1. Migrating earthquakes and faults switching on and off: a new view of intracontinental earthquakes Seth Stein Northwestern University Mian Liu University of Missouri Eric Calais Purdue University “How wonderful that we have met with a paradox. Now we have some hope of making progress.” Niels Bohr

2. Plate Boundary Earthquakes Major fault loaded rapidly at constant rate Earthquakes spatially focused & temporally quasi-periodic Past is good predictor Intraplate Earthquakes Tectonic loading collectively accommodated by a complex system of interacting faults Loading rate on a given fault is slow & may not be constant Earthquakes can cluster on a fault for a while then shift Past can be poor predictor Plate A Plate B Earthquakes at different time Stein, Liu & Wang 2009

3. You must unlearn what you have learned. Alan Kafka

4. 1900-2002 PACIFIC NORTH AMERICA New Madrid seismic zone M 7 earthquakes in 1811-12 Small quakes continue (M>6 about every 175 years) with little damage Big ones might happen again Don’t know why, when, how dangerous

5. 1991: large earthquake in next few hundred years seemed plausible because paleoseismology shows large events in 900 & 1450 AD We started GPS expecting to find deformation accumulating, consistent with large events ~500 years apart After 8 years, 3 campaigns, 70 people from 9 institutions … 0 +/- 2 mm/yr!

6. Science, April 1999 No or little motion Recent cluster may be ending Seismicity migrates Hazard overestimated

7. As data improve, maximum possible motion keeps decreasing E. Calais < 0.2 mm/yr No sign of large earthquake coming Long time needed to store up slip for future large earthquake For steady motion, M 7 at least 10,000 years away M 8 100,000

8. Large earthquake cluster in past 2000 years isn’t representative of long term NMSZ behavior Lack of significant fault topography, jagged fault, seismic reflection, and other geological data also imply that recent pulse of activity is only a few thousand years old Recent cluster may be ending ?? 9k7k6k4k12k3k1kToday Portageville CycleReelfoot CycleNew Madrid Cycle Slip Cluster Slip Cluster Slip Cluster Quiescent Holocene Punctuated Slip New Madrid earthquake history inferred from Mississippi river channels Holbrook et al., 2006

9. Tuttle (2009) Meers fault, Oklahoma Active 1000 years ago, dead now Obermeier, (1998) Wabash: M~7 6 Kybp Faults active in past show little present seismicity Seismicity migrates among faults due to fault interactions (stress transfer)

10. “Large continental interior earthquakes reactivate ancient faults … geological studies indicate that earthquakes on these faults tend to be temporally clustered and that recurrence intervals are on the order of tens of thousands of years or more.” (Crone et al., 2003) Similar behavior in other continental interiors

11. “During the past 700 years, destructive earthquakes generally occurred in different locations, indicating a migration of seismicity with time.” (Camelbeeck et al., 2007)

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16. Although small earthquakes in New Madrid area are often cited as evidence of an upcoming large earthquake, most seem to be 1811-1812 aftershocks -used to identify 1811-12 ruptures -rate & size seem decreasing -largest at the ends of presumed 1811-12 ruptures Stein & Newman, 2004

17. Rate-state friction predicts aftershock duration  1/loading rate Plate boundary faults quickly reloaded by steady plate motion after large earthquake Faults in continents reloaded much more slowly, so aftershocks continue much longer Stein & Liu, 2009 Fits general pattern of long aftershock sequences in slowly deforming continental interiors Stein & Liu 2009

18. 8/23/2011 Washington Post

20. In general terms, part of seismic zone along passive continental margin that has events up to M7 M7 M7.2 1755 M6 1933 Baffin Bay M7.3

21. Passive margin earthquakes presumably reactivate faults remaining from ocean closing and rifting

23. Virginia 8/323/11: Reverse faulting on margin-parallel NE-SW striking fault North edge of Central Virginia seismic zone, whose trend normal to the fault plane, margin, Appalachian Mountains & associated structures, has no clear geologic expression. F. Pazzaglia

24. Unclear why this and similar seismic zones have the geometry they do Unclear whether zones are more active over time, or present loci of activity that migrates. Could some reflect aftershocks of large prehistoric earthquakes? F. Pazzaglia

25. GPS shows 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 Time- and space- variable deformation can’t only reflect platewide tectonic stresses, which change slowly in space and over millions of years CAUSES OF INTRAPLATE EARTHQUAKES Earthquakes reflect reactivation at least in part by localized stress sources & fault interactions S. Marshak

26. Sella et al., 2007 POSSIBLE STRESS SOURCE FOR SEISMICITY: GIA - GLACIAL ISOSTATIC ADJUSTMENT May explain seismicity along old ice sheet margin in Eastern Canada & elsewhere (Stein et al., 1979; 1989) Effect should be less to south GPS

27. Wolin & Stein, 2010

28. Deformed stratigraphic and geomorphic markers, localized high-relief topography, and rapid river incision show uplift of Piedmont and Appalachians relative to the Coastal Plain for the past 10 Ma MidAtlantic coastal seismicity reflects active and long-term deformation whose cause’s unclear F. Pazzaglia

29. Approaching intracontinental seismic zones as a complex system is necessary to improve understanding of midcontinental tectonics, the resulting earthquakes, and the hazards they pose. Summary Unlike plate boundary faults that give quasi-periodic earthquakes, interacting fault networks in midcontinents predict complex variability of earthquakes. Conventional seismic hazard assessment, which assumes steady behavior over 500-2500 years, can overestimate risks in regions of recent large earthquakes and underestimate them elsewhere.