Rift and separate

I am not quite old enough to remember the 1811-1812 earthquakes centered on New Madrid, Missouri, though I do know this much: there have been literally thousands of smaller quakes within the New Madrid Seismic Zone, and “Madrid” is pronounced with the accent on the first syllable: “MAD-rihd.” (Yes, I have offspring living in Missouri. Why do you ask?)

What I did not even suspect, though, is that those recent rumblings may be aftershocks from The Big Ones:

The small earthquakes that sporadically rattle the central United States may actually be aftershocks from a few extremely large quakes that occurred in the region almost 200 years ago, according to a new study.

Seismically speaking, this sounds awfully slow. And yet:

For one thing, “there’s no motion across the fault now, so nothing’s going on, but yet there are still small earthquakes there,” said Seth Stein, the study’s lead author and a professor of geological sciences at Northwestern University. The small quakes also occur on the same fault plane that researchers believe is responsible for the big quakes. Furthermore, the present-day temblors are getting smaller with time, which is a characteristic of aftershocks, Stein said. And when larger quakes do occur, they happen at the corners of the fault section that scientists think broke during the 19th century earthquakes, a pattern that suggests these are aftershocks, Stein told LiveScience.

A comparison to The Other Big One works out this way:

[T]he San Andreas Fault in California, which moves at the relatively fast speed of about 1.5 inches (3.8 cm) per year, will only have aftershocks for about 10 years after a large quake, Stein said. The fast motion essentially “reloads” the fault, wiping out the effects of a previous earthquake and suppressing aftershocks.

On the other hand, the New Madrid faults, known as the “Reelfoot Rift,” move more than 100 times more slowly than the San Andreas fault, allowing the aftershocks to last much longer. The researchers found a similar pattern in faults around the world.

The most recent quake reported in Oklahoma was Monday afternoon. The epicenter was located west of Okemah; the quake’s estimated magnitude was 2.7.





2 comments

  1. fillyjonk »

    5 November 2009 · 8:18 am

    Wow, you learn something new every day. I had no idea aftershocks could come that late.

    (I admit I think a bit about the New Madrid fault a bit – and “what would happen if an earthquake happened RIGHT NOW” every time I travel across the Eads Bridge from Missouri into Illinois…)

  2. Kirk »

    5 November 2009 · 11:27 am

    When I lived in Iowa, we would occasionally feel some of the quakes centered in southern Illinois or eastern Missouri. What if, indeed.

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