NEW MADRID: A dying fault? GPS seismology geology Heat flow Recent data, taken together, suggest that the New Madrid seismic zone may be shutting down after the recent cluster of large earthquakes in the past ~1000 years. If so, it will be a very long time until the large earthquakes of recur. ?? 9k7k6k4k12k3k1kToday Portageville CycleReelfoot CycleNew Madrid Cycle Slip Cluster Slip Cluster Slip Cluster Quiescent Holocene Punctuated Slip New Madrid earthquake history inferred from Mississippi river channels Holbrook et al., 2006

GPS measurements show that the ground is barely moving or isn’t moving, so little or no strain is building up for a future earthquake The small earthquakes we see today look like aftershocks of the large earthquakes The New Madrid zone doesn’t look thermally or mechanically different from many other structures in the central U.S. Geological data show that the New Madrid zone goes through pulses of activity The simplest explanation is that the present pulse is ending A DYING FAULT? In the past 15 years we’ve learned:

GPS SITES SHOW LITTLE OR NO MOTION Motions with respect to the rigid North American plate are small, < 2 mm/yr, and generally within their error ellipses. The data do not require motion, and restrict any motion to being very slow. Thus a very long time would be needed to store up the slip needed for a future large earthquake For steady motion, M 7 is at least hundreds of years away. M 8 would thousands. Stein 2007

NEW MADRID SEISMICITY: AFTERSHOCKS? Ongoing seismicity looks like aftershocks of , as suggested by the fact that the rate & size are decreasing. Moreover, the largest are at the ends of the presumed ruptures Looks like a dying fault? Stein & Newman, 1994

Dieterich (1994) model relates ratio of aftershock length to main shock recurrence t a / t r  1/stressing rate For low intraplate stressing rate, could have 200 year aftershocks for 500 yr recurrence Hence large style events may not recur here for a very long time, but could migrate elsewhere. INTERPLATE EARTHQUAKES ROCK MECHANICS CONSISTENT WITH LONG INTRAPLATE AFTERSHOCK SEQUENCES Stein & Newman, 1994

?? 9k7k6k4k12k3k1kToday Portageville CycleReelfoot CycleNew Madrid Cycle Slip Cluster Slip Cluster Slip Cluster Quiescent Holocene Punctuated Slip New Madrid earthquake history inferred from Mississippi river channels Holbrook et al., 2006 GEOLOGY IMPLIES NEW MADRID EARTHQUAKES ARE EPISODIC & CLUSTERED The absence of significant fault topography, and other geological data, imply that the recent pulse of activity is only a few thousand years old. This is consistent with results from other continental interiors showing episodic & clustered earthquakes

o Historical o Instrumental NEW MADRID SEISMICITY: EPISODIC & MIGRATING? Why are NMSZ earthquakes concentrated on the Reelfoot Rift, when the continent contains many fossil structures that seem equally likely candidates for concentrated seismicity? Liu & Zoback (1997) suggest the NMSZ is hotter and thus weaker than surrounding regions, so the weak lower crust and mantle concentrate stress and seismicity here in the upper crust. Alternatively, if the NMSZ is not significantly hotter and weaker than its surroundings, seismicity is likely to be episodic and migrate among many similar fossil weak zones. M. Liu

NMSZ NOT HOT & WEAK Liu & Zoback (1997) argue that NMSZ heat flow is ~ 15 mW/m 2 higher than the surrounding area, so crust and upper mantle are significantly hotter and thus weaker than surroundings. Reanalysis finds the anomaly is either zero or much smaller (3+/-23 mW/m 2 ), so the NMSZ and CEUS have essentially the same temperature & strength. Hence there is no tendency for upper crustal stresses to concentrate in the NMSZ. Liu & Zoback, 1997 McKenna et al., 2007

GPS, seismological, geologic, & geothermal data are consistent with the NMSZ shutting down after the recent cluster of large events The longer geodetic data show essentially no motion, the more likely it seems that the fault is shutting down Seismicity may migrate to somewhere else Hazard from style large events may be small for thousands of years In this case, the seismic hazard from large style events should be viewed as lower and diffuse rather than high and concentrated near the rupture.