Johnny Cervo closed on his new house the morning of Monday, August 22, 2011, and moved in that afternoon. The structure sits in the rolling hills just north of Valdez, near the New Mexico border in south-central Colorado. The rural area normally enjoys a deep tranquility, but before Cervo could make it through his first night at the house, a terrifying sound jarred him awake. “I thought I heard something like an airplane going over,” Cervo says. The place began to rumble. He noticed that the water in a bottle near his bed was shaking. He fled his new home and spent the rest of the night at a property he’d leased on the far side of town.

Although he didn’t know it at first, what Cervo had experienced that night was an earthquake. The shaking caused an estimated $10,000 in damage to his water well, and for the next year, his nights were haunted by the fear that he might suddenly have to bolt to safety. Other properties in the area also suffered significant losses. The foundation of one Valdez home was knocked onto an uneven angle and sustained a reported $40,000 in damage. According to news reports, Ringo’s Super Trading Post in nearby Segundo delayed opening the next day so staff could clean up broken inventory that had been shaken off the shelves. Rocks dislodged from slopes were strewn on roads throughout the region, including I-25.

Unlike most of Colorado, the area around Trinidad, a city of about 8,500 people in Las Animas County that sits 15 miles east of Valdez, is subject to mild natural seismicity. During the lead-up to the 2011 quake, seismic activity had intensified. In the almost 30 years before 2001, there was a single earthquake in the region that registered at or above magnitude 4. Since 2001, there’d been an average of one earthquake of that magnitude or higher annually.

The Trinidad earthquake, as the 2011 event is known, registered a magnitude of 5.3, placing it fourth among the strongest earthquakes in Colorado history. It was felt as far away as Nebraska. In the days after, U.S. Geological Survey (USGS) scientists attributed the quake to natural causes. Tony Crone, a USGS geophysicist, told the local Times Independent newspaper that the earthquake was “caused by the Earth’s crust being ‘stretched’ ?” and noted that the Sangre de Cristo Fault, which lies just west of Trinidad, is a source of such crust movement. That same article also briefly suggested one alternative explanation: “Some local residents have questioned whether increased natural gas drilling in Las Animas County could trigger quakes.”

This past March, the USGS for the first time released a report, accompanied by a series of maps, that identifies potential hazards from man-made earthquakes in the Central and Eastern United States. Such maps previously showed hazards only from natural seismicity, but a recent outbreak of earthquakes believed to be induced by human activity prompted the new approach. The agency singled out six states for concern, including Colorado.

The Centennial State might not be known as a seismically volatile area in the way that, say, California is. Nevertheless, the science of man-made earthquakes—“induced seismicity,” in geophysicists’ parlance—was born in Colorado. In 1962, crews at the Rocky Mountain Arsenal, a now-defunct chemical weapons plant just north of Denver, began injecting liquid waste down a well that penetrated more than two miles of earth. Within months, Denver residents were jolted by a series of earthquakes, including one of magnitude 4.8 that caused more than $1 million in damage. Academic and governmental researchers found the disposal process at fault, and today Rocky Mountain Arsenal is considered the first recognized case of induced seismicity due to deep injection of fluid.

Two of the other most-cited historical examples of induced seismicity also come from Colorado. Starting in 1969, the USGS conducted a yearslong experiment in an oil field near the small town of Rangely, just east of the Utah border, in which scientists essentially turned earthquakes on and off by injecting or withdrawing fluid from the ground. Something similar happened in western Colorado’s Paradox Valley. In 1996, authorities began injecting brine from the valley’s groundwater into a deep well to prevent it from contaminating the Dolores River with too much salt. Those injections have continued, and according to federal research they’ve induced earthquakes, including a magnitude 4.3 quake in May 2000. “Colorado just happens to be a test bed for this whole science,” says Harley Benz, the scientist in charge of the USGS National Earthquake Information Center (NEIC) in Golden.

Little from those historical cases prepared scientists for what’s happened since the start of the 21st century. In the 30 years before 2000, there was an average of 29 earthquakes of magnitude 3 or greater a year in the Central and Eastern United States. In the three years between 2010 and 2013, the average jumped to more than 100. The increase coincides with the proliferation of horizontal oil and gas drilling and hydraulic fracturing, or fracking, a process that can produce large volumes of salty wastewater. Companies typically dispose of that water by injecting it back into the ground, which is filled with pores and fractures. Natural pressures normally keep these geological joints quiet, but in some cases, the introduction of fluids can loosen things up. “Basically, you’re lubricating the fault to make it slippery,” Benz says.

In the Raton Basin, an area that includes Trinidad, major wastewater injections related to methane gas production began in 1999. The timing of the injections matches the region’s earthquake spike. A 2014 USGS study concluded that such activity “is responsible for” the majority of the ground-shaking. Indeed, many scientists now believe the Trinidad earthquake was the second-strongest fluid-injection-induced earthquake in U.S. history. The first, a magnitude 5.6, occurred on November 5, 2011, near Prague, Oklahoma, in a region that has become the country’s primary area of concern when it comes to induced quakes. (Before 2009, the annual number of magnitude 3 or greater earthquakes in Oklahoma was about two; in 2015, there were 907. Last month, Oklahoma was hit with a second 5.6 earthquake that the USGS suggested was linked to industrial activities, though at press time, further study was pending.)

