The New Madrid seismic zone of the Central United States - Intraplate Earthquakes - page 175

( 2005 ) ( Figure 7.7 ) . Large aftershocks diminished after 1812 and the seismic threat was
largely forgotten until the 1970s when the Mississippi River valley was evaluated for the
construction of nuclear power plants. Installation of seismometers since the 1970s has
clearly defined the NMSZ ( Figure 7.1 ) (Chiu et al ., 1992 ; Mueller and Pujol, 2001 ) .
Magnitudes of the 1811-1812 earthquakes have been estimated from intensity data with
maximum magnitude estimates having ranged from M 8.1 (Johnston, 1996 ; Johnston and
Schweig, 1996 ) to low 7 magnitude (Gomberg, 1993 ; Newman et al ., 1999 ; Hough et al .,
2000 ) .

7.3.2 Geological structure of the NMSZ

NMSZ earthquakes are generally believed to occur along reactivated, and commonly
inverted, Cambrian Reelfoot Rift faults (Van Arsdale, 2009 ) . In some cases faults are
illuminated by seismic reflection data while in other cases earthquakes occur along planar
surfaces that we interpret as fault planes. Specifically, the southern NMSZ seismicity arm
is occurring along the Axial (Cottonwood Grove) fault (oriented 46 ); earthquakes

occurring along the central northwest-trending arm appear to define a southwest-dipping
Reelfoot reverse fault that is divided into the Reelfoot South fault (150

° ,90 °

° ,44 ° SW) and

Reelfoot North fault (167 ° ,30 ° SW), the Risco fault seismicity (92 ° ,82 ° N), and the New

Madrid North fault seismicity (29 SE) (Csontos and Van Arsdale, 2008 ) . Fault plane

solutions indicate that the Axial and New Madrid North faults are right-lateral strike-slip
faults, the Risco fault is a left-lateral strike-slip fault, and the Reelfoot fault is a thrust fault
at depths of 4 to 14 km (Csontos and Van Arsdale, 2008 ) . Seismic reflection data and fault
modeling (Purser and Van Arsdale, 1998 ) reveal that the Reelfoot thrust fault steepens into
a reverse fault with a southwest dip of 72

° ,72 °

° above 4 km ( Figure 7.8 ) and Champion et al .

( 2001 ) model the Reelfoot fault above 500 m depth as a trishear fault propagation fold.

The Reelfoot North fault has a hanging wall horst that is manifest at the ground surface as
the Lake County Uplift with the Tiptonville dome culmination.
Although seismically quiet in recent times there are two other faults with Quaternary
displacement within the immediate NMSZ area. The 135 km long Bootheel fault is a
transpressional right-lateral strike-slip fault with at least 13 m of strike-slip offset and

3 m of up-to-the-east displacement ( Figure 7.2 ) (Guccione et al ., 2005 ) . Immediately
northeast of the Lake County uplift is the NewMarkham fault. Odum et al . ( 1998 ) interpret
the New Markham fault to be a near-surface continuation of the Reelfoot fault. However,
the sense of displacement on this fault is up to the east, which is opposite to Reelfoot
fault displacement. Van Arsdale et al .( 1998 ) propose that the New Markham fault is a
transpressive strike-slip fault with up-to-the-east reverse displacement. The Bootheel and
New Markham faults may be linking faults (Wesnousky, 1988 ) that bypass the Reelfoot
North fault stepover zone and may eventually “straighten out” the New Madrid shear zone
(Schweig and Ellis; 1994 ; Van Arsdale et al ., 1998 ) .
Considering the regional tectonic setting it appears that the NMSZ is a right-lateral
strike-slip fault system with two compressional left stepovers ( Figure 7.2 ) (Csontos et al .,