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-periodicPast is good predictorIntraplate 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 shiftPast can be poor predictor

Plate A

Plate B

Earthquakes at different time

Stein, Liu & Wang 2009

You must unlearn what you have learned.

Alan Kafka

1900-2002

PACIFIC

NORTH AMERICA

New Madrid seismic zoneM 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

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!

Science, April 1999

No or little motion

Recent cluster may be ending

Seismicity migrates

Hazard overestimated

As data improve, maximum

possible motion keeps

decreasingE. 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

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

? ?

9k 7k 6k 4k12k 3k 1k Today

Portageville Cycle Reelfoot Cycle New Madrid Cycle

Slip

Cluster

Slip

Cluster

Slip

ClusterQuiescent Quiescent Quiescent

Holocene Punctuated Slip New Madrid earthquake history inferred from Mississippi river channels

Holbrook et al., 2006

Tuttle (2009)

Meers fault, OklahomaActive 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)

“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

“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

-used to identify 1811-12 ruptures

-rate & size seem decreasing

-largest at the ends of presumed 1811-12 ruptures

Stein & Newman, 2004

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

8/23/2011 Washington Post

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

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

Virginia 8/323/11: Reverse faulting on margin-parallel

NE-SW striking faultNorth 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 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

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

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

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

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.