Earthquakes in Iowa? Occasionally, but not "the big one"
By Molly Montag Journal staff writer | Posted: Sunday, May 04, 2008
INWOOD, Iowa -- An earthquake in the Upper Midwest would seem like a noteworthy event, but residents of one Northwest Iowa town have a hard time recalling the night geologists say the earth moved under their feet.
An earthquake with a magnitude of 4.2 on the Richter scale rattled Inwood, Iowa, on Oct. 11, 1938. It's one of only 13 quakes recorded in the state since 1867 and the only one with an epicenter in Northwest Iowa.
The others have been in the eastern and extreme southwestern parts of the state.
If it weren't for Inwood's chief engineer, Arthur Hanson, the 1938 temblor might have been swallowed by history.
According to a front-page account in the Oct. 13, 1938, edition of The Inwood Herald, Hanson noticed a change in the instrument readings at the city's municipal water works plant.
Hanson recorded the date and time: 3:37 a.m. Oct. 11, 1938.
The article and headline -- "'Baby' Quake, Central Sioux Falls, 3:30 a.m. Tuesday, felt by Inwood Citizens" -- implied that the quake occurred in the Sioux Falls area. According to modern data compiled by the Iowa Department of Natural Resources, the quake actually took place near Inwood.
However, a visit Thursday with Inwood residents who are old enough to remember an event in 1938 turned up no one at City Hall, the community center pool room, a nursing home, assisted living apartments or a downtown feed store who could remember an earthquake in the town.
Not even Arthur Hanson's son, Marlo Hanson, recalled anything about the quake.
"I don't remember even hearing about it," Marlo Hanson said.
Doug Jiskoot, secondary principal at the area's West Lyon Schools, said the earthquake isn't included in the school's curriculum.
"It's news to me," he said.
Ray Anderson, a geologist with the Iowa Geological Survey, said the earthquake measured 4.2 on the Richter scale, which means it would have caused noticeable shaking and rattling but little damage.
"This was not exactly one that was going to shake down buildings," Anderson said.
It was less powerful than the 5.2 magnitude quake that hit central Illinois on April 18 and was felt by some Iowa residents.
According to The Inwood Herald, "The shock was felt within a radius (of Sioux Falls) of 50 to 100 miles, jarring buildings and houses, shaking beds, rattling dishes and windows and swaying pictures."
The story said area residents described the sound that accompanied the earthquake as a far away thunder, the rumble of an express train and of a big bridge falling down.
The hour at which the quake hit may be one reason no one remembers it because it isn't that Inwood residents can't remember back that far.
Several of those interviewed on Thursday recalled feeling the earth shake in 1936, when criminals in Sioux Falls, S.D., attempted to conceal a murder by detonating thousands of pounds of blasting powder and dynamite to dispose of a body.
The 1938 Inwood earthquake is not recognized by the U.S. Geological Survey, but it is listed by the Iowa Geological Survey.
Anderson said the little researchers do know about the quake came from research done in the 1970s by a professor and students from St. Louis University in Missouri. They dug up what information they could find, including news accounts, to determine when and where earthquakes occurred.
Earthquakes happen when plates in the earth's crust -- which Anderson said are common across the country -- slip and crash into each other at fault lines.
"(The plates) are moving and banging into each other, and they're moving at about the same speed your fingernails grow -- about a half-inch a year," he said.
Although they might feel earthquakes from time to time, Anderson said Siouxland residents shouldn't worry about living near the epicenter of any major earthquakes. Anderson said there are no major fault lines in this part of the state.
At the time of the 1938 quake, the newspaper quoted then-South Dakota state geologist E.P. Rothrock telling a Sioux Falls newspaper that a local geologic feature called the "lost mountains" might have had something to do with the tremors.
The "lost mountains" were said in the 1938 article to have been formed at the same time as northern Minnesota's Iron Range.
The slipping of faults in the granite rock of the Sioux Falls area also was blamed for earthquakes earlier in October 1938, in Chamberlain and Wessington Springs, S.D.
The article also cited a tremor in Redfield, S.D., in 1937, and a stronger earthquake about 40 years earlier, in the late 1800s, in Vermillion, S.D.
Anderson said he'd never heard of the "lost mountains" but believed Rothrock might have been referring to what's now called the Sioux Ridge. According to information from the U.S. Geological Survey, the Sioux Ridge is a quartzite deposit that extends from west of Mitchell, S.D., to New Ulm, Minn.
It's buried under about 100 feet of sediment in most of the region, but the rock is exposed in Lyon County, Iowa, -- home to Inwood -- and in neighboring Minnehaha County, S.D.
Unlike mountains, which are thrust up out of the earth by movements of tectonic plates, Anderson said researchers now believe the Sioux Ridge is a rock deposit exposed when surrounding sediment eroded away. As such, it does not have tectonic, or earthquake-producing, capabilities.
Anderson said area residents are more likely to feel aftershocks from large earthquakes, such as the one in Illinois, than any that take place here in Siouxland.
