migrating earthquakes and faults switching on and off: a new view of intracontinental earthquakes...
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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
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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
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You must unlearn what you have learned.
Alan Kafka
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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
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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!
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Science, April 1999
No or little motion
Recent cluster may be ending
Seismicity migrates
Hazard overestimated
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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
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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
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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)
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“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
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“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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?
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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
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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
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8/23/2011 Washington Post
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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
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Passive margin earthquakes presumably reactivate faults remaining from ocean closing and rifting
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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
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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
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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
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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
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Wolin & Stein, 2010
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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
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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.