New Madrid seismic zone fault geometry

Ryan Csontos Ground Water Institute, University of Memphis, Memphis, Tennessee 38152, USA Roy Van Arsdale Department of Earth Sciences, University of Memphis, Memphis, Tennessee 38152, USA

ABSTRACT active area in the eastern United States and also on an old fault (Pratt, 1994; Schweig and Ellis, because it experienced at least three devastat- 1994; Van Arsdale, 2000). The New Madrid seismic zone of the cen- ing earthquakes in 1811–1812 (Nuttli, 1982; In this study we defi ne the geometry of New tral Mississippi River valley has been inter- Penick, 1981; Johnston, 1996; Johnston and Madrid seismic zone fault planes by analyz- preted to be a right-lateral strike-slip fault Schweig, 1996; Tuttle et al., 2002) (Fig. 1). This ing the distribution of earthquake hypocenters zone with a left stepover restraining bend seismicity has been attributed to reactivation of as has been done in studies conducted along (Reelfoot reverse fault). This model is overly basement faults within the subsurface Cambrian the San Andreas fault system (e.g., Carena and simplistic because New Madrid seismicity Reelfoot rift (Zoback, 1979; Kane et. al, 1981; Suppe, 2004; Pujol et al., 2006) and previously continues 30 km southeast of the stepover. Braile et. al, 1986; Thomas, 1989; Dart and along the Reelfoot fault (Chiu et al., 1992; Liu, In this study we have analyzed 1704 earth- Swolfs, 1998). The Reelfoot rift (Mississippi 1997; Pujol et al., 1997; Mueller and Pujol, quake hypocenters obtained between 1995 Valley Graben) is a major basement structure 2001). In this study, however, we use an addi- and 2006 in three-dimensional (3-D) space to mapped from gravity, magnetic, seismic refrac- tional 1704 earthquake hypocenters from 1995 more accurately map fault geometry in the tion, seismic refl ection, and a few deep petro- through 2006 and state-of-the-art, 3-D viewing New Madrid seismic zone. Most of the earth- leum exploration wells (Fig. 2) (Hildenbrand, capabilities to better evaluate their distribution. quakes appear to align along fault planes. 1982; Hildenbrand et al., 1982; Hamilton and When viewing New Madrid seismic zone earth- The faults identifi ed include the New Madrid Zoback, 1982; Howe and Thompson, 1984; quake hypocenters with stereoscopic 3-D soft- North (29°, 72° SE), Risco (92°, 82° N), Axial Howe, 1985; Hildenbrand and Hendricks, 1995; ware, it is evident that the New Madrid seismic (46°, 90°), Reelfoot North (167°, 30° SW), Dart and Swolfs, 1998; Parrish and Van Arsdale, zone earthquakes occur within discrete zones and Reelfoot South (150°, 44° SW) faults. A 2004; Csontos et al., 2008) that is covered by up (fault planes). Delineation of these fault planes diffuse zone of earthquakes exists where the to 6 km of Phanerozoic sediments. New Madrid provides geometric constraints for future kine- Axial fault divides the Reelfoot fault into the seismic zone seismicity is occurring along these matic analysis of the Reelfoot rift fault system Reelfoot North and Reelfoot South faults. rift faults primarily between depths of 4 and and its earthquakes (e.g., Gomberg and Ellis, Regional mapping of the top of the Precam- 14 km within the Cambrian and Precambrian 1994; Johnston and Schweig, 1996). As pre- brian crystalline basement indicates that section (Chiu et al., 1992; Pujol et al., 1997; Van sented below, we also believe that the Reelfoot the Reelfoot North fault has an average of Arsdale and TenBrink, 2000). The seismically rift illustrates the association of structural inver- 500 m and the Reelfoot South fault 1200 m of most active fault within the New Madrid seismic sion and intraplate seismicity that may apply to down-to-the-southwest normal displacement. zone is the Reelfoot reverse fault that extends other intraplate seismic zones. Since previously published seismic refl ec- northwest from near Dyersburg, Tennessee, to tion profi les reveal reverse displacement on New Madrid, Missouri (Figs. 1 and 2) (Van Ars- Geology of the New Madrid Seismic Zone top of the Paleozoic and younger strata, the dale et al., 1995a, 1999). A surface scarp (mono- Region Reelfoot North and South faults are herein clinal fl exure) due to Reelfoot fault propagation interpreted to be inverted basement nor- folding (Champion et al., 2001) can be traced The southeastern United States is under- mal faults. The Reelfoot North and Reelfoot from the southwestern margin of Reelfoot Lake, lain by several Precambrian terranes welded South faults differ in strike, dip, depth, and Tennessee, where it is ~3 m high north to a together to form the North American craton. The displacement, and only the Reelfoot North maximum of 10 m where the scarp is truncated Precambrian terrane that underlies the Reelfoot fault has a surface scarp (monocline). Thus, by the Mississippi River, to less than 3 m high rift is the 1470 ± 30 Ma Eastern Granite Rhyo- the Reelfoot fault is actually composed of two where the scarp is again truncated by the Mis- lite Province (Atekwana, 1996; Van Schmus et left-stepping restraining bends and two faults sissippi River near New Madrid, Missouri (Van al., 1996, 2007). Drill-hole samples reveal that that together extend across the entire width Arsdale et al., 1999) (Fig. 3). Holocene uplift these rocks consist of granite, granite porphyry, of the Reelfoot rift. rate on the Reelfoot fault is 1.8 mm/yr (Van and dioritic gneiss (Thomas, 1988; Dart, 1992; Arsdale, 2000), and global positioning system Dart and Swolfs, 1998). INTRODUCTION (GPS) data reveal a horizontal convergence rate The Reelfoot rift (Fig. 2) is interpreted to be across the fault of 2.7 mm/yr (Smalley et. al, a Cambrian intracratonic rift associated with the The New Madrid seismic zone, beneath the 2005). These high rates have been explained as opening of the Paleozoic Iapetus Ocean (Ervin central Mississippi River valley, is undergoing Holocene initiation of the New Madrid seismic and McGinnis, 1975; Thomas, 1989, 2006). considerable study as it is the most seismically zone or as a burst of Holocene seismic activity Reelfoot rifting occurred along northeast-striking

