A possible sonic boom heard last week by residents across southeastern Missouri and eastern Arkansas could help researchers better understand earthquake hazard.

At 2:48 p.m. last Wednesday people for more than 200 miles, from Forrest City, Ark., to Cape Girardeau, felt what they thought was an earthquake.

Scientists at the Center for Earthquake Research and Information (CERI) at the University of Memphis in Tennessee know it was not.

Though scientists were reluctant to say what caused the boom, it most probably resulted from an Air Force jet flying over the area. Whatever the reason, it created sound waves that helped the earthquake researchers.

The disturbance was first recorded on a seismic station in Pemiscot Bayou, in the lower part of the Bootheel. It then traveled southeast at about 1,115 feet per second to a station in Lennox, Tenn., and was eventually recorded on all of the CERI's 100 seismic stations in the New Madrid Fault Zone.

"(It moved) with the speed of sound in air," explained Dr. Steph Horton, seismology research scientist. "The sound interacted with the ground and we could see it each time it reached a station. The speed of sound moving through the ground is much higher."

Sound's interaction with the ground is something CERI professor Dr. Charles Langston began studying in 2004, according to his Web site. Langston was not available for comment.

Langston's project explores, in part, the potential for acoustic shock waves to be used to map soil structure, according to Horton.

The type of soil beneath a location can play a significant role in determining how well an area can withstand an earthquake.

The New Madrid zone has as many as 200 minor quakes a year and has in the last 200 years suffered damaging earthquakes. These have been felt in an area up to 20 times greater than California earthquakes because seismic waves die out much more slowly here than on the West Coast.

Scientists believe this is because the thick layers of soft soil, often found in flood plains, may amplify motion as it nears the surface. Structures built on or near bedrock tend to experience lower levels of earthquake ground shaking.

Because soil structure can vary greatly from one location to the next, seismologists say detailed mapping of an are's soil must be done to know the true seismic hazard of an area. This is not something included in current National Seismic Hazard Maps.

Soil mapping is a time-consuming process, which uses imaging techniques and requires a high concentration of borehole measurements to be taken from the area being mapped. The years-long project is currently taking place in St. Louis and Memphis.

It may be possible to use sound wave pressure increases and decreases to understand how quickly seismic waves will travel through near-surface soils, Horton said.

It appears last week's sonic boom started north of Poplar Bluff, Mo., which is on the outside of CERI's seismic instrument network, according to seismologists.