Winter break earthquake was unprecedented 'enigma,' geologist says

Thomas Beard’s morning was a bit shaky. Literally.

At 7:55 a.m. on Dec. 30, a 3.8 magnitude earthquake struck Indiana.

Beard, a retired geologist and resident of Lebanon, Ind., was watching TV in his bed while his family was still asleep.

“At first I thought the cat had just jumped on the bed. Then I thought it was a truck passing by our house,” Beard said. “But when I saw the ripples in my drink, that’s when I knew it was an earthquake.”

About 77 miles away in Bloomington, IU geology professor Michael Hamburger was awoken just before 8 a.m. by telephone calls from news stations asking for information about the earthquake.

Hamburger then went to his office as soon as he could to find out exactly what had happened on the seismograph.

“This was large enough that it actually flew a couple of the pens off of their axes, and the instruments needed a little tuning after the earthquake,” Hamburger said. “But we got some high quality recordings from the network of seismographs from across the state.”

The U.S. Geological Survey originally estimated the earthquake to be 4.2 magnitude but eventually downgraded it to 3.8.

The epicenter was in a fairly rural city near Kokomo called Greentown.

The earthquake was felt as far as eastern Kentucky and throughout north central Indiana, southern Michigan, southern Wisconsin, western Ohio and eastern Illinois, Hamburger said.

No significant damages were reported, he said.
 
“This earthquake is near the lower end for an earthquake that is capable of producing significant damage,” Hamburger said. “It carries enough energy to shake and rattle things but not enough to do any major structural damage.”

He said although this earthquake wasn’t significant in terms of damage, it was unprecedented.

“This earthquake is a little bit of an enigma because it occurred in a place that has no previous history of seismic activity. So we can’t relate it to other, better known seismic zones,” Hamburger said.

As for the question of which seismic fault caused this earthquake, Hamburger said it is harder to answer.

Figuring out exactly which seismic area caused the earthquake is difficult because earthquakes occur deep in the earth’s crust, and the faults they are associated with may or may not be visible on the earth’s surface, he said.

“So in contrast to places like California, where we can see direct evidence of active fault movement, here it’s buried under many miles of younger sedimentary layers, and we need to rely on some kind of circumstantial evidence to detect where and how active the faults are,” Hamburger said.

Hamburger said two seismic areas are near the epicenter of the December quake.

One is the Sharpsville Fault in Howard County, located near the epicenter.
 
“Whether in fact that was the causative structure or it’s just a coincidence that it occurred near a fault is yet to be determined,” Hamburger said.

Another moderately active seismic zone located in Anna, Ohio, could have caused the earthquake as well, Hamburger said.

“This particular event was kind of in a quiet spot that’s between all of the areas that have had previous earthquake activity,” Hamburger said.