DES MOINES — Iowans, for the most part, have been spared the feeling of the ground trembling beneath their feet and the sight of crumbling buildings rocked by tremors.
For this part of the country, tornadoes and floods, as opposed to earthquakes, are the greatest cause for concern when talking about natural disasters. But that’s not to say Iowans haven’t been shaken in the past, nor does it rule out that it will happen in the future.
The New Madrid Seismic Zone is the major active earthquake zone closest to the Hawkeye State. The zone is made up of hundreds of fault lines covering about 150 miles and portions of Illinois, Kentucky, Tennessee, Missouri and Arkansas. It was the epicenter of a series of quakes in 1811 and 1812, considered among the most violent in U.S. history.
But there’s potentially another, much closer, fault zone located right under the Quad-Cities.
“There’s really no good reason that the (Mississippi) river swings west where it does by Davenport, from a glacial standpoint,” Iowa state geologist Robert Libra said. “The Mississippi River is probably following a fault line right there. It makes sense especially when you look at the data.”
Most quakes
That data is a record of quakes with their epicenters in the state that goes back to 1867. Of the
13 recorded quakes — the most recent being the Shenandoah quake in 2004 — four have had their epicenters in, or just outside of, Davenport. That’s more than any other part of the state.
It includes the state’s most violent, a Nov. 12, 1934, quake that was given VI on the Mercalli Intensity scale. That’s a scale that measures the damage brought by a quake, as opposed to the Richter scale, which measures seismic magnitude. The Richter scale wasn’t developed until 1935.
A VI on the Mercalli scale means that everyone in the area felt the ground tremble, plaster crumbled off walls and dishes, books and other items fell off shelves. It works out to about a 5.0 or 5.1 on the Richter scale.
Libra said that even though it’s highly likely there’s a fault line there, it’s not very active. The last quake with an epicenter in the Quad-City area was recorded on Nov. 24, 1939, which measured between a II and III on the Mercalli scale, meaning it was “slight” or “weak.”
“What makes a seismic zone active is when there are stresses on the faults,” he said. “When you’re talking about that level of activity in the Midwest, you’re talking about the New Madrid zone and, to a lesser extent, the Wabash Valley zone.”
The Wabash Valley zone is located in southeastern Illinois, near the Indiana border.
Tornado season
Even if the chance of a major earthquake in Iowa is slim to none, the events in Japan gave state legislators pause last week. They called for more study of MidAmerican Energy’s plans to build a new nuclear reactor in the state, and they began a donation fund to aid relief efforts.
But it remains to be seen if the specter of an earthquake causes other changes.
“I can tell you we haven’t had any questions about earthquakes this week,” said Ross Bergen, director of the Scott County Emergency Management Agency. “We’ve had a couple calls about radiation because of the nuclear plants in Japan, but that’s about it for calls that we usually don’t get.”
Bergen, whose been with the agency for 18 years, said he has never heard of a fault line in the Quad-City area. He said that even if a quake hit, emergency responders would be able to coordinate a quick and effective response.
“We do multi-jurisdictional exercises once a month,” he said. “We’re the only ones that do that in the state that I know of.”
The focus these days, he said, is the upcoming tornado season.
That season is loosely defined as running between April and July, said Jeff Johnson, who is the warning coordinator meteorologist with the Des Moines office of the National Weather Service.
Johnson said the state, on average, has had 48 tornadoes a year for the past 30 years.
“You can’t predict them, although we can say what type of weather conditions are likely to cause them,” he said. “That’s why you’ll hear the terms warnings and watches. When you have a warning, it’s advised you take action.”