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?