Intraplate seismic hazard: Evidence for distributed strain and implications for seismic hazard - Intraplate Earthquakes - page 313

Figure 12.4 Seismotectonic zonation map developed for CEUS-SSC project (modified from Copper-
smith et al ., 2012 ) . The aim of this project was not to identify all failed rifts within the mid-continent
region, but only those considered to have an elevated earthquake potential relative to stable conti-
nental regions. The Commerce Geophysical Lineament tracks the northwestern edge of the Reelfoot
Rift (RR), extending from roughly the southwestern corner of the RR, continuing through the Illinois
Basin Extended Basin (IBEB).

12.3 Strain rate

12.3.1 Observed strain rate

The interiors of tectonic plates are assumed to be rigid, with strain rates 3-4 orders of
magnitude lower than active plate boundaries. In the NMSZ, increasingly precise estimates
of strain rate from GPS data have revealed an increasingly tight upper bound on the level of
strain accrual in the region. Although the initial investigation of Liu et al .( 1992 ) estimated
a strain rate of 5-7 mm/yr across the Reelfoot fault based on a combination of GPS data and
triangulation data dating back to the 1950s, Calais et al . ( 2005 ) show that no statistically
significant site motions are found within the NMSZ using GPS data collected through 2005.
Using GPS data recorded through 2005, Calais et al .( 2005 ) estimate an upper bound on
residual velocities relative to stable North America of 1.4 mm/yr, at a 95% confidence level.
Frankel et al .( 2012 ) analyze GPS data recorded through 2010 and conclude that there is
statistically significant motion of 0.37 0.07 mm/yr across the Reelfoot fault at the

surface, which they show is consistent with an interseismic deep creep rate of 4 mm/yr on
the Reelfoot fault at depths between 12 and 20 km. Their model predicts that sufficient strain
will accumulate across the zone to produce a M w

+ / −

7.3 earthquake every 500 years, which