Tectonic irregularity may cause southeast U.S. temblors

USGS map highlighting short-term earthquake risk zones. Blue boxes indicate areas with induced

Seismic images of Southeast U.S. show unstable thickness in tectonic plate could be cause of recent quakes

In 2011, a powerful earthquake centered deep under central Virginia shook Hampton Roads and was felt by millions more from Georgia to Canada.

Since then, scientists have been searching for a likely cause for that magnitude 5.8 temblor — and for dozens of lesser regional ones since — that occurred not along the volatile boundaries where tectonic plates grind and collide to cause quakes, but in the Southeastern U.S., a thousand miles from the nearest edge.

Now scientists believe they may have found the answer: In this region, pieces of the vast North American Plate are breaking off and sinking into the Earth, even as others are welling up. This means the plate's thickness is far less homogenous, and far more heterogenous or "piecemeal," than previously thought.

"This is an interesting finding, because everybody thought that this is a stable region, and we would expect regular plate thickness," seismologist C. Berk Biryol told the American Geophysical Union last week. Biryol is at the University of North Carolina in Chapel Hill and lead author of a study on their findings that appeared in AGU's "Journal of Geophysical Research — Solid Earth."

The ongoing imbalance in the plate, Biryol said, means earthquakes in the region are likely to continue. The North American Plate stretches from the West Coast to the Mid-Atlantic Ridge, and from the Caribbean to the Arctic oceans and eastern Siberia.

Within tectonic plates are individual seismic zones that exhibit various levels of activity.

The Central Virginia Seismic Zone, for instance, may see half a dozen mild earthquakes every year, said Christopher "Chuck" Bailey, a structural geologist at the College of William and Mary in Williamsburg. Bailey studies tectonics but wasn't involved in the study.

Hampton Roads is on the shoulder of what's considered a quiescent or inactive zone, he said. "There certainly have been earthquakes in Hampton Roads," Bailey said. "They are less common and less frequent than the Central Virginia Seismic Zone."

For their study, scientists focused on the Southeastern U.S., where they used a network of detectors throughout the region to measure seismic waves generated by more than 750 earthquakes around the world.

Seismic waves pass faster through older, denser rock and slower through newer, warmer rock, so they were able to scan 100 or 200 miles below the surface to create 3-D images of the interior of the Earth, much like a CAT scan of the planet.

From those images, they determined the upper mantle of the Southeastern U.S. is far more complicated than previously believed. "It's fabulous data," Bailey said. "It really does show it's much more interesting than maybe we would've thought prior to this."

In eastern North America, in particular, he said, the crust and mantle are heterogenous in part because of the region's long history.

"You have evidence of very old plate collisions, and you have evidence for parts of the upper part of the mantle ... which is dripping back inside — kind of almost like a heavy sock which is starting to founder and sink," Bailey said. "And then there are places like central Virginia, where it looks like there may actually be bits and pieces of mantle that are actually welling up or slowly rising."

The study didn't draw a direct link to the 2011 quake, he said, but explored the natural processes that produce the seismic stresses that can cause earthquakes within a tectonic plate.

The findings, he said, are impressive.

"It shows the power of modern seismology," Bailey said. "A few years back, we had an array of seismometers that slowly moved across the country. So there were literally hundreds and hundreds of seismic stations that were located in eastern North America.

"And, in essence, this is sort of the fruits of that campaign of sort of sitting, quietly listening for earthquakes. We can now put together some pretty detailed maps of what the deep Earth underneath the U.S. looks like."

The original article was written by Tamara Dietrich
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