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Earthquake hits near Chamberlain, not uncommon for area

When the walls of his rural Brule County house rumbled Friday afternoon, Dwayne Cooley did not suspect an earthquake.

Rather, he wondered about the not-so-stable wall in his basement.

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"The basement isn't really that good, so I was worried that something happened down there," he said. "I never thought it was an earthquake."

Cooley said the "strange event" was certainly loud. He was sitting with his wife in the living room of their home when it happened.

"It felt like a sonic boom of sorts and it rumbled a little bit," he said.

The earthquake, which registered as a 2.9 on the Richter scale, occurred at 2:39 p.m. Friday, about 17 miles southeast of Chamberlain. There were no official damage reports. Chamberlain Police Chief Joe Hutmacher said there was no damage reported to his station and the city received no calls related to the incident.

The U.S. Geological Service's GPS coordinates placed the event in Richland Township of Brule County and estimated the depth of the quake to be 8.9 miles into the Earth's surface. Usually, the closer to the surface of the earth, the more likely it is that the seismic event will be recorded.

Jeff Earle, a seismologist with the USGS National Earthquake Information Center in Golden, Colo., said the earthquake was probably strong enough to be felt anywhere from 25 to 50 miles away, but the center received no damage reports.

"If you're standing right on top of it, you'll certainly feel it," he said. "But it's not a sign of impending doom."

Prior to Friday, the most recent South Dakota earthquake was a 3.5-magnitude event at Custer State Park in December.

"It's just a reminder that you can have earthquakes basically anywhere," Earle said. "It's not just restricted to California or the West Coast or anything."

The USGS has a seismic hazard map for South Dakota, which gauges the hazard of items that are associated with earthquakes (surface faulting, ground shaking, tectonic plate movement, etc.). The area east of the Missouri River running from Huron down through Charles Mix County registers some of the highest seismic hazard ratings in the state, along with a stretch in southwestern South Dakota from Shannon County to Custer State Park and into Wyoming.

While the seismic hazard for the state is still relatively small, especially compared to California or Yellowstone National Park, the data shows that the south-central region of South Dakota receives more seismic activity than most people probably realize.

State geologist Derrick Iles, whose South Dakota Geological Survey Program at the University of South Dakota has kept earthquake records as far as 1872, said there's two historical events that could lend themselves to increased seismic activity in that area.

First, he said, the area east of the Missouri River is still experiencing what is known as isostatic rebound from the last glacial period roughly 12,000 years ago. Because there was thousands of feet of ice pushing down on the crust of the Earth for a long period of time, Iles said, South Dakota is still experiencing the rebounding of the Earth's surface back to where it was before the glacial period.

"We're still experiencing a form of rebound and that can cause some seismic activity, especially in that part of the state," he said.

Iles said it's also possible that the weight of the reservoirs on the Missouri River, created by the dams, can cause "microseismic" reactions.

So is it possible that could have caused Friday's earthquake?

"There's no cause and effect," Iles said. "It's so hard to tell and really know what was causing this."

This will not go down as Brule County's strongest earthquake ever. A 4.2-magnitude quake was recorded outside Chamberlain on Oct. 1, 1938, according to the SDGS data.

The largest earthquake ever recorded in South Dakota was a 4.5-magnitude event on June 2, 1911, which occurred near the Sanborn-Beadle County line. The USGS Earthquake History of South Dakota says that event was centered in the James River valley and was felt in Iowa and Nebraska, as well. In the first 140 years of records for the SDGS, 87 earthquakes were recorded in 34 different South Dakota counties.

He said being in a rural area probably increases the likelihood of someone feeling a small earthquake, as opposed to an event in a large city.

"If you're not used to it, you're certainly more likely to take note," Earle said. "If you're working on a real industrial farming operation, it's possible that you wouldn't even notice something like that. But if you're in a quiet house, you'd be able to feel it."