Some background on the New Madrid Seismic Zone debate

The New Madrid seismic zone has been the focus of an important debate in the past years, triggered in part by a controversy over the amount of strain currently accumulating in that area, as measured by geodetic techniques. The debate is of importance because the current National Seismic Hazard Maps show, for the New Madrid area, a predicted peak ground acceleration expected in 50 years at 2% probability larger than in San Francisco and a predicted highest acceleration (>1.2 g) larger than for Los Angeles or San Francisco. As noted by Newman et al. (1999), this result is largely constrained by the assumption of a 1000 year recurrence time for M8 earthquake in the New Madrid area.

Using a GPS-triangulation comparison, Liu et al. (1992) found a shear strain rate of 0.108+-0.045 microrad/yr in the southern part of the NMFZ, corresponding to a 5 to 7 mm/yr of slip rate. Snay et al. (1994), using similar data in the northern part of the NMFZ, found strain rates of 0.030+-0.019 microrad/yr, indistinguishable from zero. More recently, Weber et al. (1998) and Newman et al. (1999) used GPS data from campaigns performed between 1991 and 1997 and found a slip rate of 0.2+-2.4 mm/yr. Using permanent GPS stations in the central US, they find far-field motion less than 2mm/yr across the NMFZ, confirming earlier estimates of NOAM plate rigidity by Argus and Gordon (1996) and Dixon et al. (1996). In both cases, motion across the NMFZ does not significantly differ from zero at the 95% confidence level.

Assuming that large earthquakes such as the 1811-1812 events repeat regularly in time, these results translate into a 2500 yr recurrence interval for such events (Figure), significantly longer than the 400-1000 yr recurrence interval reported from paleoseismological studies (e.g. Russ et al., 1978; Tuttle and Schweig, 1995). Newman et al. (1999) therefore suggests that (1) the 1811-1812 events had smaller magnitudes than previously estimated, and (2) seismic hazard in the New Madrid area is overestimated. This conclusion has sparked a debate in our community and with the agencies in charge of earthquake regulations and policies in the US. The case is not yet settled, in part because of uncertainties in the geodetic estimates of strain accumulation in the central and eastern United States.

Additional difficulties arise because (1) the characteristic earthquake model (earthquakes of similar magnitude repeat regularly on a given fault segment), on which the figure below is based, is not proven to apply to all faults -- we do not know whether or not it applies to new Madrid faults, (2) seismogenic deformation in the New Madrid area may be localized to deeper crustal levels (as suggested by the current seismicity distribution), therefore resulting in small surface effects.

Figure. Relation between far-field interseismic motion and recurrence time, assuming that characteristic earthquakes repeat regularly on a given active fault. Modified from Newman et al., 1999.

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Liu, L., M.D. Zoback, and P. Segall (1992). Rapid intraplate strain accumulation in the New Madrid seismic zone, Science, 257, 1666-1669.
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Snay, R.A., J.F. Ni, and H.C. Neugebauer (1994). Geodetically derived strain across the northern New Madrid Seismic Zone, U.S. Geol. Surv. Prof. Pap. 1538-F.
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