Geosphere; October 2008; v. 4; no. 5; p. 802–813; doi: 10.1130/GES00141.1; 6 fi gures; 1 animation.

802 For permission to copy, contact [email protected] © 2008 Geological Society of America New Madrid seismic zone fault geometry lored dots with stars at esti- lored mated locations of the 16 December 1811 (south), 23 January 1812 (north), and 7 February (central) earthquakes. 1811 mated locations of the 16 December Figure 1. Reelfoot rift bounded by black lines and Mississippi embayment bounded by purple line. Microearthquake activity as co 1. Reelfoot rift bounded by black lines and Mississippi embayment purple line. Microearthquake Figure

Geosphere, October 2008 803 Csontos and Van Arsdale

Elevation (m) 005500110000kkmm

RCG 37° -90° -89° -91°

NM

M E

36°

M W

W F R A FZ 35°

Figure 2. Precambrian crystalline basement unconformity structure contour map with basement faults. Reelfoot rift extends from White River Fault Zone (WRFZ) to Reelfoot fault (RF). Contour interval—400 m; elevation relative to sea level. GRTZ—Grand River Tectonic Zone; CMTZ—Central Missouri Tectonic Zone; OFZ—Osceola Fault Zone; BMTZ—Bolivar-Mansfi eld Tectonic Zone; WRFZ—White River Fault Zone; EM—Eastern Rift Margin faults; AF—Axial fault; WM—Western Rift Margin fault; RF—Reelfoot fault; NM—New Madrid; RCG—Rough Creek Graben (from Csontos et al., 2008).

normal faults (Howe and Thompson, 1984; and Van Arsdale, 1999; Csontos et al., 2008). Lake Saint Francis Sunklands area, and Joiner Nelson and Zhang, 1991) that are segmented Quaternary displacement has been documented Ridge are interpreted to be tectonic or tectoni- by northwest-striking vertical faults (Stark, along the Eastern Rift Margin (Cox et al., 2001, cally infl uenced landforms (Fig. 4) (Csontos 1997; Csontos et al., 2008). During rifting, the 2006), Western Rift Margin (Van Arsdale et al., et al., 2008). New Madrid seismic zone faults Reelfoot rift accumulated up to 8 km of Cam- 1995b; Baldwin et al., 2005), Axial (Van Ars- have also modifi ed Mississippi River gradients brian sediment, while outside the rift 1.5 km of dale, 1998; Guccione et al., 2000), and Reelfoot and infl uenced sedimentary processes during Cambrian sediment accumulated (Howe and fault (Russ, 1982; Kelson et al., 1996; Mueller the Quaternary (Spitz and Schumm, 1997; Guc- Thompson, 1984). Late Cambrian to Middle et al., 1999; Van Arsdale et al., 1999; Cham- cione et al., 2002; Guccione, 2005; Holbrook et Ordovician regional subsidence of the rift and pion et al., 2001). In addition, the Lake County al., 2006). Johnston and Schweig (1996) have surrounding area (Howe and Thompson, 1984; uplift (essentially coincident with the seismic- proposed that the Bootheel lineament produced Howe, 1985; Dart and Swolfs, 1998) resulted in ity between the central and northern 1811–1812 one of the major earthquakes of the 1811–1812 deposition of the thick, shallow-marine Ordo- earthquakes in Fig. 1), Reelfoot Lake basin sequence, and Guccione et al. (2005) report vician Knox (Arbuckle) carbonate Supergroup (Fig. 3), a segment of the eastern Mississippi Val- Quaternary displacement on the Bootheel fault (Thomas, 1985; Lumsden and Caudle, 2001). ley bluff line (located in westernmost Tennessee (lineament). However, Csontos (2007) mapped Tectonic landforms within the central Mis- between the dark-green uplands and pale-green the Precambrian basement unconformity and sissippi River valley are directly linked to the Mississippi River fl oodplain in Fig. 1), south- the Pliocene-Pleistocene unconformity surfaces underlying Reelfoot rift faults (Fig. 3) (Mihills ern half of Crowley’s Ridge, the Big Lake and within the Reelfoot rift, and there is no evidence