Scientists have noted, as was the case in the Rangely experiments in the 1960s, that the timing of induced seismicity often correlates to the injection or withdrawal of fluids. But they’ve also seen dangerous earth-shaking occur well after injections have ceased and many miles from the initial injection point. The Rocky Mountain Arsenal earthquakes continued for two decades after authorities stopped pumping liquid waste into the ground. “If you shut off a well, that doesn’t mean you shut off seismicity,” says William Yeck, a USGS research geophysicist at the NEIC. “It takes a while for those pore pressures to dissipate, and as they dissipate, they can induce more earthquakes. So it’s not something that we can instantaneously stop.”

The USGS hazard maps and report released in March, which focus on the Central and Eastern United States, indicate that about seven million people live and work in areas that induced seismicity could strike. The report also identified “21 areas with increased rates of induced seismicity,” five of which were in Colorado: Greeley; the Rocky Mountain Arsenal; Paradox Valley; Rangely; and the Raton Basin. In January 2013, a magnitude 4.4 event rattled the town of Paradox, and two summers ago, a series of quakes that included a 3.2 hit northeast of Greeley, a historically earthquake-free area. Those rumblings are widely believed to have been induced by wastewater injection. As Yeck puts it, “There’s something going on.”

Despite the recent USGS findings and a general consensus in the scientific community that humans can cause earthquakes through fluid injection, when it comes to examining individual cases, not everyone is so quick to concede the connection.

When I asked Karen Berry, the Colorado state geologist and director of the Colorado Geological Survey, about the apparent uptick in earthquakes in Colorado, she replied in an email that part of the reason for the increase in reports is that “modern instruments are more sensitive.” In response to a question specifically about the Trinidad earthquake, Berry wrote, “Determining whether a single earthquake is natural or induced is extremely difficult.” Although USGS studies did eventually conclude that the 2011 event was man-made and that the Raton Basin in general is subject to induced seismicity, Berry explained that earthquakes in that area don’t fit the mold of induced quakes elsewhere in the United States because they occur well below the depth of wastewater injection. “Little is understood about earthquake hazards near Trinidad and the risks to public safety,” Berry wrote.

Doug Flanders, director of policy and external affairs for the Colorado Oil & Gas Association, emphasized that there are still unknowns in seismic science, and he echoed Berry in arguing that the cause of the Trinidad earthquake is undetermined. “Amongst the geologists, the experts on this issue, they’re still debating it,” Flanders says. The presence of natural seismicity in the Raton Basin, combined with recent earthquakes having originated below the level in the earth at which wastewater injection occurs, has led state officials such as Berry and Stuart Ellsworth, an engineer at the Colorado Oil and Gas Conservation Commission (COGCC), to say that insufficient evidence exists to blame the Trinidad earthquake on human activity.

Flanders, however, does acknowledge that the June 2014 earthquake near Greeley was the result of over-injection of wastewater fluids; he says that case demonstrated how state and industry officials were able to properly respond to the problem. COGCC has instituted wastewater-disposal regulations that govern maximum allowable injections by pressure, volume, and rate—and Flanders says there hasn’t been another earthquake in that area since. “We’re not disputing the fact that a [wastewater] well may cause induced seismicity,” Flanders says. “But I think from a Colorado standpoint, with the rules and regulations that we have in place, you are not seeing this as being the issue that you might find in other states.”

Even if a direct link to energy production were assumed, forming a policy response to induced seismicity is tricky because it’s difficult to make long-term predictions about man-made earthquakes. Conditions can rapidly change depending on the business decisions of energy companies; gas production near Trinidad, for example, declined significantly in recent years. Even the new USGS report states that “building-code committees are reluctant to consider induced seismicity in their current design codes because the hazard from induced earthquakes will change before the building regulations are enacted.” What’s more, the vast majority of injection wells operate problem-free. Induced earthquakes have been linked to just a few dozen of the estimated 35,000 active wastewater disposal wells in the United States.

Those challenges aside, state Representative Joe Salazar, a Democrat from Thornton, offered a political response to induced seismicity earlier this year. Salazar helped sponsor a bill that would have made it easier to sue oil and gas companies for damages caused by hydraulic fracturing activities, including earthquakes. The bill passed the Democrat-controlled House but died in the Republican-controlled Senate. Democratic Governor John Hickenlooper also opposed the bill. At the time, a spokesperson for the governor told the Denver Business Journal that Colorado “[hasn’t] seen widespread seismicity caused by oil and gas operations” and called the bill “a solution in search of a problem.”

For Salazar, the science is clear, and he points to the effects of oil and gas production in Oklahoma as a warning for Coloradans. “The more that they ramp up their operations in Oklahoma, the more induced seismicity has occurred,” says Salazar, who is running for re-election next month and is considering reintroducing the bill in 2017 if he wins. “The same thing is going to happen here,” he says. “The more they ramp it up here in the state of Colorado, we’re going to follow the same suit. I mean, Oklahoma is the miner’s canary.”