Earthquakes have been rumbling around the U.S. lately, in ways that have left seismologists scratching their heads and surprised individuals running for cover.

Late last year, 23 earthquakes were recorded in central Oklahoma in a 24-hour period, including the largest one on record in that state. And while Oklahoma isn’t a stranger to seismic activity, the number and intensity of Oklahoma earthquakes have increased in recent years.

More recently, residents in parts of Wisconsin reported hearing strange booms. After reviewing the seismic data, the U.S. Geological Survey (USGS) discovered that many, very small earthquakes had occurred.

Now this week the USGS released part of a report stating that the “remarkable” increase in the rate and magnitude of earthquakes felt within the interior of the continent is “almost certainly manmade” and likely caused by oil and gas drilling.

In the report’s abstract, USGS scientists examined correlations between the quakes and oil and gas drilling activities, particularly deep underground injection of drilling waste (the full paper will be presented next month at the Seismological Society of America).

There appears to be a strong link between the quakes and the surge in shale drilling, which produces millions of gallons of wastewater that must be disposed, often by injecting it deep underground. Repercussions are not necessarily localized and may be felt throughout a region of the country.

But the exact cause-and-effect is not clear. What must be determined is whether the rate of oil and gas production has caused the quakes or whether it is the increased use of new extraction processes.

Scientists do know that the increase in the number of earthquakes with a magnitude of 3.0 and higher is caused by humans: these events are “unprecedented outside of volcanic settings or in the absence of a main shock." The study's abstract goes on to list the links between increases in drilling activity and increases in the number and strength of earthquakes throughout the central U.S.

According to E&E Publishing:

Geologists have known for decades that deep injection of industrial waste can lubricate faults and unleash earthquakes….Earthquakes unleashed by drilling wastewater fall into a gap in federal environmental laws. Oil and gas producers are exempt from provisions of the hazardous waste statutes designed to prevent industrial waste injection wells from triggering earthquakes (EnergyWire, March 12). States can adopt stricter rules. Ohio is adding additional seismic testing in the wake of its tremors, but other states have not followed suit.

The spotlight - and especially the public's interest - on drilling activities and earthquakes like won't go away anytime soon. We can look forward to learning more about the connections this spring with the release of the National Academy of Science's report "Induced Seismicity Potential in Energy Technologies.”

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