804 Geosphere, October 2008 New Madrid seismic zone fault geometry

Figure 3. Pliocene-Pleistocene unconformity on top of the Eocene section with vertical projection of Reelfoot rift faults from the top of the Precambrian unconformity. Contour interval—2 m; elevation relative to sea level. B—Big Lake; S—Lake Saint Francis; J—Joiner Ridge; GRTZ—Grand River Tectonic Zone; CMTZ—Central Missouri Tectonic Zone; OFZ—Osceola Fault Zone; BMTZ—Bolivar-Mansfi eld Tectonic Zone; WRFZ—White River Fault Zone; EM—Eastern Rift Margin faults; AF—Axial fault; WM—Western Rift Margin fault; RF—Reelfoot fault; CR—Crowley’s Ridge (from Csontos et al., 2008).

Geosphere, October 2008 805 Csontos and Van Arsdale

Crowley’s Ridge

Restraining Bends

Uplifted Block

Dropped Block

Possible Subsidence Axis Earthquake

Figure 4. Deformation of the Pliocene-Pleistocene unconformity surface. Black lines—faults; J—Joiner Ridge; GRTZ—Grand River Tectonic Zone; CMTZ—Central Missouri Tectonic Zone; OFZ—Osceola Fault Zone; BMTZ—Bolivar-Mansfi eld Tectonic Zone; WRFZ—White River Fault Zone; EM—Eastern Rift Margin faults; AF—Axial fault; WM—Western Rift Margin fault; RF—Reel- foot fault. Inset map showing a restraining bend (from Csontos et al., 2008).

806 Geosphere, October 2008 New Madrid seismic zone fault geometry of vertical displacement or hypocenter align- nique removes the effect of the unconsolidated et al., 1997; Van Arsdale et al., 1999; Mueller ment along the Bootheel fault. This lack of ver- sediments of the Mississippi embayment, which and Pujol, 2001). The third major seismicity tical displacement may be due to a very young changes in thickness under the two networks. trend, called the New Madrid North fault (John- Bootheel fault experiencing relatively minor This change in thickness and the low P- and ston and Schweig, 1996; Baldwin et al., 2005) displacement, and/or the displacement is strike- S-wave velocities of the post Paleozoic Missis- appears to occur along a segment of the north- slip. However, in this study we focus on seis- sippi embayment sediments introduce errors in western Reelfoot rift margin. A closer look at mogenic faults and identifi ed deformation and the earthquake locations when the earthquakes the earthquake hypocenters reveals that there therefore do not consider a possible aseismic are located individually. The error in depth, in is also a seismicity trend extending west from Bootheel fault. particular, may be as large as 1 km. The errors New Madrid herein called the Risco fault for the that may affect JHD locations have been esti- town of Risco, Missouri, and that the Reelfoot New Madrid Seismic Zone Faults mated by analyzing synthetic data generated fault actually consists of three discrete, seismi- for a velocity model that includes the variations cally defi ned segments (Fig. 5). Many of the Reelfoot rift faults (Fig. 2) have in thickness of the unconsolidated sediments Five faults and a diffuse earthquake zone were large normal displacements within the crystal- (Pujol et al., 1997). Based upon this JHD analy- delineated based upon 3-D earthquake hypo- line basement and Cambrian section (Howe sis, it was found that the average depth errors are center trend surface analyses (Animation 1). 1985; Parrish and Van Arsdale, 2004; Csontos likely to be less than 0.5 km, with the epicentral The Axial fault (r2 = 0.9752) is essentially verti- et al., 2008). The Western Rift Margin fault is a error considerably smaller. cal, and so a regression line fi t to the earthquakes steeply eastward-dipping normal fault, displac- Earthquake hypocenters were imported into defi nes a fault that strikes 46°. The New Madrid 2 ing the Precambrian surface up to 3 km (Howe, Landmark’s GeoprobeTM and Schlumberger’s North fault strikes 29° and dips 72° SE (r =

