year Month Day hr:min Long. Lat. Mw
NM1 1811 12 16 02:15 –90.0 36.0 7.2
NM1-A 1811 12 16 07:15 –89.5 36.3 7.0
NM1-B 1811 12 17 noon –89.2 34.6 6.0
NM2 1812 1 23 08:45 –89.7 36.6 7.0
NM2-A 1812 1 27 09:00 –84.0 38.9 NE
NM3 1812 2 7 03:45 –89.6 36.4 7.4
NM3-A 1812 2 7 20:30 –84.0 38.9 ≈ 4.5
NM3-B 1812 2 7 22:40 –84.0 38.9 5.0-5.5
Event; year, month, day; local time; crudely estimated longitude
and latitude in decimal-degrees north and west;preferred moment
magnitude estimate, NE: no estimate.
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532
Susan E. Hough
Fig. 3. Map showing inferred locations of principal and remotely
triggered earthquakes that occurred duringthe 1811-1812 «New
Madrid» sequence. Black stars indicate locations given in table I;
gray star indicates pos-sible source zone for event NM2 as proposed
recently by Mueller et al. (2004).
event that occurred near noon (LT) on 17 De-cember 1811. They
obtain a preferred magni-tude estimate of 6.0 and a location well
southof the NMSZ (table I; fig. 3).
6. Discussion and conclusions
The magnitude of the principal NewMadrid mainshocks has been the
matter ofsome debate in recent years. As is often thecase with
historic earthquakes, macroseismic
data provide the most direct constraint on mag-nitude. As
summarized in this paper, the key tointerpreting such data in this
case is twofold:
1) to assign intensity values based on objec-tive observations
such as damage to structuresrather than on the apparent drama of
anecdotalaccounts, and 2) to consider the earthquakes’ ef-fects in
light of their historic context. The pre-ferred estimates for the
moment magnitudes ofthe three principal events are Mw 7.2, ≈ 7,
and7.4-7.5, respectively, and 7.0 for the dawn after-shock. These
values are more consistent with
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533
Scientific overview and historical context of the 1811-1812 New
Madrid earthquake sequence
other lines of evidence, such as geomorphology,that provide
indirect constraint on event size.
The geometry and extent of the CottonwoodGrove Fault, which is
assumed to have generat-ed NM1, is established primarily from
recentmicroseismicity (e.g., Gomberg, 1993; John-ston and Schweig,
1996). Event NM2 is diffi-cult to analyze in any detail because the
in-ferred causative fault, the northern strike-sliplimb of the NMSZ
(Johnston, 1996), is the leastwell-understood part of the zone.
Also, al-though the hour of the event provided a
bettercharacterization of the low-intensity (MMI III-IV) field,
reporting of the event was likely ham-pered by a cold spell that
had frozen the riverand halted boat traffic along the Ohio River
andMississippi River until 22 January 1812. Veryrecently, Mueller
et al. (2004) have reexaminedthis event and questioned whether it
occurredwithin the NMSZ at all. At a minimum, thisevent remains the
least well-understood of thethree principal mainshocks.
However, recent investigations have provid-ed significant
constraint of the Reelfoot Fault,the thrust fault in between the
two strike-sliplimbs of the NMSZ that is inferred to have pro-duced
NM3 (e.g., Russ, 1982; Kelson et al.,1992; Johnston, 1996).
Structure of theReelfoot fault has been elucidated in recentyears
with seismic reflection profiling. Odum etal. (1998) infer an
overall fault length of at least30 km and constrain the dip to be ≈
31°. Morerecently, Champion et al. (2001) concluded thatthe
Reelfoot fault does not exhibit clear geo-morphic expression to the
southeast of thenorthern terminus of the Cottonwood Grovefault.
Given their inferred spatial extent of theactively deforming
Reelfoot fault, they con-clude the fault could host plausibly a
low- tomid-Mw 7 earthquake. Gomberg (1993) reachedsimilar
conclusions based on the fault area esti-mated from current
microseismicity.
Although no direct measurements of faultscarp height are
available for NM3, contempo-rary accounts from boats on the
Mississippi de-scribe waterfalls forming on the river. As
dis-cussed by Odum et al. (1998), these observa-tions correspond to
points where the inferredfault rupture crossed the river. The
height ofthese waterfalls is not well constrained, although
some information can perhaps be gleaned fromavailable reports.
In light of these accounts andthe established geometry of the
Reelfoot Fault,one obtains an average slip of 4-5 m.
One can account for a Mw of 7.5 on theReelfoot fault with
plausible rupture parame-ters: a length L of 40 km, a width W of 30
km(consistent with the maximum depth of micro-seismicity, see
Gomberg, 1993), and an averageslip D of 5.0 m. These rupture
parameters areconsistent with established scaling
relationshipsderived from worldwide events (Wells and Cop-persmith,
1994). Given a rupture length of 40km and an average slip of 5 m,
one obtains Mw6.9 and Mw 7.5 from the scaling relationshipsfor
rupture length versus magnitude and slipversus magnitude,
respectively.
