New Madrid

America’s Heartland is Earthquake Country      

Posted: 30 Sep 2013 06:30 AM PDT

New research provides insight on why the New Madrid Seismic Zone is unique and may continue to pose a higher earthquake risk than adjacent areas in the central United States.

Using innovative and sophisticated technology, scientists now have high-resolution imagery of the New Madrid Seismic Zone, allowing them to map the area in more detail than ever before. The maps allow for greater understanding of the weak rocks in this zone that are found at further depths in the Earth’s mantle compared to surrounding areas. Scientists also determined that earthquakes and their impacts are likely to be narrowly concentrated in this zone.

U.S. Geological Survey scientists led this research and recently published their findings in the journal, Earth and Planetary Science Letters.

A swarm of some of the largest historical earthquakes in the nation occurred in the New Madrid Seismic Zone, in particular three earthquakes greater than magnitude 7 occurred from 1811 to 1812. There have been several smaller, yet still significant, earthquakes in the area since then. This zone extends about 165 miles from Marked Tree, Ark., to Paducah, Ky. and the southern end of the zone is about 35 miles northwest of Memphis, Tenn.

“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.”

Prior to this research, the New Madrid Seismic Zone has been mapped by the USGS as an area of high seismic hazard, but those assessments were a consequence of a short (about 4,500 years) earthquake record for the area.

This research specifically investigated the Reelfoot Rift area, which is a 500-million-year-old geologic feature that contains the New Madrid Seismic Zone in its northernmost part. Scientists imaged rocks deep beneath Earth’s surface to see their characteristics and understand their mechanical behavior, especially their ability to withstand the huge stresses constantly placed on them.

A surprising finding was that weak rocks underlie the fault lines in the crust of the Reelfoot Rift and extend more than 100 miles down into the mantle. In contrast, weak rocks in other ancient rift zones in the central and eastern United States bottom out at much shallower depths.  These weak mantle rocks have low seismic velocity, meaning that they are more susceptible to concentration of tectonic stress and more mobile.

USGS scientists used data from USArray, which is a large network of seismometers that is a component of the EarthScope program of the National Science Foundation. These seismometers provide images of the crust and mantle down to 120 miles (200 kilometers) beneath the surface using the methods employed by these scientists.

“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.”

In the future, USGS scientists plan to map the seismic structure of the entire nation using USArray.  This effort started in California in 2004, is focusing on the east coast next, and will then move to Alaska. All of the USArray and other Earthscope efforts will also help inform the USGS National Seismic Hazard Maps.

Advertisement

Not So Constant: Atomic Wights Changed for Five Chemical Elements

 

---

 

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communications and Publishing
12201 Sunrise Valley Dr, MS 119
Reston, VA  20192

Standard atomic weights for chemical elements have commonly been considered as constants of nature, along with the speed of light and the attraction of gravity. Hold on to your Newtonian hat and prepare for the possibility of elementary nuances. The International Union of Pure and Applied Chemistry (IUPAC) Commission on Isotopic Abundances and Atomic Weights has published a new table that expresses the standard atomic weights of magnesium and bromine as intervals, rather than as single standard values. In addition, improved standard atomic weights have been determined for germanium, indium, and mercury. This new table is the result of cooperative research supported by the U.S. Geological Survey, IUPAC, and other contributing Commission members and institutions. Modern analytical techniques can measure the atomic weight of many elements with such precision that small variations in an element’s atomic weight serve as markers for certain physical, chemical, and biological processes. “The USGS has a long history of research in this field,” said acting USGS Director Suzette Kimball. “Through isotopic analysis, USGS scientists detect slight variations in atomic weights of various elements, which can be applied to a wide variety of mission-critical investigations, such as the identification of the geographic origin of materials, quantification of surface-water groundwater interaction, and understanding paleoclimatic conditions.” “We are pleased to partner with the International Union [IUPAC] in this vital work,” Kimball added. Atoms of the same element that have different masses are called “isotopes.” The atomic weight of an element depends upon how many stable isotopes it has and the relative amounts of each stable isotope present in a sample containing the element. Elements with only one stable isotope do not exhibit variations in their atomic weights. For example, the standard atomic weights for fluorine, aluminum, sodium, and gold are constant. Their values are known to better than six decimal places. Variations in atomic weight occur when an element has two or more naturally occurring stable isotopes that vary in abundance, depending on the sample. The standard atomic weights of magnesium and bromine will now be expressed as intervals to more accurately convey this variation in atomic weight. For example, bromine commonly is considered to have a standard atomic weight of 79.904. However, its actual atomic weight can be anywhere between 79.901 and 79.907, depending on where the element is found. IUPAC previously adjusted the standard atomic weights of the elements hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium as intervals to reflect variations in their atomic weights. “For more than a century and a half, many students have been taught to use standard atomic weights — a single value — found on the inside cover of chemistry textbooks and on the periodic table of the elements,” said Ty Coplen, director of the USGS Stable Isotope Laboratory in Reston, Va. “Though this change offers significant benefits in the understanding of chemistry, one can imagine the challenge now to educators and students who will have to select a single value out of an interval when doing chemistry calculations.” Practical applications of this research can be easily found in daily life. For example, precise measurements of the abundances of isotopes of carbon can be used to determine the purity and source of food products, such as vanilla and honey. Isotopic measurements of nitrogen, chlorine and other elements are used for tracing pollutants in streams and groundwater. In investigations of sports doping, performance enhancing testosterone can be identified in the human body because the atomic weight of carbon in natural human testosterone is different from that in pharmaceutical testosterone. The importance of determining precise atomic weights has long been recognized. As far back as 1882, Frank W. Clarke, then a professor at the University of Cincinnati, prepared a table of atomic weights for use in science, industry, and trade. He carried on this work as Chief Chemist of the USGS (1883-1924). Clarke was a founder of the American Chemical Society and a member of the National Academy of Sciences. Recently, IUPAC has overseen the periodic evaluation and dissemination of atomic-weight values. The report, published in Pure and Applied Chemistry, also includes educational material and a Periodic Table of the Isotopes illustrating the relationship between isotopes and atomic weights. Not So Constant: Atomic Weights Changed for Five Chemical Elements