Geologists discover widespread seismic activity

Using a 140-station seismic network, geologists with Indiana University have discovered widespread seismic activity along the Ste. Genevieve Seismic Zone, suggesting a greater possibility of earthquakes in the Tri-State.

In addition to being near populated areas, the Ste. Genevieve Seismic Zone is an area of geological interest. The area underlies the Ste. Genevieve fault zone, near the divide between the billion-year-old Ozark Dome and the younger sedimentary rock of the Illinois basin.

The study, published in the November/December issue of Seismological Research Letters, is the first published research from the collaborative Ozark, Illinois, Indiana and Kentucky geophysical research project, the Evansville Courier & Press reports (https://bit.ly/1HNIcy2). OIINK is a collaborative effort among Indiana University, Purdue University, the University of Illinois and the Illinois and Indiana state geological surveys.

"It's an underappreciated zone of seismic activity that has potential implications for St. Louis and other populated areas in Eastern Missouri and Southern Illinois," said Michael Hamburger, professor of geological sciences at IU Bloomington and co-author of the study. "It's comparable to the Wabash Valley Seismic Zone in Western Indiana, which is capable of producing moderate-sized earthquakes every few decades and, perhaps, has the potential for larger earthquakes."

The most recent significant earthquake along the Wabash Valley Fault was in 2008 when a magnitude 5.2 tremor struck near Mount Carmel, Illinois. That quake caused damage from St. Louis to Louisville, Ky.

Researchers believe the Ste. Genevieve and Wabash Valley fault systems are not interconnected.

"In essence, one of the contributions of our paper was to separate the broad area of seismic activity into two somewhat distinct zones — the Wabash Valley zone in the east and the Ste. Genevieve zone in the west," Hamburger said. "They are probably associated with two distinct fault systems."

The first phase of the $1.3 million OIINK project involved deployment of 140 state-of-the-art seismographs across a broad swath of the Midwest, from Missouri through Southern Illinois and Indiana, and through Western and Central Kentucky. The devices were deployed in 2011 and 2012.

"The sites in Missouri and Illinois, and some in Indiana that were the target of the first phase of the study have mostly been abandoned and replaced by new sites in Indiana and Kentucky," Hamburger said. "We now have sites near Richland City, Patronville, Evanston, Dale and Huntingburg."

Moderate to severe earthquakes along the Ste. Genevieve fault system have the potential to impact a large area of the Tri-State, including cities along the Ohio River.

"In principle, because seismic waves propagate very efficiently through the Earth's crust in the Midwest, earthquakes in this zone could affect a broad area of the region," Hamburger stated. "The specific effects would depend, of course, on the size and location of potential earthquakes. Because it is somewhat further afield, moderate earthquakes would have a limited effect in our area."

Geologists monitoring the OIINK seismograph array in early 2012 made a stunning discovery.

"The array detected unusual seismic signals above those associated with the deadly tornadoes that struck Southern Missouri and Illinois," Hamburger said. "The seismograms showed a strong, low frequency pulse beginning around 4:45 a.m. on Feb. 29. Preliminary interpretation, based on other seismic records of tornadoes, suggests that the recording was not of the tornado itself, but a large atmospheric pressure transient related to the thunderstorms that spawned the tornadoes."

Dubbed the "Leap Year Outbreak," twisters in the Ohio Valley left 15 people dead, including eight in one neighborhood in Harrisburg, Illinois.

"To be honest, this was a tantalizing initial discovery and did attract some interest from atmospheric scientists," Hamburger said. "There are others working on this problem, using data from similar stations elsewhere in the country."

The supersensitive OIINK seismic network is also capable of recording distant earthquakes (or teleseisms) with magnitudes greater than 5.0 from virtually anywhere in the world. Because the network is in an active mining area, the array also records numerous "artificial" quakes produced by blasting from neighborhood quarries and surface mines. TimesUnion