The New Madrid seismic zone of the Central United States - Intraplate Earthquakes - page 181

Movement on one of the southeastern Reelfoot Rift margin faults is believed responsible
for the Marianna liquefaction. Further southwest near Monticello, Arkansas, but still within
the projection of the Reelfoot Rift, Cox et al .( 2000 ) have identified Holocene faulting and
earthquake liquefaction (Cox et al ., 2004 ) along the northwest-trending Saline River fault
zone ( Figure 7.9 ) . Although this area is also underlain by the Ouachita-Appalachian thrust
belt, Cox et al .( 2000 ) and Cox ( 2010 ) believe that the northwest-trending basement fault
responsible for this Quaternary faulting is the Alabama-Oklahoma transform fault ( Figure
7.4 ) - the probable southern terminus of the Reelfoot Rift.

7.3.6 New Madrid seismic zone fault activation models

Various models have been proposed to explain the NMSZ. Gomberg and Ellis ( 1994 )
numerically model far-field (plate tectonic scale) and locally derived driving strains for the
NMSZ faults. Applying elastic dislocation theory Tavakoli et al .( 2010 ) model the NMSZ as
a 240 km long transpressional flower structure with the principal fault being a right-lateral
shear zone rooted in the lower crust that is located along the axis of the Reelfoot Rift (Axial
fault). The driving mechanism for the Tavkoli et al . model is a drag force at the base of
the North American plate. Pratt ( 2012 ) combines an analog sandbox model and computer
models of a restraining stepover within a N28

E-trending right-lateral shear zone to analyze
the NMSZ ( Figure 7.10 ) . Deformation displayed in Figure 7.10 is the predicted map view
of surface displacements caused by three N47

°

° E upper-crustal faults above a N28 ° Elower

crustal shear zone under a uniform N70 ° E regional compression. The driving mechanism

for the Pratt model is the horizontal N60 E maximum compressive stress due to

ridge push. Van Arsdale and Cupples (in press) believe that ridge push and/or basal drag is
the driving force, but using hundreds of well logs they show that Quaternary right-lateral
shear extends across the entire Reelfoot Rift to include the outboard Commerce and Big
Creek faults ( Figure 7.11 ) . This shear has produced east-west-trending normal faults and
north-south-trending compressional stepovers.
Within the Reelfoot Rift region of the Eastern United States are the seismically quiet
Rough Creek graben, Rome Trough, Mid Continent Rift, and SouthernOklahoma aulacogen
( Figure 7.4 ) . This raises the fundamental questions: why is the Reelfoot Rift seismically
active when its neighbors are not, and why does activity within the Reelfoot Rift migrate
among different faults during the Quaternary? A number of contributing factors have
been cited, some of which are unique to the Reelfoot Rift. Two of these factors are (1)
the Reelfoot Rift faults have broken completely through the crust (Nelson and Zhang,
1991 ; Bartholomew and Van Arsdale, 2012 ) , and (2) these faults are favorably oriented
to experience shear in the regional stress field (Zoback and Zoback, 1989 ; Ellis, 1994 ;
Heidbach et al ., 2008 ) . Regionally, a N60

° EtoN80 °

E horizontal maximum compressive
stress is due to the plate driving force of ridge push (Zoback, 1992 ; Richardson, 1992 )
and/or basal drag from lower mantle flow (Liu and Bird, 2002 ) .
A third factor argued by a number of authors is that the rift pillow in the base of the
crust beneath the Reelfoot Rift causes a local stress concentration. Grana and Richardson

° EtoN80 °