Some fracking operations in western Canada are producing a surprising—and for many Canadians, troubling—side effect: earthquakes.

How widespread is the problem? According to a new study published Tuesday, at least 39 fracked wells in Alberta and British Columbia are suspected of triggering about 65 earthquakes between 2010 and 2015.

“We now confirm that many induced earthquakes in Western Canada can be linked to hydraulic fracturing operations,” study author Honn Kao wrote in an email to InsideClimate News. Kao is a scientist at Natural Resources Canada, the country’s main environmental regulatory agency.

The Canadian study is the latest in a string of investigations focusing on man-made quakes tied to the oil and gas industry in North America. Earlier this week, the United States Geological Survey published new maps that took into account the threat of man-made, or “induced,” events for the first time.

Unlike in Canada, however, the main culprit for man-made quakes in the central and eastern U.S. is the deep underground disposal of oil-and-gas-related wastewater—not fracking.

Kao, along with his 12 colleagues from academia and government, also examined the link between wastewater disposal wells and earthquakes in Canada. They found wastewater activities had a smaller impact on earthquakes there, identifying 17 disposal wells with possible links to past seismicity, according to their work published in the journal Seismological Research Letters.

To track down the activity causing recent earthquakes in Alberta and British Columbia, the researchers compiled a database of oil and gas activity and earthquakes from 1985 to June 2015. They ran an initial analysis to find fracking and wastewater disposal wells within 20 kilometers of any earthquakes large enough to be felt, magnitude 3.0 or above.

After narrowing the pool of potential wells, researchers conducted a more detailed analysis to pinpoint earthquakes that occurred only when nearby fracking wells were active or within three months after operations ended. Past studies have shown that man-made quakes don’t occur before fracking begins or long after operations cease. Researchers also looked for earthquakes that occurred near active waste injection wells. They ran additional statistic calculations to rule out natural seismicity.

Of the more than 12,000 fracked wells in Canada reviewed for this study, at least 39 wells—or 0.3 percent—showed a likely connection to earthquakes. Researchers also reviewed approximately 1,200 disposal wells and found roughly 1 percent of them, or 17 wells, have possibly triggered earthquakes in Canada.

Hadi Ghofrani, a study author and researcher at Western University in Ontario, told InsideClimate News the study potentially underestimates the fracking-earthquake phenomenon. Due to a six-month delay in reporting of well activity by Canadian oil-and-gas operators, the study took into account only wells known to be active up to June 2015. Since then, additional cases of fracking-linked earthquakes have been identified by Canadian regulators and scientists.

For example, Alberta Energy Regulator ordered the shutdown of fracking operations indefinitely in Fox Creek in January following a magnitude 4.8 event. Regulators told InsideClimate News they are still investigating the event and have not yet confirmed the earthquake was caused by fracking. This region has experienced several fracking-suspected events in the past, including a magnitude 4.4 event in 2015.

If confirmed, the January earthquake in Alberta could set a world record for the largest fracking-linked earthquake. Currently, the record belongs to a 4.6 earthquake that struck British Columbia in August 2015, another event not included in the recent Canadian study.

Fracking-linked earthquakes are far less common in the United States. There are a few suspected cases in Ohio, none of which generated the same level of shaking and intensity as observed in Canada, Stanford earthquake researcher Mark Zoback told InsideClimate News.

Meanwhile, several wastewater-linked earthquakes have reached magnitudes 4.0 and above already this year in the United States. In early February, Oklahoma experienced its third-largest earthquake ever—a 5.1 magnitude event likely connected to the region’s wastewater disposal activities. The earthquake struck the rural northwestern corner of the state and caused little damage.

Oklahoma is one of six central and eastern states that the USGS targeted in new maps out this week as having a heightened risk of experiencing a damaging earthquake year.

According to USGS researchers, approximately 7 million people—mainly in Oklahoma City and the Dallas-Fort Worth area—are at risk of experiencing damaging man-made wastewater quakes this year.

“The potential for damage is mostly from cracking in walls,” Mark Petersen, head of the USGS Natural Seismic Hazard Mapping Project, said at a press event on Monday. “I don’t want people to feel like their houses will fall down tomorrow,” he added.

Oklahoma experienced a massive uptick in earthquakes starting in 2009, around the time oil and gas development—and likewise waste disposal activities—expanded. In 2014, Oklahoma surpassed California as having the highest number of earthquakes a year. Similarly, western Canada also saw a rise in earthquakes in 2009 in conjunction with the region’s expansion of fracking operations for oil and gas.

Now local regulators in both countries are increasingly using what’s called a “traffic-light” approach in response to likely man-made earthquakes. This means regulators will order companies to either reduce or shut down their operations if an earthquake of a certain magnitude strikes nearby.

In the recent paper, however, Canadian researchers questioned the traffic-light approach, noting the potential for suspected fracking earthquakes to strike weeks to months after operations have finished. They wrote that there’s been at least one instance in Canada where a fracking-linked quake occurred eight days after nearby operations ended.

No one knows for sure why man-made earthquakes in Canada are mostly tied to fracking while those in the United States are predominantly linked to waste disposal. According to one theory, it has to do with proximity to faults. In Oklahoma, for example, scientists told InsideClimate News that the region’s faults are especially deep underground. When fracking occurs there, it’s at shallower depths, well above the faults. Wastewater, by contrast, is injected deeper into the earth and closer to faults.

The Canadian researchers, coming from Alberta Geological Survey, British Columbia Oil and Gas Commission, Natural Resources Canada, McGill University, Western University and the University of Calgary, questioned whether the United States should be so quick to dismiss the threat that fracking poses for causing earthquakes.

“It is possible that a higher-than-recognized fraction of induced earthquakes in the United States are linked to hydraulic fracturing,” they wrote, but that these events are being missed due to the attention given to wastewater-related events.