Franklin, TN Boom Was "Unusual" Shallow Earthquake[Original headline: 'Shallow earthquake' described by UT]
The red dot shows the epicenter of the seismic activity 
Williamson County residents who heard a loud rumbling sound early this month probably heard an earthquake five to 10 kilometers underground about four miles southeast of Franklin.
"Our assessment is that it was a shallow earthquake and not a cave collapse, but we can't rule out that it was a collapse," says University of Tennessee geophysics Professor Rick Williams.
Tennessee Valley Authority seismographs measured the "felt earthquake" at 2.6 on the Richter scale at 10:05 p.m. July 7. TVA's monitors automatically page an official when the measurement is more than 3.0 on the Richter scale. Since the tremor 18 days ago was lower, no page was issued and the record was reviewed later.
A map issued with a TVA report on the earthquake indicates the epicenter of the seismic activity was northeast of Carothers Road and west of Arno Road. Clearview Drive and Saddleview Lane are in that area some four miles southeast of Franklin.
"The earthquake that occurred near Franklin is of interest because it's unusual," Williams said. "And it was looked at relatively closely to see if it was caused by other things."
The seismograph reading rules out sonic boom, the professor said.
"In most cases we can tell the difference," Williams said. "In the first place, sounds in the area - sonic booms, setting off dynamite, rumbles of thunder - don't normally register very well in our seismic network.
"Things in the air don't normally couple with things in rock. The fact that we can see it [on the meter's record] seems to rule that out."
Residents hearing the rumble heard a result of the movement of rock. Compared to other earthquakes, which would be some 20 kilometers deep, this one was shallow. The movement of rock starts a sound wave which travels upward and is carried further by the air.
Since the event was at about 10:05 p.m., there were few other sounds to compete with the sound of the earthquake, so more people heard it. Typically, people call law enforcement agencies which forward the reports to other agencies and those are called "felt earthquake" reports.
"About half turn out not to be earthquakes," Williams said. "In most cases, we try not to run those down and occasionally we are successful. Several years ago, a Space Shuttle landing sonic boom was tracked down."
Sonic booms, however, would result in a series of reports in a line which would approximate the path of the aircraft.
"This event ... was unusual," Williams said.
Tennessee has two major seismic zones. One in the west, the New Madrid seismic zone, is named after New Madrid, Mo., because of an historic event which created Reelfoot Lake.
The other seismic zone is the East Tennessee seismic zone, which is disconnected from the New Madrid zone.
The two are not connected, so the July 7 event is unusual," Williams said.
"I don't know if this event was large enough to create any special concern," he said. "For an event this small, we would not conduct any aftershock studies."
Because this area is not recognized as having a propensity for earthquakes, the premiums on earthquake insurance here are inexpensive.
However, the Middle Tennessee area has Karst topography, meaning it has caves, so the collapse of a cave roof to its floor is a possibility, Williams said.
"We record those kinds of things on our seismic network and sometimes they can be relatively large in terms of the seismic effect," he said.
However, seismic monitors aren't in this area. One would need to be closer than 10 miles to get a good measurement of the depth of rock shifting underground.
"We cannot rule out a cave collapse, but we can rule out a sonic boom.
Meanwhile, "Karst can be serious business, not because of earthquake but because it affects the surface, with some possibility of a house being damaged by the falling of the ground. That's not an earthquake. That would be a sinkhole, a cave that fell in."
That can be very expensive for a homeowner, and it's not covered by earthquake insurance, he said.
"Our assessment was that it was a shallow earthquake and not a collapse," he said. "But we can't completely rule out that it was a collapse."
• Story originally published by •
The Review Appeal & Brentwood Journal / TN | Clint Confehr - July 25 2001
