1. 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

2. Interplate Earthquakes Intraplate Earthquakes Unclear 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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)

13. 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