Attempting to Reconcile Holocene And Long-Term Seismicity Rates in the New Madrid Seismic Zone
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Attempting to Reconcile HoloceneAnd Long-Term Seismicity Rates in the
New Madrid Seismic ZoneMark Zoback – Stanford University
NASA World Wind
looking west
10X vertical exaggeration
Reelfoot Lake
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?
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.
Paleoseismic Study Areas
Paleoseismic Data Indicates 2 to 4 Large Earthquakes Prior to 1811-1812 (~3-5 Large Events in Past 2600 Years)
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.
1811/1812Events and
Modern Seismicity Occur
Within aFailed Rift of Late
PreCambrian/ Early Paleozoic
Age
Following Rifting there was
Late CretaceousVolcanism and
Faulting
Late Cretaceous Volcanism and Faulting
No Significant Fault Offset in Post Late-Cretaceous Sediments
Hamilton and Zoback (1991)
New Madrid SeismicityNew 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?
Localized Weak Mantle ModelLocalized Weak Mantle Model
Holocene Strain Rate ~ 1 x 10-9 y-1
Grollimund and Zoback (2001)
Crustal Structure in the New Madrid Seismic Zone
Geodetic Estimates of StrainGeodetic Estimates of StrainA 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 14*10-9 y-1
Geomorphic estimates of strain:Surface Uplift
~10 m (in the last 2400 years)
Reelfoot Lake
NASA World Wind
looking west
10X vertical exaggeration
Earthquakes Fault Slip
Consistent with Regional
NE-SW Compression
Reelfoot Fault
~100 km
~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
Assume the Reelfoot fault accommodates all the motion in a transpressional system which dominates the local strain budget.
Seismic Moment Summation I:Seismic Moment Summation I:Large Historical EarthquakesLarge Historical Earthquakes
• December 16, 1811: Mw~7.3 M0~1*1020
• January 23, 1812: Mw~7.0M0~4*1019
• February 7, 1812: Mw~7.5M0~2*1020
M0 = 10(3/2*Mw + 9.1)
Using magnitudes from Hough et al., (2000)
ε ~ (cos30º)ΣM0j (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
ε ~ 1.3x10-5
_________ 2*G*V
How much strain per earthquake cycle?How much strain per earthquake cycle?
Strain rate ~ 1.3 10-5 = 26 x10-9 yr-1
Strain rate ~ 1.3 10-5 = 13 x10-9 yr-1___x____ 1000 years
___x ____ 500 years
Moment rate = b/(1.85 – b) x10[a + 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
_________ 2*G*V
Seismic Moment Summation From Recent Earthquakes
Seismic Moment Rates:Ask 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.
Anderson, 1986
Expert values within 1σ
Moment rates: 4*1016 – 1*1018 N-m/yr Areas: 6000 – 60,000 km2 Strain rates: 1.6*10-9 – 25*10-9 per year
Strain rates on maps are in picostrain/year.
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 < 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?