September 30, 2013
American Heartland At High Risk For Sizable Earthquakes
[ Watch the Video: Earthquake Risks In America's Heartland ]
Lawrence LeBlond for redOrbit.com - Your Universe Online
An earthquake zone that extends from Marked Tree, Arkansas to Paducah, Kentucky and as far south as Memphis, Tennessee has a higher earthquake risk than adjacent areas within the United States, according to new research from the US Geological Survey (USGS).
Using sophisticated technology, USGS scientists have developed new high-resolution images of the New Madrid Seismic Zone (NMSZ), which allows them to map the area in more detail than ever before. These images allow for greater understanding of the weak rocks in this region, found at much greater depths in the Earth’s mantle compared to those in surrounding zones.
The USGS-led research was published in the journal Earth and Planetary Science Letters.
Some of the largest earthquakes in the US have occurred in the NMSZ, including three earthquakes greater than magnitude 7 on the Richter scale that occurred in 1811 and 1812. Smaller temblors that have occurred in this region since those more powerful ones have been significant in their own right.
"With the new high-resolution imagery, we can see in greater detail that the New Madrid Seismic Zone is mechanically weaker than surrounding areas and therefore concentrates movement and stress in a narrow area," said USGS scientist Fred Pollitz, who is the lead author of this research. "The structure beneath this zone is unique when compared to adjacent areas in the central and eastern United States. A more in-depth understanding of such zones of weakness ultimately helps inform decisions such as the adoption of appropriate building codes to protect vulnerable communities, while also providing insight that could be applied to other regions across the world."
The USGS has mapped the NMSZ before and had concluded that it is a region of high seismic hazard. However, previous assessments included earthquake records over a much shorter time span -- 4,500 years.
This mapping project looked at a much larger area – the 500-million-year-old Reelfoot Rift – specifically, with the NMSZ being located at the northernmost part of that region. The USGS team imaged rocks deep beneath the Earth’s surface to get a good sense of their characteristics and an understanding of their mechanical behavior, especially their ability to withstand a constant stream of stress and pressure.
Surprisingly, the team found that the weak rocks under the Reelfoot Rift fault lines extend more than 100 miles down into the mantle, much farther than weak rocks found in other ancient rift zones in the central and eastern US. The weak mantle rocks in the Reelfoot Rift area are more susceptible to concentration of tectonic stress and more mobile due to their low seismic velocity.
For the mapping project, the team relied on data from USArray, a large network of seismometers that make up one part of the EarthScope program operated by the National Science Foundation (NSF). These seismometers provide USGS and other scientists with images of the crust and mantle as far down as 120 miles.
"Our results are unexpected and significant because they suggest that large earthquakes remain concentrated within the New Madrid Seismic Zone," said USGS scientist Walter Mooney, the co-author of the report. "There are still many unknowns about this zone, and future research will aim to understand why the seismic zone is active now, why its earthquake history may be episodic over millions of years, and how often it produces large quakes."
The USGS research team said they hope to map the seismic structure of the entire country using data from USArray. The effort, which started in California nearly 10 years ago, is now focusing on the east coast and will later map out zones under Alaska. The team noted that all the USArray and Earthscope data will also help inform future USGS National Seismic Hazard Maps.