SCIENTISTS SEND SEISMIC WAVES ALONG FAULT

DEXTER -- Scientists with the United States Geological Survey (USGA) hope a major seismic study of the New Madrid fault zone will help determine earthquake hazards in the fault region, and what safety measures need to be taken.

The study, scheduled to begin at 1 a.m. today, was discussed at a press conference Saturday at the study's central site near Dexter. In all, the study involves 45 scientists, 15 trucks and 500 seismic recorders.

The launch of the study was to be signaled by the release of three artificially-generated, seismic waves along a line extending just northeast of Belleville, Ill. to several miles south of Memphis, Tenn., said Robert L. Wesson, chief of the USGA's Office of Earthquakes, Volcanoes and Engineering.

Charges buried about 200 feet into the ground were to be set off at each end of the line, with a third fired at the center, about five miles west of Dexter.

By studying the shock waves emitted from the three explosions, scientists hope to determine the amount of destruction that would take place in St. Louis and Memphis if a large-scale earthquake were to occur along the fault zone.

"This experiment is going to primarily focus on determining how strongly the ground will shake in Memphis and St. Louis if we had a quake centered in the zone," Wesson said.

"In other words, we'll focus on determining how quickly the shock waves would die away with distance from the earthquake's epicenter."

Rufus D. Catchings, co-leader of the study, explained that shock waves resulting from the explosions will be received and recorded by 500 portable seismographs set up along the line at intervals of between 300 feet and several miles.

In addition to today's explosions, other smaller charges are scheduled for release Tuesday and Thursday, Catchings said.

The explosions, caused by ammonium nitrate, would be comparable to blasts commonly used in quarry operations or roadwork, he said.

Catchings said the explosions will be used to map the fault zone, which consists of not a single large fault, as is commonly believed, but of several small faults.

By compiling this map, the team hopes to find structures present in the zone that would allow them to monitor quake activity.

According to Eugene Schweig, a USGA geologist, the study will also allow a more accurate calibration of seismographs in the central U.S.

"For the seismic instruments we have in the region, we'll be able to calibrate the instruments much more accurately because we know the exact size and location of these explosions," Schweig said.

The ammonium nitrate explosions posed no risk of triggering an earthquake, he added.

"The point in the experiment," he said, "is to gain an understanding of the earthquake hazard so that reasonable kinds of measures can be taken over a long period of time.

"We're not saying there's going to be an earthquake next week or next month," he said, adding that attention should be focused on building codes, earthquake insurance and other safety measures "so that until we can predict an earthquake accurately, we can be prepared for the time when one does occur."

Catchings said the test shots would be similar to shotgun blasts and that a small amount of ground motion would be felt within a few hundred feet of each shot.

The New Madrid seismic zone continues to be the most active seismic zone east of the Rocky Mountains. The fault runs roughly from Marked Tree, Ark. to Cairo, Ill.

Last year, national attention was focused on the New Madrid Fault when climatologist Iben Browning predicted a major earthquake would strike the region in early December. Schools closed and a mass of reporters descended on the town, but no quake occurred.