1985; Nelson and Zhang, 1991). The Eastern PetrelTM for 3-D point rendering using a per- 0.9452), and the Risco fault strikes 92° and dips Rift Margin faults consist of two down-to-the- sonal computer (PC) workstation. Active stereo 82° N (r2 = 0.49). Planes were fi t to the two seg- northwest steep normal faults with as much as vision in GeoprobeTM and PetrelTM was achieved ments of the Reelfoot fault—the Reelfoot North 3 km of normal displacement on the western using Sharper Technology’s CrystalEyes stereo fault, 167°, 30° SW (r2 = 0.4485), and Reelfoot fault, and 0.5 km on the eastern fault (Howe, goggles and emitter. The 3-D image was also South fault, 150°, 44° SW (r2 = 0.9121). A diffuse 1985; Parrish and Van Arsdale, 2004). projected onto a screen for large-format viewing zone of seismicity, with no apparent structure, Several seismogenic faults have been mapped using a DepthQ Stereo3D projector. Earthquake divides the Reelfoot North and Reelfoot South within the Reelfoot rift (Figs. 1–3). The Axial hypocenters appearing to lie on planes (as vis- faults at the intersection with the Axial fault. fault, which trends down the center of the rift, ible in the 3-D model) were placed into separate is a near vertical fault (Howe, 1985; Sexton and data subsets. These subsets were exported to DISCUSSION

Jones, 1986; Stephenson et al., 1995; Csontos, ArcMapTM and ZmapTM, and a fi rst-order trend 2007). The southwest-dipping Reelfoot reverse surface (fault plane) was calculated for each The Reelfoot Rift and the Reelfoot Fault fault produces most of the New Madrid seismic hypocenter subset. In this way, location, strike, zone seismicity (Sexton, 1988; Chiu et al., 1992; dip, and r2 value (a statistical measure of how Based primarily on gravity and magnetic Pujol et al., 1997; Mueller and Pujol, 2001). well the plane fi ts the hypocenter distribution) data (Hildenbrand et al., 1982), the Reelfoot The Reelfoot fault appears to be a segment of were calculated for each fault plane in the New rift steps up (shallows) at the Reelfoot fault and the Grand River tectonic zone, and the Reelfoot Madrid seismic zone. The faults and earthquakes appears to close to the north toward the Rough fault has been interpreted to form the northern were then imported into a 3-D GeoprobeTM and Creek graben (Fig. 2). This step is interpreted to margin of the Reelfoot rift (Fig. 2) (Csontos et PetrelTM scene to ensure goodness of fi t to the be the result of Cambrian down-to-the-south- al., 2008). The third major fault along which calculated plane and to determine if there were west displacement on the Grand River tectonic earthquakes are occurring is the New Madrid any outliers. Any obvious hypocenter outliers zone (Csontos et al., 2008). Since the Reelfoot North fault (Baldwin et al., 2005), which is a were removed from the subset, and a fi nal planar fault essentially overlies the Grand River tec- portion of the Western Rift Margin fault north of trend surface (fault plane) was then created. tonic zone, we believe that the Reelfoot fault, New Madrid, Missouri (Fig. 5). To create Reelfoot fault cross sections, the within the Cambrian and Precambrian section, top of the crystalline Precambrian basement was has down-to-the-southwest normal separation METHODS taken from Figure 2, and the top of the Paleo- (Fig. 6) (Csontos et al., 2008). However, seis- zoic and Cretaceous sections were obtained mic refl ection profi les reveal that the Reelfoot The earthquake hypocenters used in this study from Van Arsdale et al. (1998) and Purser and fault displaces the post-Paleozoic section ~70 m were obtained from two data sets. One included Van Arsdale (1998). in a reverse sense (Sexton, 1988; Van Arsdale 568 events recorded between October 1989 and et al., 1998; Champion et al., 2001). To allevi- August 1992 using a portable seismic network NEW MADRID SEISMIC ZONE FAULTS ate this apparent contradiction, we propose that (PANDA, Chiu et al., 1992). This is the data set the Reelfoot fault was a rift-margin normal fault used in Mueller and Pujol (2001). The second The majority of New Madrid seismic zone during Cambrian extension that was subse- data set includes 1136 events recorded from 2001 seismicity falls along three major trends (Figs. 1 quently inverted into a thrust and reverse fault to 2006 by stations of the New Madrid seismic and 5). The longest trend is the 100-km-long in late Paleozoic-Appalachian Orogeny and zone permanent network (data available through southern section, extending northeast along younger compression. the Center for Earthquake Research and Informa- the Axial fault (Chiu et al., 1992; Stephenson The Axial fault separates the differently ori- tion of the University of Memphis). The earth- et al., 1995). The second trend extends 70 km ented Reelfoot North and Reelfoot South faults. quakes in the two data sets were located using northwest from near Dyersburg, Tennessee, At the level of the top of the Precambrian, the joint hypocentral determination (JHD) tech- to New Madrid, Missouri, and corresponds the Reelfoot North fault has 500 m of throw. nique described in Pujol et al. (1997). This tech- to the Reelfoot fault (Chiu et al., 1992; Pujol whereas the Reelfoot South fault has 1200 m of

