Science Nugget - By Charles Langston (University of Memphis), Christine Powell (University of Memphis), Steve Horton (University of Memphis), Heather Deshon (Southern Methodist University), Charles Ammon (Pennsylvania State University), and Robert Herrman (Saint Louis University) - NSF # EAR 1053530 | 2011-2016
A recent study using analysis techniques similar to those found in medical CT imaging finds that the upper 300 kilometers (186 miles) of the Earth under the active New Madrid Seismic Zone (NMSZ) are highly unusual suggesting that the deep Earth is both hot and contains fluids that could be driving the earthquake activity and may result in episodes of magma intrusion in portions of Tennessee, Missouri, Kentucky and Arkansas.
The researchers studied waves from over 300 distant earthquakes that were recorded by a network of seismometers covering eight states and encompassing the New Madrid seismic zone. The seismic experiment is named the Northern Embayment Lithosphere Experiment (NELE) and is part of the National Science Foundation EarthScope program.
Rocks with unusually low seismic velocities are present below NELE. The region of low velocity extends upward from great depth to the near surface close to the location of present day earthquakes within the NMSZ. Low velocity rocks are weak compared to surrounding material in the mantle and concentrate stress. Strong, high velocity rocks located below the NMSZ allow the elevated stress to extend to shallow parts of the crust, ultimately causing the earthquakes. The low velocities are produced by the presence of hot,wet conditions, both of which increase the likelihood that magmas could be produced.
Top Figure: Seismic velocity in the Earth along the slice BB’. Red area contains material with low velocities that is rising up from deep in the Earth.
Nyamwandha, C. A., C. A. Powell, and C. A. Langston (2016), A joint local and teleseismic tomography study of the Mississippi Embayment and New Madrid Seismic Zone, J. Geophys. Res. Solid Earth, 121, doi:10.1002/2015JB012761.
Nyamwandha, C. A. and C.A. Powell (2016), Seismic anisotropy beneath the Mississippi Embayment and New Madrid seismic zone: a study of shear wave splitting, J. Geophys. Res. Solid Earth (in review).