A new study explains how just four wells forcing massive amounts of drilling wastewater into the ground are probably shaking up Oklahoma.

Those wells seem to have triggered more than 100 small-to-medium earthquakes in the past five years, according to a study published Thursday by the journal Science. Many of the quakes were much farther away from the wells than expected.

The wastewater is leftover from unconventional wells that drill for oil and gas with help of high pressure liquids — nicknamed fracking — and from the removal of water from diluted oil. These new methods mean much more wastewater has to be discarded.

Combined, the four wells in the study daily pour more than 19 million litres of water around two or three kilometres underground into rock formations, the study found. That buildup of fluid creates more pressure that "has to go somewhere," said study lead author Cornell University seismologist Katie Keranen.

Researchers originally figured the water diffused through underground rocks slowly. But instead, it is moving faster and farther and triggers quake fault lines that already were likely ready to move, she said.

"You really don't need to raise the pressure a great deal," she added.

The study shows the likely way in which the pressure can trigger fault lines — which already existed yet were not too active— but researchers need more detail on the liquid injections themselves to absolutely prove the case, Keranen said.

While there are about 8,000 deep injection wells in the region, the amount of water injected at the four wells — named Chambers, Deep Throat, Flower Power and Sweetheart — has more than doubled since the drilling boom started about a decade ago.

From 1976 to 2007, Oklahoma each year averaged about one quake of magnitude 3 or more — strong enough to feel locally but too weak to cause damage. But from 2008 to 2013, the state averaged 44 earthquakes of that size every year. So far this year there have been another 233, Keranen said, getting her earthquake figures from the U.S. Geological Survey database.

Push for restrictions

The rattling has led some Oklahomans to push for restrictions on the use of injection wells.

While past research has shown more quakes in Oklahoma, Texas, Arkansas and Kansas and correlated it statistically to injection wells, this study used computer simulations to identify the mechanism of how massive amounts of water travel as much as 32 kilometres from the well. The pressure then triggers existing small faults — or previously unknown ones. In the past, scientists thought wells could only jump-start quakes within five kilometres) or so.

Austin Holland, a seismologist at the Oklahoma Geological Survey said Keranen's study confirms what he is seeing in the field and will help better understand what's happening in Oklahoma.

"It's a study that needed to be done," said U.S. Geological Survey seismologist Elizabeth Cochran. "That changes how we might look at the hazard for a particular well."