High Resolution Seismic Imaging Study in the Mississippi Embayment

USGS and its NEES Partner Conduct Earthquake Study in the New Madrid Seismic Zone
Using a 12,000-pound vibrating truck named Thumper, USGS studies earthquake history and hazard

WHAT: In 2006 the U.S. Geological Survey (USGS), in collaboration with the University of Memphis and the Network for Earthquake Engineering Simulation (NEES) at the University of Texas-Austin, explored the sediment beneath the Mississippi River valley near Lepanto and Crowley's Ridge, Arkansas. By vibrating the ground with Thumper and listening to the echoes, this high-resolution seismic-imaging study can 'see' into sedimentary layers that are hidden beneath a blanket of river deposits laid down over the past few thousand years. This blanket of sediments masks nearly all evidence of faulting and can dramatically alter the way the ground shakes when earthquakes occur. The data can reveal evidence of past earthquakes (or lack of) and lead to a better understanding of how the soil responds to shaking. The work in 2006 was the first of a multi-year project to acquire a continuous seismic cross section to examine past earthquake history in the New Madrid seismic zone.

2007 SSA abstract

View of study area showing the two seismic profile locations (red lines) and proposed profiles shown (yellow dashed lines) that would complete the Transect across the Reelfoot rift. Seismicity from Wheeler et al. (2003).

WHAT: In 2006 the U.S. Geological Survey (USGS), in collaboration with the University of Memphis and the Network for Earthquake Engineering Simulation (NEES) at the University of Texas-Austin, explored the sediment beneath the Mississippi River valley near Lepanto and Crowley's Ridge, Arkansas. By vibrating the ground with Thumper and listening to the echoes, this high-resolution seismic-imaging study can 'see' into sedimentary layers that are hidden beneath a blanket of river deposits laid down over the past few thousand years. This blanket of sediments masks nearly all evidence of faulting and can dramatically alter the way the ground shakes when earthquakes occur.

The data can reveal evidence of past earthquakes (or lack of) and lead to a better understanding of how the soil responds to shaking. The work in 2006 was the first of a multi-year project to acquire a continuous seismic cross section to examine past earthquake history in the New Madrid seismic zone.

Thumper working on County Rd 800 on the west side of Crowley's Ridge, November 2006

HOW: A small, rubber-tired vibroseis truck (Thumper) generates vibrations to create small waves that will travel into the earth, be reflected by layered sediment, and then be recorded by a 720-m-long string of seismometers spread out along the survey line. The vibroseis truck is preferred over explosions as an energy source near cities and towns because the vibrations are spread out over 10 to 20 seconds, rather than being released suddenly, and because no drilling of shot holes is needed. The truck shakes the ground slightly, then moves on and leaves nothing behind -- only if you stand close to the truck can you feel the slight shaking in the bottom of your feet.

Thumper drives slowly along the survey line and stops every 5 m to vibrate. A 1.2-m-wide pad is pressed against the ground beneath the truck to carry part of the truck's 12,000-lb weight, and then the pad is vibrated for about 10-12 seconds. This 10-second "sweep" is repeated several times, and then the truck moves forward 5 m to repeat the process at the next vibration point. The end result is a 6-mile long sonogram-like image of the earth from which earthquake hazard assessments can be made.


Map of middle Mississippi River valley in the central U.S. showing topography

Map of middle Mississippi River valley in the central U.S. showing topography (purple at about 47 m up to orange at about 360 m elevation). White heavy lines bound the Reelfoot rift. New Madrid seismic zone seismicity shown by white and blue circles, are evidence that earthquakes threaten the Mississippi, Ohio, and Wabash River valleys of the Central United States. Several of the largest historical earthquakes to strike the continental United States occurred in the winter of 1811–1812 along the New Madrid seismic zone, which stretches from just west of Memphis, Tenn., into southern Illinois. Black box outlines Transect study area. (Base topographic map courtesy of Robert Smalley, Univ. of Memphis.)

WHEN: May 8-19 and November 4-18, 2006

WHERE: Lepanto, AR on State Highway 14. Located about 42 miles NW of Memphis, TN. Also, across Crowley's Ridge between Jonesboro and Harrisburg, AR. This site was chosen because of: pre-existing data at the site that indicated a good target, proximity to the Blytheville Arch and the New Madrid seismic zone, and field acquisition logistics to make future data acquisition along this cross section feasible.

HISTORY: The study area is located at the southern end of the New Madrid seismic zone and was seriously affected by great shocks of 1811-1812. The Mississippi River valley earthquakes of 1811-1812 rank among the most significant events in U.S. history. The area of strong shaking associated with these shocks is two to three times larger than that of the 1964 Alaska earthquake and 10 times larger than that of the 1906 San Francisco earthquake.