Impact of Deglaciation on Stress

1
Attempting to Reconcile Holocene And Long-Term
Seismicity Rates in the New Madrid Seismic
Zone Mark Zoback Stanford University
Reelfoot Lake
NASA World Wind looking west 10X vertical
exaggeration
Reelfoot Scarp
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Strain Rate Estimates in the New Madrid Seismic
Zone

• Holocene Strain Rates
• Seismic moment summation
• 1811-1812 events
• Recent seismicity
• Deglaciation model (Holocene trigger)
• Geomorphic constraints
• Geodetic estimate on upper bound strain rate
• Ask the Experts!

How do we reconcile the fast Holocene rate with
the slow Cenozoic rate?
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New Madrid Seismicity

• Intraplate region in the vicinity of a late
  Pre-Cambrian or early Paleozoic rift system.
• High level of background seismicity.
• 3 very large earthquakes in 1811-1812.
• Paleoseismic data indicates recurrence of large
  earthquakes every 500-1,000 years in late
  Holocene.

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Paleoseismic Study Areas
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Paleoseismic Data Indicates 2 to 4 Large
Earthquakes Prior to 1811-1812 (3-5 Large
Events in Past 2600 Years)
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New Madrid Seismicity

• Intraplate region in the vicinity of a late
  Pre-Cambrian or early Paleozoic rift system.
• High level of background seismicity.
• 3 very large earthquakes in 1811-1812.
• Paleoseismic data indicates recurrence of large
  earthquakes every 500-1,000 years in late
  Holocene.
• Seismic reflection profiles show small cumulative
  fault offset in post Late Cretaceous Mississippi
  embayment sediments.

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1811/1812 Events and Modern Seismicity Occur
Within a Failed Rift of Late PreCambrian/ Early
Paleozoic Age
Following Rifting there was Late
Cretaceous Volcanism and Faulting
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Late Cretaceous Volcanism and Faulting
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No Significant Fault Offset in Post
Late-Cretaceous Sediments
Hamilton and Zoback (1991)
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New Madrid Seismicity

• Intraplate region in the vicinity of a late
  Pre-Cambrian or early Paleozoic rift system.
• High level of background seismicity.
• 3 very large earthquakes in 1811-1812.
• Paleoseismic data indicates recurrence of large
  earthquakes every 500-1,000 years in late
  Holocene.
• Seismic reflection profiles show small cumulative
  fault offset in post Late Cretaceous Mississippi
  embayment sediments.
• Recent seismic activity appears to have been
  triggered in Holocene time. Triggered by
  deglaciation?

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Localized Weak Mantle Model
Holocene Strain Rate 1 x 10-9 y-1
Grollimund and Zoback (2001)
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Crustal Structure in the New Madrid Seismic Zone
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Geodetic Estimates of Strain
A majority of GPS studies have concluded that
strains in the New Madrid area are below the
limits of detection. Calais et al. (2006)
conclude that displacements are less than 1.4
mm/yr., with 95 confidence. Over a 100 km
baseline, this limits strain rates to be less
than 1410-9 y-1
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Geomorphic estimates of strainSurface Uplift
10 m (in the last 2400 years)
Reelfoot Lake
NASA World Wind looking west 10X vertical
exaggeration
Reelfoot Scarp
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Earthquakes Fault Slip Consistent with
Regional NE-SW Compression
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Assume the Reelfoot fault accommodates all the
motion in a transpressional system which
dominates the local strain budget.
250 km
10 m vertical separation on a 73º fault
corresponds to 3 m strike slip on the overall
system. 3 m / 100 km 3x10-5 Over 2400 years,
this represents 3x10-5/2400 12x10-9 per year
Reelfoot Fault
100 km
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Seismic Moment Summation ILarge Historical
Earthquakes
Using magnitudes from Hough et al., (2000)

• December 16, 1811 Mw7.3 M011020
• January 23, 1812 Mw7.0 M041019
• February 7, 1812 Mw7.5 M021020
• M0 10(3/2Mw 9.1)

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How much strain per earthquake cycle?
e (cos30º)SM0j (cos30º added because
moment tensors are not all parallel) Shear
modulus G 30 GPa Crustal volume V 25,000
km2 x 15 km 375,000 km3 e 1.3x10-5
_________ 2GV
Strain rate 1.3 10-5 26 x10-9 yr-1
___x ____ 500 years
Strain rate 1.3 10-5 13 x10-9 yr-1
___x____ 1000 years
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Seismic Moment Summation From Recent Earthquakes
Moment rate b/(1.85 b) x10a 6.5 (1.85
b)Mmax 6.5x1016 N-m/yr Strain rate
(cos30º) M0 (cos30º added because
moment is not all parallel) Strain rate
2.5x10-9 per year
_________ 2GV
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Seismic Moment RatesAsk the Experts?
As part of a report for the Nuclear Regulatory
Commission (Bernreuter et al., 1984), thirteen
experts in seismicity of the eastern U. S. were
asked to evaluate earthquake hazards to nuclear
power plants. Anderson (1986) used the
earthquake recurrence relations provided by these
experts to calculate total moment rates and the
corresponding strains.
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Expert values within 1s Moment rates 41016
11018 N-m/yr Areas 6000 60,000 km2 Strain
rates 1.610-9 2510-9 per year
Strain rates on maps are in picostrain/year.
Anderson, 1986
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Strain Rate Estimates in the New Madrid Seismic
Zone

• Holocene Strain Rates
• Seismic moment summation
• 1811-1812 events 13-26 x10-9 y-1
• Recent seismicity 2.5 x10-9 y-1
• Deglaciation model (Holocene trigger) 1x10-9 y-1
• Geomorphic constraints 12 x10-9 y-1
• Geodetic estimates lt 14x10-9 y-1
• Ask the Experts! 1.6-25 x10-9 y-1

How do we reconcile the fast Holocene rate with
the slow Cenozoic rate?