WICHITA, Kan. (AP) — Geologists are blaming a spate of small Kansas earthquakes on an underground pressure wave that is slowly creeping northward into the state from the Oklahoma border region.

There were 13 earthquakes that originated in Kansas last month and were strong enough to feel. There were just a combined total of 10 for the first three months of the year.

On the Oklahoma side of the border, the U.S. Geological Survey recorded another 57 quakes 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.

Quakes in the two states have been tied to underground injections of wastewater from oil and gas drilling.