Geosphere, October 2008 807 Csontos and Van Arsdale ). (A) New Madrid seismic zone seismicity with faults mapped by Stephenson et al. (1995) within the of diffuse seismic- continued on next page A Figure 5 ( 5 Figure ity. A–A' is line of cross section for B. Cross-section B–B' and C–C' located in Figure 6. B–B' and C–C' located in Figure B. Cross-section section for A–A' is line of cross ity.

808 Geosphere, October 2008 New Madrid seismic zone fault geometry '

fault—red; and the arcuate and the arcuate fault—red;

Fault

Ridgely Ridge Ridgely

Grove Fault Grove

Cottonwood

Fault

Unnamed Axial Fault Axial ). (B) Northwest-southeast A–A’ Reelfoot thrust faults showing earthquake foci. Reelfoot fault segments—orange and blue; Axial Reelfoot thrust faults showing earthquake foci. fault segments—orange and blue; A–A’ ). (B) Northwest-southeast continued B Figure 5 ( Figure 6. diffuse zone—black. Dashed lines denote primary depth to seismicity coincident with change in dip on Figure

Geosphere, October 2008 809 Csontos and Van Arsdale throw (Fig. 6). Thus, the Axial fault experienced between the Reelfoot North and Reelfoot South surface. Alternatively, the Reelfoot North fault down-to-the-southeast displacement during Cam- faults is that there is no surface scarp along the may be more complex than presented and may brian rifting (Csontos, 2007). Reelfoot South fault (Van Arsdale et al., 1999). contain more than one fault plane. In summary, The relatively low regression (r2 = 0.4485) value the Reelfoot North and Reelfoot South faults New Madrid Seismic Zone Earthquakes for the Reelfoot North fault plane has at least differ in strike, dip, basement displacement, sur- two possible explanations. The Reelfoot North face deformation, and earthquake depths. Trend surface analyses of earthquake hypo- fault may be slightly curved (Animation 1), The seismicity of the Axial fault, where it centers illustrate that most earthquakes within which would show a poor fi t to a single planar divides the Northern and Southern Reelfoot the New Madrid seismic zone occur along well- defi ned fault planes (Figs. 3–5, Animation 1). Specifi cally, earthquakes defi ne the Reelfoot B Cretaceous Top -430m B' North, Reelfoot South, Axial, Risco, and New Paleozoic Unconformity 80˚ -530m Madrid North faults. The deep, seismically active portion of the Reelfoot fault is divided into the Reelfoot North fault that dips 30° SW and the Reelfoot South fault that dips 44° SW (Fig. 6). However, seis- mic refl ection data reveal that the Reelfoot fault dips 73° (Van Arsdale et al., 1998) to perhaps as much as 80° (Champion et al., 2001) at depths less than 1 km, and the change in fault dip 30˚ -3200m has been placed at the top of the Precambrian 500m crystalline basement (Purser and Van Arsdale, Precambrian Unconformity 1998). This fault dip change was estimated by Purser and Van Arsdale (1998) to be at 4 km depth, which compares reasonably well with the footwall elevation of the Precambrian surface Fault lfoot defi ned in Figure 6. The elevation of the top of Ree 0 500 1000m the Precambrian in the hanging wall of the Reel- foot normal fault is −3700 m across the Reelfoot − North fault and 4800 m across the Reelfoot C Cretaceous Top -430m C' South fault. It is also apparent that the minimum Paleozoic Unconformity 80˚ -530m depth of seismicity on the Reelfoot South fault is deeper than on the Reelfoot North fault (Fig. 5). In both of these faults the seismicity is occur- ring on the deeper shallow-dipping portion of the fault planes. An additional major difference