"Your chances of experiencing one as an epicenter or source from this area is very unlikely," he said.
Earthquake map
Click to see all the earthquakes that have shaken Iowa.
An earthquake with a magnitude of 4.2 on the Richter scale rattled Inwood, Iowa, on Oct. 11, 1938. It's one of only 13 quakes recorded in the state since 1867 and the only one with an epicenter in Northwest Iowa.
The others have been in the eastern and extreme southwestern parts of the state.
If it weren't for Inwood's chief engineer, Arthur Hanson, the 1938 temblor might have been swallowed by history.
According to a front-page account in the Oct. 13, 1938, edition of The Inwood Herald, Hanson noticed a change in the instrument readings at the city's municipal water works plant.
Hanson recorded the date and time: 3:37 a.m. Oct. 11, 1938.
The article and headline -- "'Baby' Quake, Central Sioux Falls, 3:30 a.m. Tuesday, felt by Inwood Citizens" -- implied that the quake occurred in the Sioux Falls area. According to modern data compiled by the Iowa Department of Natural Resources, the quake actually took place near Inwood.
However, a visit Thursday with Inwood residents who are old enough to remember an event in 1938 turned up no one at City Hall, the community center pool room, a nursing home, assisted living apartments or a downtown feed store who could remember an earthquake in the town.
Not even Arthur Hanson's son, Marlo Hanson, recalled anything about the quake.
"I don't remember even hearing about it," Marlo Hanson said.
Doug Jiskoot, secondary principal at the area's West Lyon Schools, said the earthquake isn't included in the school's curriculum.
"It's news to me," he said.
Ray Anderson, a geologist with the Iowa Geological Survey, said the earthquake measured 4.2 on the Richter scale, which means it would have caused noticeable shaking and rattling but little damage.
"This was not exactly one that was going to shake down buildings," Anderson said.
It was less powerful than the 5.2 magnitude quake that hit central Illinois on April 18 and was felt by some Iowa residents.
According to The Inwood Herald, "The shock was felt within a radius (of Sioux Falls) of 50 to 100 miles, jarring buildings and houses, shaking beds, rattling dishes and windows and swaying pictures."
The story said area residents described the sound that accompanied the earthquake as a far away thunder, the rumble of an express train and of a big bridge falling down.
The hour at which the quake hit may be one reason no one remembers it because it isn't that Inwood residents can't remember back that far.
Several of those interviewed on Thursday recalled feeling the earth shake in 1936, when criminals in Sioux Falls, S.D., attempted to conceal a murder by detonating thousands of pounds of blasting powder and dynamite to dispose of a body.
The 1938 Inwood earthquake is not recognized by the U.S. Geological Survey, but it is listed by the Iowa Geological Survey.
Anderson said the little researchers do know about the quake came from research done in the 1970s by a professor and students from St. Louis University in Missouri. They dug up what information they could find, including news accounts, to determine when and where earthquakes occurred.
Earthquakes happen when plates in the earth's crust -- which Anderson said are common across the country -- slip and crash into each other at fault lines.
"(The plates) are moving and banging into each other, and they're moving at about the same speed your fingernails grow -- about a half-inch a year," he said.
Although they might feel earthquakes from time to time, Anderson said Siouxland residents shouldn't worry about living near the epicenter of any major earthquakes. Anderson said there are no major fault lines in this part of the state.
At the time of the 1938 quake, the newspaper quoted then-South Dakota state geologist E.P. Rothrock telling a Sioux Falls newspaper that a local geologic feature called the "lost mountains" might have had something to do with the tremors.
The "lost mountains" were said in the 1938 article to have been formed at the same time as northern Minnesota's Iron Range.
The slipping of faults in the granite rock of the Sioux Falls area also was blamed for earthquakes earlier in October 1938, in Chamberlain and Wessington Springs, S.D.
The article also cited a tremor in Redfield, S.D., in 1937, and a stronger earthquake about 40 years earlier, in the late 1800s, in Vermillion, S.D.
Anderson said he'd never heard of the "lost mountains" but believed Rothrock might have been referring to what's now called the Sioux Ridge. According to information from the U.S. Geological Survey, the Sioux Ridge is a quartzite deposit that extends from west of Mitchell, S.D., to New Ulm, Minn.
It's buried under about 100 feet of sediment in most of the region, but the rock is exposed in Lyon County, Iowa, -- home to Inwood -- and in neighboring Minnehaha County, S.D.
Unlike mountains, which are thrust up out of the earth by movements of tectonic plates, Anderson said researchers now believe the Sioux Ridge is a rock deposit exposed when surrounding sediment eroded away. As such, it does not have tectonic, or earthquake-producing, capabilities.
Anderson said area residents are more likely to feel aftershocks from large earthquakes, such as the one in Illinois, than any that take place here in Siouxland.
"Your chances of experiencing one as an epicenter or source from this area is very unlikely," he said.
Earthquake map
Click to see all the earthquakes that have shaken Iowa.
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HAMILTON wrote on May 4, 2008 8:43 PM: