A US Geological Survey research team says a remarkable increase in earthquake occurrence in the US in the past decade is “almost certainly man-made.”

­According to the study by USGS, oil and natural gas extraction activity have possibly provoked a series of recent earthquakes from Alabama to the Northern Rockies.

However, USGS authors did not estimate a direct cause-effect relationship between oil and gas activity and earthquakes.

“It remains to be determined how they are related to either changes in extraction methodologies or the rate of oil and gas production,” says the abstract for USGS study, published by the Seismological Society of America.

However they gave a possible explanation for it. They relate it to drilling, which requires the disposal of millions of gallons of wastewater for each well. The number of wells drilled has increased over the past decade.