44˚ -3600m

1200m

Precambrian Unconformity

lt u

lfoot Fa Animation 1. Three-dimensional model Ree 0 500 1000m of New Madrid seismic zone with best-fi t fault planes. You will need Adobe Acro- bat or Adobe Reader Version 8 or later to view and rotate this fi le. If you are viewing the PDF of this paper or reading it offl ine, Figure 6. Generalized cross-section B–B' of the inverted Reelfoot fault across please visit http://dx.doi.org/10.1130/ the Reelfoot North fault segment. Generalized cross-section C–C' of the GES00141.S1 (Animation 1) or the full- inverted Reelfoot fault across the Reelfoot South fault segment. Cross-section text article on www.gsajournals.org to scales are 1:1 and are located in Figure 5. Minor hanging-wall deformation is view Animation 1. not shown due to scale.

810 Geosphere, October 2008 New Madrid seismic zone fault geometry faults, is a structureless cloud of earthquakes two reasons. If there is a signifi cant component gests that there should be earthquakes along (Fig. 5). The only seismicity pattern within this of strike-slip on the Reelfoot North fault, then a northeastern continuation of the Axial fault zone is a 15-km-wide arched distribution around the ~1.8 m of coseismic reverse slip (Merritts beyond the Reelfoot faults. To this we can only the intersection of the Axial and Reelfoot faults. and Hesterberg, 1994; Mueller and Pujol, 2001) speculate that the northeastern extension of the This cloud of seismicity corresponds with the determined for the 7 February 1812 Reelfoot Axial fault is currently aseismic or that the dis- northeast-striking vertical basement Axial fault, fault earthquake may be only a component of placement is occurring as reverse movement on which locally includes the Unnamed, Cotton- greater oblique slip. Secondly, this “crowding” the Reelfoot North fault. wood Grove, and Ridgely faults mapped within around the Reelfoot North fault may be respon- Van Arsdale et al. (1999) and Mueller and the overlying Phanerozoic section (Stephenson sible for the Risco fault. We believe that the left- Pujol (2001) argue that the Reelfoot fault is et al., 1995; Van Arsdale et al., 1998), appar- lateral Risco fault is an escape fault caused by continuous from New Madrid, Missouri, south ently refl ecting a wide zone of distributed shear this crowding around the Reelfoot North fault to near Dyersburg, Tennessee. However, our (Odum et al., 1998). However, this zone of stepover (Csontos, 2007). research indicates that the Reelfoot North and structureless seismicity is wider than these four Right-lateral shear on the New Madrid North Reelfoot South are discrete fault segments that faults, thus suggesting that additional parallel and Axial faults explains the Reelfoot North fault may not rupture as one fault. These faults differ right-lateral strike-slip faults may exist south- (Odum et al., 1998) but does not explain the seis- in strike, dip, depth, area, basement displace- east of the Ridgely Ridge fault and northwest of micity occurring southeast of the Axial fault on ment, and the Reelfoot South fault does not have the Axial fault within this zone (Fig. 5). the Reelfoot South fault. It is necessary to modify a surface scarp. Absence of a fault scarp along The Risco fault was defi ned by fi tting a plane the current model for the New Madrid seismic the Reelfoot South fault could be due to the fact to the earthquake hypocenters west of New zone. We propose that the Reelfoot South fault that this fault is east of the Mississippi River Madrid (Fig. 5). Based on these hypocenter is also a compressional left stepover between the fl oodplain and to displace the ground surface locations, this fault strikes 92° and dips 82° N. right-lateral Axial fault and the right-lateral East- the fault would have to pass through an addi- Fault plane solutions (Johnston and Schweig, ern Rift Margin faults (Cox et al., 2001, 2006; tional 30 m of unconsolidated sediment includ- 1996) reveal left-lateral strike-slip motion on Csontos et al., 2008). This is supported by the ing a thick loess deposit. Although this may be the Risco fault. right-lateral fault plane solutions for the Axial an important factor as to why there is no scarp and Eastern Rift Margin faults and the reverse along the Reelfoot South fault, we believe this is CONCLUSIONS fault plane solutions for the Reelfoot South fault not the primary reason. (Chiu et al., 1992, 1997; Liu, 1997). The contem- The Reelfoot North fault scarp (fault propa- New Madrid seismic zone seismicity is porary stress