NEW MADRID: A dying fault?

GPSseismology

geologyHeat 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 1811-12 recur.

? ?

9k 7k 6k 4k12k 3k 1k Today

Portageville Cycle Reelfoot Cycle New Madrid Cycle

Slip

Cluster

Slip

Cluster

Slip

ClusterQuiescent Quiescent Quiescent

Holocene Punctuated Slip New Madrid earthquake history inferred from Mississippi river channelsHolbrook et al., 2006

Precise 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 1811-12 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: 1811-12 AFTERSHOCKS?

Ongoing seismicity looks like aftershocks of 1811-12, as

suggested by the fact that the rate & size are decreasing. Moreover, the largest are at the ends of the presumed

1811-12 ruptures

Looks like a dying fault?

Stein & Newman, 1994

Dieterich (1994) model relates ratio of aftershock length to main shock recurrence

ta/ tr 1/stressing rate

For low intraplate stressing rate, could have 200 year aftershocks for 500 yr recurrence

Hence large 1811-12 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

? ?

9k 7k 6k 4k12k 3k 1k Today

Portageville Cycle Reelfoot Cycle New Madrid Cycle

Slip

Cluster

Slip

Cluster

Slip

ClusterQuiescent Quiescent Quiescent

Holocene Punctuated SlipNew 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

IS NMSZ HOT & WEAK?

Liu & Zoback (1997) argue that NMSZ heat flow ~ 15 mW/m2 higher than the surrounding area, so crust and upper mantle are significantly hotter and thus weaker than surroundings.

NMSZ heat flow, 55+/-7 mW/m2, at most slightly higher than mean eastern U.S. heat flow of 52 +/-22 mW/m2

Anomaly is either absent or much smaller (3+/-23 rather than 15 mW/m2) than assumed

The resulting geotherms predict that temperature differences between the NMSZ and the average CEUS are trivial in theseismogenic crust, and small in the mantle, rather than the large differences predicted by Liu & Zoback (1997)

LZ are geotherms plotted in Liu & Zoback (1997), LZC denote geotherms computed from Liu & Zoback (1997) values. MSS denotes geotherms for our models. BDS is basalt dry solidus.

For our model of a much smaller temperature contrast, the NMSZ and CEUS have essentially the same strength.

Hence there would be no tendency for upper crustal stresses to be concentrated in the NMSZ.

NMSZ IS NOT HOT & WEAK

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 1811-12 style large events may be small for thousands of years

In this case, the seismic hazard from large 1811-12 style events should be viewed as

lower and diffuse rather than high and concentrated near the 1811-12 rupture.