Tuesday, March 9, 2010

360 Analysis: US Small Quakes Are Aftershocks of 19th Century Quakes; One Quake Triggers Another

New research has shown that most small earthquakes occurring in the US are actually aftershocks of big earthquakes that happened in the 19th century.

Researchers from Northwestern University and the University of Missouri-Columbia conducted the study.

When small earthquakes shake the central US, citizens often fear the rumbles are signs a big earthquake is coming.

Now, new research has shown that most of these earthquakes are aftershocks of big earthquakes (magnitude 7) in the New Madrid seismic zone that struck the Midwest almost 200 years ago.

“This sounds strange at first. On the San Andreas fault in California, aftershocks only continue for about 10 years. But in the middle of a continent, they go on much longer,” said the study’s lead author, Seth Stein, the William Deering Professor of Geological Sciences in the Weinberg College of Arts and Sciences at Northwestern.

There is a good reason, according to co-investigator Mian Liu, professor of geological sciences at Missouri.

“Aftershocks happen after a big earthquake because the movement on the fault changed the forces in the earth that act on the fault itself and nearby. Aftershocks go on until the fault recovers, which takes much longer in the middle of a continent,” he added.

The difference, according to Stein, is that the two sides of the San Andreas fault move past each other at a speed of about one and a half inches in a year - which is fast on a geologic time scale.

This motion “reloads” the fault by swamping the small changes caused by the last big earthquake, so aftershocks are suppressed after about 10 years.

The New Madrid faults, however, move more than 100 times more slowly, so it takes hundreds of years to swamp the effects of a big earthquake.

“A number of us had suspected this because many of the earthquakes we see today in the Midwest have patterns that look like aftershocks. They happen on the faults we think caused the big earthquakes in 1811 and 1812, and they’ve been getting smaller with time,” Liu said.

To test this idea, Stein and Liu used results from lab experiments on how faults in rocks work to predict that aftershocks would extend much longer on slower moving faults.

“Instead of just focusing on where small earthquakes happen, we need to use methods like GPS satellites and computer modeling to look for places where the earth is storing up energy for a large future earthquake,” said Stein.

One Quake Can Trigger Another

The recent cluster of large earthquakes in the South Pacific has led some scientists to wonder if one quake triggers another.

Last week, a series of seven earthquakes rocked a small area near Vanuatu and the Santa Cruz Islands in less than 24 hours.

The unusual swarm of four magnitude 6 and three magnitude 7 events, and many smaller aftershocks, has a lot of people asking what it means and whether the quakes have any connection to the very recent large quake in Samoa or even the 9.2 Sumatra-Andaman monster earthquake in 2004.

“It’s not unprecedented, but getting three events above 7 is unusual,” seismic researcher Dr Susan Hough of the US Geological Survey told ABC News.

Hough has investigated similar, more powerful quake swarms - most notably the five large 7 and 8-magnitude quakes that struck the New Madrid area of Missouri in the winter of 1811-1812.

“What New Madrid and other clusters of large quakes have taught seismologists is that it’s largely a game of chance,” said Hough.

“Since we don’t have enough data on the exact happenings inside the Earth’s crust to be more precise, we must resort to working out the odds of one shock being a foreshock of a larger quake or not, based on what’s happened before,” she added.

“For instance, anytime an earthquake happens there’s a one-in-20 chance something bigger will follow,” said Hough.

In the case of the Vanuatu swarm, there was first a 4.9 foreshock, then hours later a mighty 7.6 shock.

That second quake might have been mistaken for a main event had it not been followed 15 minutes later by 7.8 magnitude show stopper.

An hour later, there was a vigorous 7.3 magnitude aftershock, then a couple of dozen more aftershocks ranging from magnitude 4.9 to 6.8.

“The good news from all this is that despite the rarity of the swarm, these quakes, individually, are not the worst sorts of events produced in the collision zone of tectonic plates in that part of the world,” said Hough.

She said that a genuinely big quake for a subduction zone is more in the range of a magnitude 9.

Then there is the matter of whether the South Pacific cluster is in any way “teleconnected,” as seismologists say, to the 8.0 magnitude Samoan event of September 29 or the 9.2 magnitude Sumatra-Andaman event of 2004.

“It’s well within the realm of possibility,” said Hough. “The (seismic) waves come tumbling through and disturb the fault zone and it causes little earthquakes,” she added.

Recent earthquakes around the U.S.

Time Magnitude Location
3 days ago 3.1 Oklahoma Map
6 days ago 2.5 Oklahoma Map
6 days ago 3.7 Southeastern Missouri Map

USGS; Northwestern University; University of Missouri-Columbia; ABC News.

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