The faulting parameters illustrated in fig. 1are thus consistent
with the lateral and depth ex-tent of the NMSZ faults as inferred
from micro-seismicity. The static stress drop values are
alsoconsistent with those inferred from finite fault in-versions of
more recent large earthquakes (e.g.,Hough, 1996). Clearly one
cannot prove that theNew Madrid earthquakes did not have
excep-tionally high static stress drop values. However,Hanks and
Johnston (1992) showed that high-frequency shaking, and thus
isoseismal areas,will depend on stress drop. Thus if the NewMadrid
events had higher stress drop values thanthe average of those used
to obtain the regressionresults of Johnston (1996), the magnitudes
of theNew Madrid earthquakes would be lower thanthe estimates
derived from these results.
Because the regression results of Johnston(1996) were calibrated
with similarly subjectivedata, one critical question is the extent
to whichthe Hough et al. (2000) assignments are consis-tent with
those on which the regressions werebased. To answer this question,
one must con-sider both our general approach to the MMI
as-signments as well as our treatment of site re-sponse issues. In
general, there is some prece-dent for keying an MMI assessment on
the mostdramatic effects described. However, consider-ing the MMI
assignments made for the 1968mb 5.3 southcentral Illinois
earthquake (Gor-don et al., 1970) as an example, it is clear thatan
MMI of VI is typically assigned when thereare multiple instances of
the damage usually as-
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534
Susan E. Hough
sociated with this level of intensity: brokenwindows, cracked
plaster, damage to brickchimneys, etc. At some locations the
specificreport suggesting a high MMI value in the NewMadrid
sequence is one that suggests relativelylong-period shaking
effects. There is ampleprecedent for not assigning an MMI
valuebased on such a report when the effects relatedto
higher-frequency shaking (i.e. toppling ofsmall objects and
furniture) indicate a muchlower value (e.g., Armbruster and
Seeber,1987).
In general, there is a fundamental distinc-tion between the
1811-1812 New Madridevents and those used by Johnston (1996) to
de-rive the isoseismal area-moment magnitude re-gressions: the
latter events are those for whichinstrumental magnitudes are
available, whichmeans they are from the 1900s (1925 onward).The New
Madrid sequence is upward of 100years less recent, and so its
collection of felt re-ports is considerably more sparse than the
oth-ers. Systematic differences in sampling of siteconditions can
clearly introduce substantial bi-ases. In 1811-1812, logistical
constraints in-duced most of the population to live along
riverbanks (or coasts), which are often characterizedby alluvial
near-surface geological conditions.Later in the nineteenth century,
the introductionof round transportation allowed settlement toshift
to higher ground, away from potentialflooding hazard.
Sediment-induced amplifica-tion is therefore much more likely to
affect re-ports from the early part of the nineteenth cen-tury than
those from the twentieth century (oreven the mid-nineteenth
century). Although thisprobably results in a systematic bias in
the1811-1812 intensity data, I do not correct for itsystematically
in our assignment of MMI val-ues. It would, indeed, not be
appropriate to«correct» MMI values for site response andthen apply
the Johnston (1996) regressions be-cause the MMI data used to
derive the regres-sions were not similarly corrected.
I have, however, addressed the issue of siteresponse in two
ways: i) by revising the MMIassignments where contemporary accounts
dodocument significant site response, which weview as consistent
with the usual practice of as-signing site-specific MMI values
based on site-
specific information, and ii) by using judge-ment in choosing
preferred isoseismal contours.
The subjective contouring approach is con-sidered to be the most
reasonable proxy for theideal procedure, which one is unable to do
inthis case: to allow the MMI values to define theshape of the
contours, with clear definition ofhigh-intensity lobes. We conclude
that it wouldclearly be inappropriate to allow the contours
to«balloon» out, as was done by Nuttli (1973),based on values that
surely represent «spokes»of anomalously high ground motions.
A systematic site response correction couldbe done via a careful
consideration of intensitydistributions from more recent events.
Hopperet al. (1983) present a map of isoseismals ex-pected from a
repeat of a New Madrid main-shock in which site response is
included im-plicitly. I note, however, that site corrections forthe
1811-1812 data would require a very de-tailed analysis because
settlement patternschanged so drastically in the decades
followingthe New Madrid sequence.
Hough et al. (2000) focused on the moder-ate intensity contours
because their isoseismalareas are the critical inputs to the
area-basedmagnitude determination method of Johnston(1996).
Isoseismal contours for MMI levels IV-VII can be constrained by
relatively objectivereports of damage to structures and the
percep-tions of individuals who (it can generally be as-sumed) were
asleep at the time of events NM1and NM3. The felt reports closer to
the NewMadrid seismic zone are relatively incontro-vertible in
documenting the extent of damageand ground failure. However,
interpretation ofthese reports is greatly complicated by the
vastextent of poorly consolidated and largely water-saturated
sediments within the Mississippi em-bayment. Once again, the
natural settlementpatterns would have resulted in a strong
corre-lation between population density and proximi-ty to the
Mississippi River.
According to the results of Hough et al.(2000), NM1 was also
similar in magnitude to the1886 Charleston, South Carolina,
earthquake,which Johnston (1996) estimates to have been Mw 7.3.
Although perhaps at odds with «conven-tional wisdom» regarding the
relative sizes of thetwo events, we note a striking degree of
reciproc-
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Scientific overview and historical context of the 1811-1812 New
Madrid earthquake sequence
ity between our results and the isoseismal con-tours from the
Charleston event determined byBollinger (1977). Both events
generated values ofMMI ≈ V for areas midway between NewMadrid and
Charleston, and Charleston generat-ed a small swatch of MMI ≈ VI
values in the im-mediate vicinity of New Madrid.
Clearly, magnitude estimates for the NewMadrid mainshocks will
always be plagued by acertain level of uncertainty; a level that
is, more-over, difficult to even quantify. We argue that thecentral
issue is not one of precisely determinedmathematical uncertainties
but rather overall con-sistency and credibility. Magnitudes of
7.2-7.3,7.0, and 7.4-7.5 for the three principal main-shocks, NM1,
NM2, and NM3, respectively, areconsistent with both known and
plausibly in-ferred faulting parameters and the shaking
distri-bution as inferred from our reinterpreted MMIvalues.
Magnitude values significantly lower thanthese estimates strain
credulity for two reasons: i)comparisons with more recent,
better-con-strained events, such as the 1886 Charleston,South
Carolina, and 1926 Grand Banks earth-quakes (Bent, 1995), and ii)
the evidence, dis-cussed above, that NM3 was associated
withsignificant surface faulting.
On the other hand, significantly larger mag-nitude values are
problematic for reasons thathave been addressed at length in other
studies;primarily, the lack of sufficient fault area andslip to
generate three separate events with Mwclose to 8.0. (Although one
could plausibly ar-gue for a greater depth extent of large
earth-quake ruptures in the NMSZ, even a factor of 2increase in
fault width would increase Mw esti-mates by only 0.3 units).
One interesting consequence of our reinter-pretation concerns
the relative magnitudes ofthe three principal mainshocks. In the
interpre-tation of Johnston (1996), NM1 is larger thanNM3, and NM2
is of appreciable size. In con-trast, our results reveal NM3 to be
substantiallylarger than the other two. This implies thatrather
than being a mostly strike-slip systemwith thrust faulting
associated with a compres-sional stepover, thrust faulting may have
beenthe dominant mechanism associated with the1811-1812 New Madrid
sequence. At least, thiswould be the case if the mechanism of
NM3
was predominantly thrust, as has generally beeninferred. A
predominant thrust mechanism isconsistent with the hypothesis that
post-glacialrebound provides the driving force for large
lateHolocene earthquakes in the NMSZ (e.g., Wuand Johnston,
2000).
Although the results of Hough et al. (2000)represent a
«down-grading» of the magnitudesof the principal New Madrid
mainshocks, sev-eral lines of evidence argue for substantial
dis-tributed hazard throughout the North Americanmid-continent.
First, the hazard is a function ofthe expected ground motions,
which, in the caseof the New Madrid sequence, appear to havebeen
significantly elevated in many cases bysite response. An evaluation
of site responsemay therefore be critical for seismic hazard
as-sessment at many locations in the central/east-ern United
States, particularly those immediate-ly adjacent to major rivers
and the Atlanticseaboard. Secondly, remotely triggered earth-quakes
potentially represent an additionalsource of distributed hazard.
Finally, the recentearthquake history of western India reveals
thatlarge earthquakes can occur close together intime, not on the
same fault but on neighboringfaults (e.g., Hough and Bilham, 2003).
Al-though the 1811-1812 New Madrid sequenceprovides a unique and
critically important dataset, a more thorough investigation of
potentialneighboring and regional source zones in themidcontinent
appears to be warranted.
Acknowledgements
I thank Jim Dewey, Lucy Jones, HirooKanamori, Bill Bakun, Jim
Dolan, Paul Bodin,Ruth Harris, and Jerry Hough, for helpful
com-ments and suggestions. I also appreciatively ac-knowledge the
constructive criticism from twoanonymous reviewers, as well as
substantial ef-fort on the part of the editors to make this vol-ume
happen.
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