If you feel like the ground's been moving around a little more than usual, it's not your imagination.
April brought a spate of small earthquakes to rattle your china as an underground pressure wave slowly creeps northward from the Oklahoma border region toward Hutchinson.
In fact, the U.S. and Kansas geological surveys have recorded more felt earthquakes in Kansas this month than in the first three months of the year put together.
The USGS picks up quakes from about 2.5 magnitude on up, roughly the threshold where people at ground level notice the shaking. A more comprehensive catalog of April's quakes will be available from the Kansas Geological Survey by about mid-May after a data review.
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The KSGS reported 10 quakes originating in Kansas in the three-month period of January, February and March. There were 13 in April, the most recent a 2.8-magnitude shaker on Sunday near Conway Springs.
That's not counting the 57 quakes on the Oklahoma side of the border that the USGS reported for April, many of which were felt in southern Kansas. Oklahoma's quake rate has stayed consistent so far this year, averaging about 50 temblors of 2.5 and above per month.
As with earlier quake swarms of the past few years, the culprit is underground disposal of millions of gallons of wastewater from oil fields in southern Kansas and northern Oklahoma.
While the wastewater itself is not moving very far, it is percolating around underground, altering the pressure within deep rock formations and pushing a pressure wave northward toward earthquake faults that have been under stress for a long time, said Rick Miller, senior scientist and geophysics section chief with the Kansas Geological Survey.
In some cases, those faults only need a small push to start slipping and cause a quake, he said.
It's not actually the wastewater, but the pressure associated with it that has formed a "pressure wave" moving slowly underground toward Hutchinson, Miller said.
"When you add water someplace, you add pressure someplace else," he said. "It's the pressure that's being transmitted, it's not the water molecules."
Essentially, it's like a giant water balloon deep underground.
"You take a water balloon and you push on one side, the other side comes up," Miller said. "You're not actually exchanging the water within the balloon, you're simply changing the pressure within the balloon."
Miller emphasizes that the quakes are not caused by the process of "fracking," where high-pressure liquids are used to fracture rock pockets and free the oil and gas trapped inside.
But along with every barrel of oil, drillers bring up about 16 barrels of salty, oily water that has to be disposed of. And the only economical way to do that now is to inject it back underground in deep rock formations.
Industry is working on technology to treat the salty, oily water to a level of purity where it could at least be used for watering crops — which would keep it from causing any quakes — but it's not there yet, Miller said.
In the meantime, he praised the Kansas Corporation Commission for taking a lead role in limiting the amount of wastewater that can be injected into seismically sensitive areas. Oklahoma lagged but is now catching up, he said.
Another factor working toward quake prevention is that oil prices have fallen and production has gone down.
The combination of regulation plus economics will help give the underground fluid pools and rock structures a chance to settle into a new state of equilibrium, at which point the quakes will taper off to a minimum, Miller said.
The data that seismologists have now indicate that quakes will continue to be small, more a matter of annoyance than danger, Miller said.
Quake prediction remains an iffy business, but Wichita, the population center of south-central Kansas, doesn't appear to be in the crosshairs for a California-style major quake, he said.
"We're monitoring the area and we've seen earthquakes in the southeastern part of Sedgwick County and way over in the western side of Sedgwick County we've seen a few," Miller said. "There are some small earthquakes that are occurring but the lion's share of these are well below what's felt."
While there are many faults in the area, only a small fraction would be expected to have the conditions of pressure and orientation for a quake to be triggered. The problem is sometimes the only way to find a fault is when it quakes, he said.
"The pressure is still trying to find equilibrium, so there's a lot we don't know yet, but of the data that we've recorded so far and that we are watching very carefully, we're not seeing anything here that says you're going to see those kind of (major quake) events in the area," Miller said. "Now, as soon as I say that, tomorrow we'll have one. But right now, as best as anybody can predict, there's nothing here that says that we should be throwing up the alarms."