2010 Metro Mitigation Plan

Likelihood of Future Occurrences
According to the Tennessee Emergency Management Agency, instead of a prediction of
when an earthquake will strike, an estimate of the likelihood of an earthquake recurring
within a given time frame should be given:
In all of western Tennessee, an event of magnitude greater than 5.0 can be expected
once every year, a magnitude of 6.0 or greater should occur ever 50 years, and a
magnitude 7.0 or greater should occur every 600 years.
The highest recurrence rate of large earthquakes in Tennessee occurs in the
northwestern quadrant of the state.
New zones of relatively small seismicity have been identified near the Georgia-
Tennessee border at Chattanooga, and roughly along Interstate 75 between
Chattanooga and Knoxville. This area has not been studied enough to ascertain the
expectancy of seismic event histories or likelihoods.
The New Madrid Fault is an active fault, averaging more than 180 events per year that
measure 1.0 or more on the Richter scale. This is equivalent to approximately 15 events per
month. Events measuring 2.5-3.0 on the Richter scale includes tremors large enough to be
felt and are noted annually. Every 18 months, the New Madrid Fault releases a shock of 4.0
or more, capable of local minor damage. Magnitudes of 5.0 or greater occur approximately
once per decade, can cause significant damage, and are felt in several states. A damaging
earthquake in the New Madrid area (6.0 or greater) occurs about every 80 years (the last one
occurred in 1895).
A major earthquake in the New Madrid area (7.5 or greater) happens every 200-300 years (the
last one occurred in 1812). It is predicted that there is a 25 percent chance of a disastrous
major earthquake by 2040. A New Madrid Fault rupture of this size would be felt throughout
half the United States and damage would be expected in 20 states or more. Events measuring
6.0-7.6 have more significant probabilities in the near future. A 6.0 shock has a 90 percent
chance of occurring by the year 2040.
Only one or two earthquakes with magnitudes equal to or greater than 3.0 are expected in the
SASZ per year. The extrapolated, expected recurrence time for earthquakes with magnitudes
of 6.0 or greater in the SASZ is 186 years (Bollinger et al., 1989).
Metropolitan Nashville - Davidson County
Hazard Identification
Multi-Hazard Mitigation Plan Page 4-47
April 2010