Earthquake felt stronger to Wichitans than year-ago quake that was larger, closer

Thursday morning’s 4.7-magnitude earthquake near Cherokee, Okla., seems to have rocked Wichita harder than a larger earthquake a year ago at Milan, Kan. — and geophysicists say they’re not sure why.

“Every earthquake is different,” said Rob Williams, a quake expert with the U.S. Geological Survey’s National Earthquake Information Center. “They’re not all duplicates of each other.”

The Milan quake, on Nov. 12 of last year, hit 4.9 magnitude and was about 36 miles from downtown Wichita.

Thursday’s quake was a slightly lower magnitude and was 95 miles from Wichita, more than twice as far away.

“I can’t say off the top of my head why this one felt stronger to you in Wichita,” Williams said. “I’d have to look into it a little bit, and even then it might just be some arm-waving.”

He said one of the possible causes might be a difference in the type of underground fault that moved, which could have focused more energy toward Wichita than the Milan quake did.

Also, the time of day might have affected people’s perception of the movement they experienced, experts said.

The Milan quake hit late afternoon when people were up and around, while the Cherokee quake hit after midnight and people were jolted awake.

The afternoon “is not as good a time to feel them as the middle of the night when everything’s quiet,” explained Rex Buchanan, director of the Kansas Geological Survey.

Buchanan was in Manhattan on Thursday for a state water conference, coincidentally the same conference at the same hotel he was at last year when the Milan quake hit.

He said he didn’t feel the Milan quake, but he did feel the Cherokee quake Thursday.

“It clearly got people’s attention here,” he said.

The USGS, based in Golden, Colo., makes “intensity maps” of quakes built up from people’s reports on how strongly they felt them.

That’s different from and in addition to the magnitude scale, which measures the energy released by ground movement and is the number most often used to describe the strength of earthquakes.

The magnitude is measured with a seismograph, while intensity is based on people’s perceptions of what they felt.

The intensity maps and underlying data on Thursday indicated that more Wichitans felt the Cherokee quake than felt the Milan quake — and rated the shaking they got as a stronger one.

With the Milan quake, 1,513 Wichita residents reported to the USGS that they felt the shaking. For the Cherokee quake, the number of Wichita reports was 1,763.

Wichitans also reported a stronger shake Thursday than they had reported for the Milan quake.

On average, Wichitans rated Thursday’s quake about 3.8 on the 10-point intensity scale. Milan averaged a little over 3.2.

Of the 26 Wichita ZIP codes reporting in on the Cherokee quake, residents in 16 zones ranked their personal shaking-up as worse than the Milan quake. Residents in only one ZIP code ranked Milan as a bigger shakeup, the USGS data showed.

Williams said the difference might have been in the type of fault. Different faults cause different ground movement and the fault that slipped in the Cherokee quake could have radiated more energy toward Wichita than the Milan quake.

He noted that intensity reports from the Milan quake were stronger in the Dallas-Fort Worth area than they were for the Cherokee quake.

In addition to the initial jolt, the Cherokee quake spawned four aftershocks of more than magnitude 2 Thursday, Williams said.

The first was a 3.1-magnitude aftershock at 3:46 a.m., followed by a 3.7 at 6:05 a.m., a 2.8 at 9:06 a.m., and a 2.6 at 11:13 a.m., he said.

He said a quake the size of the Cherokee one would be expected to generate five to 10 aftershocks in the three-magnitude range and 50 to 100 in the two range.

“People can expect aftershocks to come, to be felt,” he said. “You can’t rule out the possibility of a larger earthquake at this location.”

This story was originally published November 19, 2015 at 6:30 PM with the headline "Earthquake felt stronger to Wichitans than year-ago quake that was larger, closer."