Geology

Shaking Clues from the Mississippi

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Science  04 Aug 2006:
Vol. 313, Issue 5787, pp. 592
DOI: 10.1126/science.313.5787.592b

The seismic hazards associated with fault zones removed from plate boundaries are particularly difficult to assess. The New Madrid fault zone in the central United States produced a series of large earthquakes around the year 1812, and trenching has identified an episode of activity starting approximately 1000 years ago (900 C.E.), extending to the 1812 quakes. However, the earlier activity of the fault zone has been enigmatic, posing a problem in assessing risks.

One major fault in this seismic zone—the Reelfoot thrust fault—straddles the Mississippi River, and Holbrook et al. have therefore looked for past changes in the course of the river as an indicator of prehistoric quakes. Large quakes on the fault would have produced uplift to the south, thereby reducing the gradient of the river north of the fault. A lower gradient would then cause the river to straighten its course rather than meander. The river straightened approximately 1000 years ago, coincident with the known seismic activity. The authors also identify a second episode, between 3600 and 4200 years ago, when the river cut off many meandering channels. Thus, two episodes of faulting spanning roughly 1000 years seem to be separated by a several-thousand-year interval of fewer large quakes. The results suggest that another period of more frequent earthquakes could arise after long quiescence. — BH

Tectonophysics 420, 431 (2006).

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