8/23/2011 washington post
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Mineral, VA, earthquake illustrates seismicity of a passive-aggressive margin. Seth Stein 1 , Frank Pazzaglia 2 , Emily Wolin 1 , Anne Meltzer 2 , Alan Kafka 3 1 Northwestern University 2 Lehigh University 3 Boston College. 8/23/2011 Washington Post. - PowerPoint PPT PresentationTRANSCRIPT
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8/23/2011 Washington Post
Mineral, VA, earthquake illustrates seismicity of a passive-aggressive margin
Seth Stein1, Frank Pazzaglia2, Emily Wolin1, Anne Meltzer2, Alan Kafka3
1Northwestern University 2Lehigh University3Boston College
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8/23/2011 Washington Post
“Passive Aggressive behavior is a form of covert abuse. Covert abuse is subtle and veiled or disguised by actions that appear to be normal, at times loving and caring.”
e.g., earthquakes in plate interior where idealized plate tectonics says they shouldn’t happen
http://divorcesupport.about.com/od/abusiverelationships/a/Pass_Agg.htm
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Margins like ENAs are “passive” in not being plate boundaries, but are seismically active
"Nearly all stable masses exhibit marginal features which are seismically active." Gutenberg and Richter (1954)
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Many passive margin earthquakes presumably reactivate faults remaining from ocean closing and rifting
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2011 VA quake is generally part of ENA’s most active seismic zone
The passive continental margin has events up to M7
M7
M7.2
1755 M6
1933 Baffin Bay M7.3
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• Some passive margin earthquakes generate large landslides and/or tsunamis
• Can large landslides occur without a seismic trigger?
• Important for evaluating seismic & tsunami hazards
1929 Grand Banks, Mw=7.2
1933 Baffin Bay, Mw=7.41920-2009
Events from Schulte & Mooney (2005), ANSS, and
Earthquakes Canada
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1929 Grand Banks Mw 7.2
Earthquakes Canada; Hasegawa & Kanamori (1987)
Enormous (~100 km3) submarine landslide cut trans-Atlantic telegraph cables & caused tsunami responsible for 27 deaths
Source studies (Hasegawa & Kanamori (1987), Bent (1995)) differ as to whether landslide triggered by earthquake
Other approach: consider aftershocks
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Plate boundary faults quickly reloaded by steady plate motion after
large earthquake
Faults in continents
reloaded much more slowly, so aftershocks continue much
longer
Passive margin may be similar
Stein & Liu, 2009Stein & Liu 2009
Rate-state friction predicts aftershock duration
1/loading rate
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Despite challenge of non-uniform detection, aftershocks of both earthquakes continue
for decades, suggesting tectonic earthquakes rather
than landslidesBaffin BayGrand Banks
detection?
detection?
Wolin & Stein, 2010
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1929 shaking similar to 1811-12 New Madrid
Propagation in ENAM crust
Similar magnitude
Hough et al, 2000
GSC
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Virginia 8/323/11: Reverse faulting on margin-parallel NE-SW striking,
SE dipping 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. PazzagliaR. Hermann
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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?
Are they related to regional uplift?
F. Pazzaglia
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M7
M7.21755 M6
1933 Baffin Bay M7.3
CAUSES?
Continental margin & plate interior contain many fossil faults developed at different times with different orientations but only a few appear active today
Platewide tectonic stresses, which change slowly in space and over millions of years, can’t account for variability
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Compression predicted across margin W.
Holt
Includes lithospheric structure, topography & mantle flow
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Sella et al., 2007
Additional possible stress source : 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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Extensions of Fracture Zones hypothesis (Sykes, 1978)
Inadequately describes locations
A. Kafka
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Long record needed to see real hazard
Swafford & Stein, 2007
1933 M 7.3
1929 M 7.2
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“Our glacial loading model suggests that earthquakes may occur anywhere along the rifted margin which has been glaciated.”
Stein et al., 1979
1985
Concentrated hazard bull's-eyes at historic earthquake sites2005
Diffuse hazard along margin
GSC
Map depends greatly on assumptions & thus has
large uncertainty
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Exciting Challenge: Find deformation and understand its causes & hazard
GPS, InSAR (Plate Interior Observatory)
Seismology, structural geology (map active & potentially seismogeneic structures)
Geomorphology (long term deformation)
Paleoseismology (look for past earthquakes)
Modeling (explain complexities)
Present Plate Boundary Observatory