The New Madrid seismic zone of the Central United States - Intraplate Earthquakes - page 163

Figure 7.1 Small earthquakes recorded between 1979 and 2006 in the New Madrid seismic zone
are illustrated in colored dots with the three largest 1811-1812 earthquakes marked with gold stars.
Southern star, December 16, 1811; northern star, January 23, 1812; and central star, February 7, 1812.
The earthquakes primarily occur within the underlying Reelfoot Rift, which is bound by the black
lines (from Csontos et al ., 2008 ) . For color version, see Plates section.
seismic zone of the Eastern United States. Microseismic activity (Chiu et al ., 1992 ; Mueller
and Pujol, 2001 ) is occurring along reactivated basement faults within the Reelfoot Rift: a
Cambrian aulacogen (Hildenbrand, 1985 ; Hildenbrand and Hendricks, 1995 ) ( Figure 7.2 ) .
Although contemporary seismicity rarely exceeds M (moment magnitude) 4.0, the NMSZ
has generated very large earthquakes; most recently during 1811-1812 when a minimum
of four earthquakes (Hamilton and Johnston, 1990 ) , estimated to have been in the mid M
7 range (Nuttli, 1973 ; Gomberg, 1993 ; Hough et al ., 2000 ; Mueller and Pujol, 2001 ; Hough
and Page, 2011 ) , occurred over a three-month period. The threat of this seismic zone to
the United States has prompted numerous studies and expensive retrofitting, an example of
which is the 276million dollar retrofit of the Interstate 40Bridge across theMississippi River
at Memphis. Debates continue as to the hazard posed by future New Madrid earthquakes
(Stein, 2010 ) . Many scientists argue that the late Holocene 500-year recurrence interval
determined from NMSZ paleoseismic studies implies continuing hazard, whereas some
of those studying GPS data argue that there is not enough strain currently accumulating
to produce a very large earthquake in the near future. In this chapter I summarize the
geological history of the NMSZ and its earthquakes.