Attempting to Reconcile Holocene And Long-Term Seismicity Rates in the New Madrid Seismic Zone Mark 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 – 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 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 (~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 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.

1811/1812 Events and Modern Seismicity Occur Within a Failed Rift of Late PreCambrian/ Early Paleozoic Age Following Rifting there was Late Cretaceous Volcanism and Faulting

Late Cretaceous Volcanism and Faulting

No Significant Fault Offset in Post Late-Cretaceous Sediments Hamilton and Zoback (1991)

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 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 Model Holocene Strain Rate ~ 1 x y -1 Grollimund and Zoback (2001)

Crustal Structure in the New Madrid Seismic Zone

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 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: Large Historical Earthquakes December 16, 1811: M w ~7.3M 0 ~1*10 20 January 23, 1812: M w ~7.0M 0 ~4*10 19 February 7, 1812: M w ~7.5M 0 ~2*10 20 M 0 = 10 (3/2*M w + 9.1) Using magnitudes from Hough et al., (2000)

ε ~ (cos30º)ΣM 0j (cos30º added because moment tensors are not all parallel) Shear modulus G = 30 GPa; Crustal volume V = 25,000 km 2 x 15 km = 375,000 km 3 ε ~ 1.3x10 -5 _________ 2*G*V How much strain per earthquake cycle? Strain rate ~ = 26 x10 -9 yr -1 Strain rate ~ = 13 x10 -9 yr -1 ___x____ 1000 years ___x ____ 500 years

Moment rate = b/(1.85 – b) x10 [a (1.85 – b)*M max ] = 6.5x10 16 N-m/yr Strain rate ~ (cos30º) M 0 (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*10 16 – 1*10 18 N-m/yrAreas: 6000 – 60,000 km 2 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 – 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! x10 -9 y -1 How do we reconcile the fast Holocene rate with the slow Cenozoic rate?