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