fi eld for the eastern United States gation fold) is symmetric in that it diminishes occurring along Cambrian fault planes of the is a horizontal maximum compressive stress in structural amplitude north and south (Van Reelfoot rift. Below ~4 km, the Reelfoot fault oriented N60°E with some central Mississippi Arsdale et al., 1999) from its maximum fold is interpreted to be the reactivation of a Cam- River valley measurements indicating due east amplitude of 11 m to the northwest of Reelfoot brian down-to-the-southwest normal fault that (Zoback and Zoback, 1989; Zoback, 1992; Ellis, Lake (Champion et al., 2001). The fold ampli- may form the northern boundary of the Reelfoot 1994). This stress fi eld, and resultant right-lateral tude is less than 3 m within the large Missis- rift. More specifi cally, the Reelfoot North, Reel- shear on the N45°E-oriented Reelfoot rift faults, sippi River bend (Kentucky Bend) area of Fig- foot South, and Axial faults bound Reelfoot rift has led numerous authors to invoke right-lateral ure 3 and at its southern end where it intersects subbasins (Fig. 2). Earthquakes are occurring shear as being responsible for the New Madrid the Axial fault (Van Arsdale et al., 1999). Van primarily between 4 and 14 km along the 30°- seismic zone. However, we believe that contem- Arsdale et al. (1999) argue that the southern dipping Reelfoot North and 44°-dipping Reel- porary right-lateral shear is not restricted to the end of the Reelfoot scarp may have been ero- foot South faults. However, above 4 km, both of Axial–Reelfoot North–New Madrid North fault sionally removed by Mississippi River fl ood these faults dip ~80°, are largely aseismic, and system, but is occurring over the full width of waters prior to the construction of the Missis- have reverse displacement. We believe that these the Reelfoot rift at its northeastern end (Odum et sippi River levees. Although we agree with this, faults at depth are normal faults that have been al., 1998; Csontos et al., 2008). We also believe when we extrapolate a diminishing fold ampli- structurally inverted probably initially during that recent GPS data support these conclusions tude (scarp height) south from its maximum of Paleozoic Appalachian-Ouachita Orogeny con- (Smalley et al., 2005). 11 m, the fold would diminish to near zero at tinental compression. Perhaps most diffi cult to explain in our pro- the eastern edge of the Mississippi River fl ood- The New Madrid seismic zone has been posed New Madrid seismic zone shear model plain. If the Reelfoot fault has ruptured over explained to be a right-lateral shear zone with are parts that at fi rst glance appear to be missing. its entire 70 km of its mapped bedrock length a compressional left stepover (Russ, 1982; Sch- For example, the southeastern Reelfoot rift mar- as one fault plane during its Holocene history, weig and Ellis, 1994). The faults within the gin is only moderately active, but commensurate we would not expect the fold amplitude (scarp current model are the seismogenic right-lateral with our model, the portion north of Memphis height) to diminish to zero at the southern end Axial fault, the compressional Reelfoot reverse is most active (Fig. 1). If the Axial fault is act- of the Reelfoot North fault. Quite the contrary: fault (our Reelfoot North fault), and the right- ing as a right-lateral fault for the Reelfoot South if the Reelfoot fault is truly continuous and has lateral New Madrid North fault. This model fault stepover, then the Axial fault should extend ruptured along its entire 70 km length during the requires that crustal rock is sliding northeast northeast of the Reelfoot faults. Van Arsdale et late Quaternary, we would expect its fault prop- along the Axial fault and being forced over and al. (1998) have proposed that the Cottonwood agation fold amplitude to be a maximum near around the Reelfoot North fault to continue slid- Grove and Ridgely faults may extend northeast its center (Van der Pluijm and Marshak, 2004) ing northeast along the New Madrid North fault. of the Reelfoot fault (Fig. 5), and Figures 4–5 where the Reelfoot North and Reelfoot South Indeed, in trench excavations across the Reelfoot and Animation 1 suggest right-lateral displace- faults meet. North fault scarp, Kelson et al. (1996) cite evi- ment of the Reelfoot North fault with respect to Van Arsdale et al. (1999) document subtle dence of right-lateral slip. This is important for the Reelfoot South fault. Our model also sug- tectonic geomorphic features along the Reelfoot

