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

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 B Plate A Earthquakes at different time

Stein, Liu & Wang 2009

You must unlearn what you have learned.

Alan Kafka

New Madrid seismic zone

M 7 earthquakes in 1811-12 Small quakes continue (M>6 about every 175 years) with little damage

1900-2002 NORTH AMERICA

Big ones might happen again Don’t know why, when, how dangerous

PACIFIC

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!

No or little motion Recent cluster may be ending Seismicity migrates Hazard overestimated

Science,

April 1999

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

12k

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

Holocene Punctuated Slip 9k Quiescent 7k Slip Cluster 6k Quiescent 4k 3k Slip Cluster Quiescent 1k Today Slip Cluster ?

Portageville Cycle Reelfoot Cycle New Madrid Cycle Holbrook et al., 2006

New Madrid earthquake history inferred from Mississippi river channels

Tuttle (2009) Wabash: M~7 6 Kybp

Obermeier, (1998)

Faults active in past show little present seismicity Seismicity migrates among faults due to fault interactions (stress transfer) Meers fault, Oklahoma Active 1000 years ago, dead now

Similar behavior in other continental interiors

“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)

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

?

Although small earthquakes in New Madrid area are often cited as evidence of an upcoming large earthquake, most seem to be 1811-1812 aftershocks

Stein & Newman, 2004 -used to identify 1811-12 ruptures -rate & size seem decreasing -largest at the ends of presumed 1811-12 ruptures

Rate-state friction predicts aftershock  duration 1/loading rate

Fits general pattern of long aftershock sequences in slowly deforming continental interiors

Plate boundary faults quickly reloaded by steady plate motion after Stein & Liu, 2009

Stein & Liu 2009

large earthquake Faults in continents reloaded much more slowly, so aftershocks continue much longer

8/23/2011 Washington Post

In general terms, part of seismic zone along passive continental margin that has events up to M7

1933 Baffin Bay M7.3

M7 1755 M6 M7.2

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

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

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

F. Pazzaglia

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

CAUSES OF INTRAPLATE EARTHQUAKES

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

POSSIBLE STRESS SOURCE FOR SEISMICITY: GIA - GLACIAL ISOSTATIC ADJUSTMENT

GPS May explain seismicity along old ice sheet margin in Eastern Canada & elsewhere (Stein et al., 1979; 1989) Effect should be less to south Sella et al., 2007

Wolin & Stein, 2010

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

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.

Approaching intracontinental seismic zones as a complex system is necessary to improve understanding of midcontinental tectonics, the resulting earthquakes, and the hazards they pose.