Intraplate seismic hazard: Evidence for distributed strain and implications for seismic hazard - Intraplate Earthquakes - page 308

evidence, including the appearance of waterfalls along the Mississippi River (e.g., Russ,
1982 ; Odum et al ., 1998 ) . NM1 is commonly associated with the strike-slip Cottonwood
Grove fault, largely based on the prominent lineation illuminated by microseismicity as
well as the distribution of liquefaction. Direct evidence from written archival accounts
points only to a location somewhere south of the Reelfoot fault; i.e., unlike NM3, there
are no accounts suggestive of surface rupture or accounts suggesting near-fault effects
(e.g., localized extremely strong ground motions). The rupture scenario for NM1A is
similarly uncertain. Two different scenarios have been proposed for NM1A: a rupture on
the northern Cottonwood Grove strike-slip fault (Johnston and Schweig, 1996 ) , or a rupture
on the southeastern extension of the Reelfoot thrust fault (Hough and Martin, 2002 ) . For
this event direct evidence - the shaking distribution as inferred from written accounts -
suggests only a location to the north of NM1. The rupture scenario of NM2 is the least well
constrained: although associated by most authors with strike-slip motion on the northern
limb of the NMSZ (e.g., Johnston and Schweig, 1996 ) , Mueller et al .( 2004 ) and Hough
et al .( 2005 ) summarize several lines of evidence from written accounts suggesting the
event might have been centered in the Wabash Valley, well north of the NMSZ, in a region
where a contemporaneous account describes “wagon loads” of sand erupting at the surface
during the 1811-1812 sequence (although the account does specify when in the sequence
the sand blows occurred).
Owing to the incomplete historical and paleoseismic records, the various 1811-1812 rup-
ture scenarios and our general understanding of the NMSZ are heavily guided by the dis-
tribution and interpretation of present-day seismicity; I therefore now review this issue
briefly. The long-lived aftershock hypothesis (e.g., Ebel et al ., 2000 ; Stein and Liu, 2009 )
has recently been called into question by Page et al .( 2012 ) , who used simulated sequences
based on the epistemic triggering of aftershock (ETAS) model to show that no plausible set
of sequence parameters can explain the robust known features of the early 1811-1812 after-
shock sequence and the current rate of small earthquakes (assuming them to be aftershocks).
It remains unclear, then, why present-day background seismicity does appear to illuminate
the primary 1811-1812 ruptures. One might question this assumption; however, there is
no question that present-day microseismicity illuminates the Reelfoot fault and, as noted,
there is compelling evidence that this fault produced NM3. It reasonably follows, then, that
microseismicity may very well illuminate the overall sequence. An alternative hypothesis
is that present-day microseismicity is driven by low levels of deep creep on continuations
of faults below the brittle seismogenic crust, as Frankel et al . ( 2012 ) conclude is occurring
on the Reelfoot and that perhaps continues at lower levels on the Cottonwood Grove fault.
This would explain why the complex geometry of the NMSZ, with its well-defined side
limbs, is consistent with predicted Coulomb stress increase associated with mainshock
rupture on these two faults (Mueller et al ., 2004 ) . (So far as we know, the temporal decay
of aftershocks universally follows Omori's law (e.g., Omori, 1895 ) . Since rigorous statis-
tical tests reveal that NMSZ seismic activity since the early nineteenth century cannot be
explained by Omori's law (Page et al ., 2012 ) and are therefore not aftershocks, a plausible
alternative hypothesis is that ongoing creep at depth is not a consequence of post-seismic