Triggering of New Madrid Seismicity by Late Pleistocene Erosion
Eric Calais & Andy Freed Purdue University
Roy Van Arsdale, University of Memphis
Seth Stein, Northwestern University
Interplate Earthquakes
Intraplate EarthquakesUnclear what controls
activation of a particular mid-continental fault and the duration of its seismic
activity
Plate A
Plate B
Earthquakes at different time
Stein, Liu & Wang 2009
Plate motions steadily & quickly reload faults,
making locations of large earthquakes and average
time between them consistent with faults’
geological, paleoseismic, and seismic histories
M 7 events in 1811-12
Small earthquakes continue,
outlining faults thought to have
ruptured in 1811-1812
Paleoseismology shows large events ~ 500 years apart
in past 2,000 years
Previously taken as evidence that
strain accumulates steadily and is periodically
released during large infrequent
events
New Madrid
However, twenty years of GPS measurements find no detectable deformation with
progressively higher precision, constraining present motions across the NMSZ to be slower
than 0.2 mm/yr
Because the recent earthquakes correspond to strain release at a rate equivalent to a slip of at least 1-2 mm/yr over the past
~2,000 years, deformation varies
with time
Hence, the NMSZ must have been recently
activated, consistent with the lack of
significant topography, the jagged fault, and seismic reflection and trenching studies that find an increase in slip rate on the Reelfoot
fault by four orders of magnitude about 10,000 years ago
This recent reactivation of the NMSZ argues
against Holocene fault activity being a direct
manifestation of tectonic stresses, which change on timescales of millions of years.
Forte et al., 2007
Similar conclusion from GPS data showing at most slow platewide
deformation
Plate interior contains many fossil faults developed at different times with different orientations but only
a few appear active today
Marshak and Paulson, 1997
Although New Madrid earthquakes
probably reactivate
favorably oriented faults associated with Palaeozoic
rifting, a stress source
localized in space & time must have
recently triggered these particular
faults
Sella et al., 2007
GIA – Glacial Isostatic Adjustment - is unlikely stress source for seismicity
May explain seismicity along old ice sheet margin in Eastern Canada & elsewhere (Stein et al., 1979; 1989; Mazzotti et al., 2005)
GPS shows nothing unusual at New Madrid
Stresses decay rapidly away from ice margin, so can’t explain NMSZ (Wu and Johnson, 2000) unless order of magnitude weaker than surroundings (Grollimund and Zoback, 2001)
No evidence for such weakening
NMSZ not hot or weak
NMSZ heat flow no higher than surroundings
NMSZ and surroundings have
essentially the same temperature &
thermally-controlled strength
No strength reason for platewide stresses to
concentrate in NMSZ rather than other
faults
McKenna, Stein & Stein, 2007
Similar difficulty for models in which earthquakes result from
-sinking of “rift pillow” ancient high density mafic body (Grana and Richardson, 1996; Stuart et al., 1997) due to weakening of the lower crust in past 9 kyr (Pollitz et al., 2001)
-sudden recent weakening of lower crust (Kenner & Segall, 2000)
Braile et al., 1986
Problems: no evidence for weak zone and no obvious reason for why weakening
occurred here at this time
Local stress source for seismicity: postglacial erosion in Mississippi Embayment
Flexure caused by unloading from river incision 16 - 10 ka reduces normal stresses sufficiently to unclamp pre-existing faults
Fits location & timing of recent seismicity
Doesn’t require assumption of weak zone
Model predicts NMSZ faults continue being unclamped by
relaxation even 10,000 years after alluvial
denudation stopped, although at a slow and
decaying rate
Maximum stress that can be transferred into the upper crust from viscoelastic relaxation following a
large earthquake more than one order of magnitude less than typical stress drop
value
After a large earthquake releases stresses on an intraplate fault segment, flexure and viscoelastic relaxation are inefficient at
bringing the rupture back to failure equilibrium unless faults weaken with time
Fault segments that ruptured are unlikely to
fail again soon, although stress changes from erosional unloading or large earthquakes may
eventually bring to failure nearby segments
that have not yet ruptured
This process may be how NMSZ seismicity migrated in the past and may
eventually activate yet unruptured segments
Other localized stress sources may have or will generate earthquakes
elsewhere in midcontinent
Marshak and Paulson, 1997
Tuttle (2009)
Stress due to Late Pleistocene erosion could have triggered New Madrid
seismicity
Localized mechanism consistent with recent initiation and localization in
NMSZ
Doesn’t require assuming sudden localized crustal weakening for which no evidence
Fault segments that ruptured unlikely to fail again soon
Stress changes from erosion or large
earthquakes may eventually cause failure on nearby segments that have not yet
ruptured