Geologists are blaming a spate of small Kansas earthquakes on an underground pressure wave that is slowly creeping northward from the Oklahoma border region.

The Wichita Eagle reports there were 13 earthquakes that originated in Kansas last month and were strong enough to feel. That compares to a total 10 quakes during the first three months of this year.

On the Oklahoma side of the border, the U.S. Geological Survey recorded 57 quakes in April, many of which were felt in southern Kansas. Oklahoma's rate of earthquakes has remained consistent so far this year, averaging about 50 temblors of 2.5 or above per month.

Earthquakes in both states have been tied to underground injections of wastewater from oil and gas drilling.

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Information from: The Wichita (Kan.) Eagle, http://www.kansas.com