Geosphere, October 2008 811 Csontos and Van Arsdale

Csontos, R., Van Arsdale, R., Cox, R., and Waldron, B., 2008, Howe, J.R., and Thompson, T.L., 1984, Tectonics, sedimen- South fault and the short-lived impoundment of Reelfoot rift and its impact on Quaternary deformation tation, and hydrocarbon potential of the Reelfoot rift: the Obion River during the 12 February 1812 in the central Mississippi River valley: Geosphere, v. 4, Oil and Gas Journal, v. 82, p. 179–190. New Madrid earthquake that may have been p. 145–158, doi: 10.1130/GES00107.1. Johnston, A.C., 1996, Seismic moment assessment of sta- Csontos, R.M., 2007, Three dimensional modeling of the Reel- ble continental earthquakes, Part 3: 1811–1812 New caused by fault displacement. However, based foot rift and New Madrid seismic zone [Ph.D. thesis]: Madrid, 1886 Charleston, and 1755 Lisbon: Geo- on the above arguments, we believe that the Memphis, Tennessee, University of Memphis, 92 p. physical Journal International, v. 126, p. 314–344, doi: Dart, R.L., 1992, Catalog of pre-Cretaceous geologic drill- 10.1111/j.1365-246X.1996.tb05294.x. paleoseismic history documented along the hole data from the upper Mississippi embayment: A Johnston, A.C., and Schweig, E.S., 1996, The enigma of Reelfoot North fault (Kelson et al., 1996; Muel- revision and update of Open-File Report 90-260: U.S. the New Madrid earthquakes of 1811–1812: Annual ler et al., 1999) probably does not apply to the Geological Survey Open-File Report, 253 p. Review of Earth and Planetary Sciences, v. 24, p. 339– Dart, R.L., and Swolfs, H.S., 1998, Contour mapping of relic 384, doi: 10.1146/annurev.earth.24.1.339. Reelfoot South fault. We close by speculating structures in the Precambrian basement of the Reelfoot Kane, M.F., Hildenbrand, T.G., and Hendricks, J.D., 1981, that the Holocene earthquake history of the rift, North American Midcontinent: Tectonics, v. 17, Model for the tectonic evolution of the Mississippi Reelfoot South fault may have begun with the p. 235–249, doi: 10.1029/97TC03551. embayment and its contemporary seismicity: Geology, Ellis, W.L., 1994, Summary and discussion of crustal stress data v. 9, p. 563–568, doi: 10.1130/0091-7613(1981)9<563 12 February 1812 earthquake and that ongoing in the region of the New Madrid seismic zone: U.S. Geo- :MFTTEO>2.0.CO;2. seismicity reveals that this southeastern stepover logical Survey Professional Paper 1538-A–C, p. B1–B13. Kelson, K.I., Simpson, G.D., Van Arsdale, R.B., Harris, Ervin, C.P., and McGinnis, L.D., 1975, Reelfoot rift: Reac- J.B., Haraden, C.C., and Lettis, W.R., 1996, Mul- zone is an area of strain accumulation (Cox et tivated precursor to the Mississippi embayment: Geo- tiple Late Holocene earthquakes along the Reelfoot al., 2001, 2006). logical Society of America Bulletin, v. 86, p. 1287– fault, central New Madrid seismic zone: Journal of 1295, doi: 10.1130/0016-7606(1975)86<1287:RRRP Geophysical Research, v. 101, p. 6151–6170, doi: ACKNOWLEDGMENTS TT>2.0.CO;2. 10.1029/95JB01815. Gomberg, J., and Ellis, M., 1994, Topography and tec- Liu, Z., 1997, Earthquake modeling and active faulting in tonics of the central New Madrid seismic zone: the New Madrid seismic zone [Ph.D. thesis]: St. Louis, We wish to thank the National Science Founda- Results of numerical experiments using a three- Missouri, St. Louis University, 164 p. tion Mid America Earthquake Center, Memphis Stone dimensional boundary element program: Journal of Lumsden, D.N., and Caudle, G.C., 2001, Origin of massive and Gravel, and the University of Memphis Dunavant Geophysical Research, v. 99, p. 20,299–20,310, doi: dolostone: The upper Knox model: Journal of Sedimen- Grant for funding this research. We also wish to thank 10.1029/94JB00039. tary Research, v. 71, p. 400–409, doi: 10.1306/2DC4094E- Landmark Graphics Company and Schlumberger for Guccione, M.J., 2005, Late Pleistocene and Holocene paleo- 0E47-11D7-8643000102C1865D. their contribution of software and technical support, seismology of an intraplate seismic zone in a large allu- Merritts, D., and Hesterberg, T., 1994, Stream networks and vial valley, the New Madrid seismic zone, central USA: long-term surface uplift in the New Madrid seismic and the North American Coal Company for their bore- Tectonophysics, v. 408, p. 237–264. zone: Science, v. 265, p. 1081–1084, doi: 10.1126/sci- hole logs. We also acknowledge Jose Pujol for his Guccione, M.J., Van Arsdale, R.B., and Lynne, H.H., 2000, ence.265.5175.1081. contribution of earthquake hypocenter data and exper- Origin and age of the Manilla high and associated Big Mihills, R.K., and Van Arsdale, R.B., 1999, Late Wiscon- tise and comments by Randel Cox. 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