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Archive for Monday, July 5, 1993

T WORRY ABOUT QUAKES HERE, KU GEOPHYSICIST SAYS

July 5, 1993

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Douglas County is in two earthquake seismic zones, but a Kansas University seismologist says there's not much chance The Big One will hit here.

Little ones, however, are jolting Kansas on a regular basis, said Don Steeples, McGee Professor of Geophysics in KU's geology department and a seismologist with the Kansas Geological Survey.

Kansas averages 12 to 20 micro-earthquakes annually, with magnitudes ranging from 1 to 3 on the Richter scale -- hardly big enough for most people to notice that an earthquake has occurred, Steeples said.

"There's really not any reason for concern around here," he said. "The houses and barns are built by the same measures to withstand a 100 mph straight wind. They can withstand any earthquake we're likely to have."

More than 98 percent of the world's earthquakes are caused by plate tectonics, the forces that produce the structure of the earth's crust. Earthquakes occur frequently in California, for example, because the westernmost part of the state lies atop the Pacific Ocean plate and the rest atop the North American plate. As the plates move in different directions, their edges rub together and energy -- in the form of earthquakes -- is released.

The rest of the world's earthquakes -- including the Kansas shakers -- occur when energy is released from stress built up within plates.

Douglas County lies in the midcontinent seismic zone, designated a hazard zone 1 with the potential for minor earthquake damage.

But the county also lies along the eastern edge of a banana-shaped seismic zone that stretches from Omaha to Oklahoma City and is designated a hazard zone 2, with the potential for moderate earthquake damage.

"If you plot quakes that produced moderate damage in the last 125 to 130 years, they all plot within the banana shape," Steeples said.

The zone lies atop the Nemaha Ridge, an underground mountain range that is deeply buried under sedimentary rocks, the geophysicist said.

"If you were to strip off the sedimentary rocks and walk from Hiawatha to Seneca (in northeast Kansas), you could see something not a whole lot different from the front range of the Rockies," he said. "There would be a wall of granite 3,000 to 4,000 feet high."

The buried mountain range is about 260 million years old, or "about four to five times the (time) range they're talking about in 'Jurassic Park,' " Steeples said.

With 15 seismographs operating in Kansas and Nebraska from 1977 to 1989, seismologists were able to plot micro-earthquakes and show that a fair number occur along the Nemaha Ridge, which stretches from Washington, Marshall and Nemaha counties in northeast Kansas to Sumner and Cowley counties in southcentral Kansas.

But the data also showed a band of micro-earthquakes that stretched from northwest Kansas nearly to Wichita. That area, part of the central Kansas uplift stretching from Wichita through Nebraska to the Black Hills, was the location of Kansas' most recent big quake, Steeples said.

"It happened in western Rooks County, my home county, about five miles from where I grew up," he said. "It was about 4.0 on the Richter scale. ... There's still people out there driving vehicles with bumper stickers that say 'Palco: Earthquake Capital of Kansas.' "

Kansas' biggest quake on record occurred in Manhattan in 1867, long before the Richter scale was developed, Steeples said.

On the Mercalli scale, which measures earthquakes by their effect on people and structures, the Manhattan quake had an intensity of 7 -- strong enough to topple chimneys in Manhattan and be detected as far away as Dubuque, Iowa.

The quake would have measured "about five and a quarter (on the Richter scale), estimated from newspaper reports," he said.

Seismologists have plotted earthquake activity in Kansas over the past 100 years, based on newspaper reports.

In 1902 southeastern Douglas County was hit by an earthquake that had an intensity of 2 on the Mercalli scale, but swinging chandeliers were about the only clue.

Twenty years earlier, southern Leavenworth County near the Douglas and Johnson county lines was hit with an earthquake that measured at 3 on the Mercalli scale and was felt noticeably indoors.

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