Scientific overview and historical context of the 1811-1812 …...New Madrid sequence of 1811-1812: three principal mainshocks and the so-called «dawn aftershock» following the first

523

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]

524

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-

525

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.

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-

527


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,

528

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

529


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

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

530

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

531


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.

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

533


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-

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-

535


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.

REFERENCES

ARMBRUSTER, J. and L. SEEBER (1987): Seismicity 1886-1889 in the southeastern United States: The aftershocksequence of the Charleston, S.C. earthquake, U.S. Nu-

536

Susan E. Hough

cl. Regul. Comm., Washington, DC, Rep. NUREG/CR-4851, pp. 153.

BENT, A. (1995): A complex double couple source mecha-nism for the Ms 7.2 1929 Grand Banks earthquake,Bull. Seismol. Soc. Am., 85, 1003-1020.

BOLLINGER, G.A. (1977): Reinterpretation of the intensitydata for the 1886 Charleston, South Carolina, earth-quake, U.S. Geol. Surv. Prof. Pap. 1028, 17-32.

BRACKENRIDGE, H.M. (1817): Views of Louisiana: Contain-ing Geographical, Statistical and Historical Notices ofthat Vast and Important Portino of America (Schaefferand Maund, Baltimore), Md.

BRADBURY, J. (1819): Travels in the Interior of America inthe Years 1809, 1810, and 1811 (Sherwood, Neely, andJones, London).

CHAMPION, J, K. MUELLER, A. TATE and M. GUCCIONE(2001): Geometry, numerical models and revised sliprate for the Reelfoot Fault and trishear fault-propaga-tion fold, Eng. Geol., 62, 31-49.

DRAKE, D. (1815): Natural and Statistical View, or Pictureof Cincinnati and the Miami County, Illustrated byMaps (Looker and Wallace, Cincinnati).

FRANKEL, A., C. MUELLER, T. BARNHARD, D. PERKINS, E.V.LEYENDECKER, N. DICKMAN, S. HANSON and M. HOP-PER (1996): National seismic hazard maps: documenta-tion, U.S. Geol. Surv. Open File Rep. 96-532, pp. 69.

FULLER, M.L. (1912): The New Madrid earthquakes, U.S.Geol. Surv. Bull., 494.

GOMBERG, J.S. (1993): Tectonic deformation in the NewMadrid seismic zone: Inferences from map view andcross-sectional boundary element models, J. Geophys.Res., 98, 6639-6664.

GOMBERG, J. and S. DAVIS (1996). Strain changes and trig-gered seismicity at The Geysers, California, J. Geo-phys. Res., 101, 733-749.

GORDON, D.W., T.J. BENNETT, R.B. HERRMANN and A.M.ROGERS (1970): The south-central Illinois earthquakeof November 9, 1968: Macroseismic studies, Bull.Seismol. Soc. Am., 60, 953-971.

HANKS, T.C. and A.C. JOHNSTON (1992): Common featuresof the excitation and propagation of strong ground mo-tion for North-American earthquakes, Bull. Seismol.Soc. Am., 82, 1-23.

HILL, D.P., P.A. REASENBERG, A. MICHAEL, W.J. ARABAZ, G.BEROZA, D. BRUNMBAUGH, J.N. BRUNE, R. CASTRO, S.DAVIS, D. DEPOLO, W.L. ELLSWORTH, J. GOMBERG, S.HARMSEN, L. HOUSE, S.M. JACKSON, M.J.S. JOHNSTON,L. JONES, R. KELLER, S. MALONE, L. MUNGUIA, S. NA-VA, J.C. PECHMANN, A. SANFORD, R.W. SIMPSON, R.B.SMITH, M. STARK, M. STICKNEY, A. VIDAL, S. WALTER,V. WONG and J. ZOLLWEG (1992): Seismicity remotelytriggered by the magnitude 7.3 Landers, California,earthquake, Science, 260, 1617-1623.

HOPPER, M.G., S.T. ALGERMISSEN and E.E. COBROVOLNY(1983): Estimation of earthquake effects associatedwith a great earthquake in the New Madrid seismiczone, U.S. Geol. Surv. Open File Rep. 83-179, pp. 94.

HOUGH, S.E. (1996): Observational constraints on earth-quake source scaling: Understanding the limits in reso-lution, Tectonophysics, 261, 83-96.

HOUGH, S.E. (2001): Triggered earthquakes and the 1811-1812 New Madrid, central US earthquake sequence,Bull. Seismol. Soc. Am., 91, 1574-1581.

HOUGH, S.E. and R. BILHAM (2003). A shock heard ‘roundthe world, Natural History, February.

HOUGH, S.E. and H. KANAMORI (2002): Source propertiesof earthquakes near the Salton Sea triggered by the10/16/1999 M 7.1 Hector Mine, California earthquake,Bull. Seismol. Soc. Am., 92, 1281-1289.

HOUGH, S.E. and S. MARTIN (2002): Magnitude estimates oftwo large aftershocks of the 16 December 1811 NewMadrid earthquake, Bull. Seismol. Soc. Am., 92, 3259-3268.

HOUGH, S.E., J.G. ARMBRUSTER, L. SEEBER and J.F. HOUGH(2000): On the Modified Mercalli Intensities and Mag-nitudes of the 1811-1812 New Madrid, Central UnitedStates earthquakes, J. Geophys. Res., 105, 23, 839-23,864.

JOHNSTON, A.C. (1996): Seismic moment assessment ofearthquakes in stable continental regions III, NewMadrid 1811-1812, Charleston 1886, and Lisbon 1755,Geophys. J. Int., 126, 314-344.

JOHNSTON, A.C. and E.S. SCHWEIG (1996): The enigma ofthe New Madrid earthquakes of 1811-1812, Annu. Rev.Earth Planet. Sci. Lett., 24, 339-384.

KELSON, K.I., G.D. SIMPSON, R.B. VANARSDALE, C.C. HA-RADEN and W.R. LETTIS (1992): Multiple Holoceneearthquakes along the Reelfoot fault, central NewMadrid seismic zone, J. Geophys. Res., 101, 6151-6170.

MCMURTRIE, H. (1819): Sketches of Louisville and its envi-rons; including, among a great miscellaneous matter, aFlorula Louisvillensis; or, a catalogue of nearly 400Genera and 600 Species of plants, that grow in thevicinty of the town, exhibiting their generic, specific,and vulgar English names, S. Penn, Jun. Main-street,Louisville.

MILNE, J. (1898): Seismology (Kegan Paul, Trench, Trubn-er and Co. Ltd., London), 1st edition, pp. 320.

MITCHILL, S.L. (1815): A detailed narrative of the earth-quakes which occurred on the 16th day of December,1811, Trans. Lit. Philos. Soc. NY, 1, 281-307.

MUELLER, K., S.E. HOUGH and R. BILHAM (2004):Analysing the 1811-1812 New Madrid earthquakeswith recent instrumentally recorded aftershocks, Na-ture, 429, 284-288.

NUTTLI, O.W. (1973): The Mississippi Valley earthquakesof 1811 and 1812: Intensities, ground motion, andmagnitudes, Bull. Seismol. Soc. Am., 63, 227-248.

ODUM, J.K., W.J. STEPHENSON, and K.M. SHEDLOCK (1998).Near-surface structural model for deformation associ-ated with the February 7, 1812 New Madrid, Missouri,earthquake, Geol. Soc. Am. Bull., 110, 149-162.

PENICK, J.L., Jr. (1981): The New Madrid Earthquakes(Univ. of Mo. Press, Columbia), rev. edition.

ROZIER, F.A. (1890): Rozier’s History of the Early Settle-ment of the Mississippi Valley (G.A. Pierrot and Son,Saint Louis).

RUSS, D.P. (1982): Style and significance of surface defor-mation in the vicinità of New Madrid, Missouri, in In-vestigations of the New Madrid, Missouri, earthquakeregion, edited by F.A. MCKEOWN, L.C. PAKISER, U.S.Geol. Surv. Prof. Pap. 1236, 95-114.

SEEBER, L., and J.G. ARMBRUSTER (1987): The 1886-1889aftershocks of the Charleston, South Carolina, earth-quake: a widespread burst of seismicity, J. Geophys.Res., 92, 2663-2696.

537


SINGH, S.K., J. LERMO, T. DOMINGUEZ, M. ORDAZ, J.M. ES-PINOSA, E. MENA and R. QUASS (1988): The Mexicoearthquake of September 19, 1985 – A study of amplifi-cation of seismic waves in the Valley of Mexico with re-spect to a hill zone site, Earthquake Spectra, 4, 653-673.

STOVER, C.W. and J.L. COFFMAN (1993): Seismicity of theUnited States, 1568-1989, U.S. Geol. Surv. Prof. Pap.1527 (revised).

STOVER, C.W. and C.A. VON HAKE (Editors) (1982): Unit-ed States Earthquakes, 1980, (US Dep. of Inter., Gold-en, Colo.).

STREET, R. (1982): A contribution to the documentation ofthe 1811-1812 Mississippi Valley earthquake sequence,Earthquake Notes, 53, 39-52.

STREET, R. (1984): The historical seismicity of the centralUnited States: 1811-1928, Final Report, Contract 14-08-0001-21251, Appendix A, U.S. Geol. Surv., Wash-ington, DC, pp. 316.

TAYLOR, K., W. STAUDER and R. HERRMANN (1991): A com-prehensive unified data set for the New Madrid, seis-mic array, Seismol. Res. Lett., 62, 187.

TUTTLE, M.P. and E.S. SCHWEIG (1996): Archaeological andpedological evidence for large prehistoric earthquakesin the New Madrid seismic zone, Central United States,Geology, 23, 253-256.

TUTTLE, M.P., E.S. SCHWEIG, J.D. SIMS, R.H. CAFFERTY,L.W. WOLF and M.L. HAYNES (2002): The earthquakepotential of the New Madrid seismic zone, Bull. Seis-mol. Soc. Am., 92, 2080-2089.

WELLS, D.L. and K.J. COPPERSMITH (1994): New empiricalrelationships among magnitude, rupture length, rupturewidth, rupture area, and surface displacement, Bull.Seismol. Soc. Am., 84, 974-1002.

WU, P. and P. JOHNSTON (2000): Can deglaciation triggerearthquakes in North America?, Geophys. Res. Lett.,27, 1323-1326.

Documents

Scientific overview and historical context of the 1811-1812 …...New Madrid sequence of 1811-1812: three principal mainshocks and the so-called «dawn aftershock» following the first