That assessment was in contrast to Oklahoma's state seismologists, who said it was natural. The federal group says a slightly smaller quake in an old oil well used to get rid of wastewater triggered the bigger one, and then a third smaller aftershock.
One thing both groups agree on; the waste was from traditional drilling, not hydraulic fracturing, commonly called fracking.
The evidence for waste water at all being the cause is rather circumstantial, thus the 'probably' qualifier. The area has been drilled since the 1950s and wastewater injection has been done since 1993. Still, the quake happened at 'the right place' to implicate conventional drilling waste, one of the authors told the AP's Seth Borenstein.