In the past decade, Oklahoma has turned heads as it has joined the list of places where earthquake insurance is a prudent investment. The sudden uptick in seismic activity is due to injections of foul wastewater into deep disposal wells—triggering what are known as “human-induced earthquakes.”

The Okie state hasn’t been entirely alone in this, though. Because geology doesn’t respect state lines, the oil and gas wells producing all this wastewater—and the deep, salty aquifer it has been pumped down into—extends into Kansas. And a new study led by Justin Rubinstein of the US Geological Survey confirms that they have been causing earthquakes there, too.

While Oklahoma actually experienced some structural damage in several earthquakes, Kansas hasn’t seen anything stronger than a magnitude 4.9 that shook some items off of shelves. But between 2012 and 2016, southern Kansas had six magnitude 4 or larger quakes, which seems far from normal.

To find out, the researchers relied on past records of earthquakes in the state and an array of seismometers that were set out in early 2014 to better measure the recent activity. The simplest analysis is to look for correlations in time and location between the drastic uptick in earthquakes and wastewater injections in the state. These injections have gone on for a long time, but very little wastewater was disposed of this way until 2012. At that point, the amount of injections doubled in a year and more than doubled again by 2014, peaking at over 100 million barrels per year.

Between 1974 and 2012, seismometers recorded just 15 earthquakes of at least magnitude 3, none of which were over magnitude 4. Over the next four years, there were six magnitude 4+ events and a whopping 127 magnitude 3+.

Kansas actually began to limit wastewater injections in reaction to noteworthy earthquakes. This started in early 2015; together with the influence of lower oil prices that slowed production in the area, the injection rate declined nearly as quickly as it rose, falling to less than half the peak rate by the end of 2016. And while the relationship between injections and quakes can be complicated by a time lag between injections and earthquakes, the number of quakes also dropped off rapidly.

So the correlation in time is pretty striking, but what about the location of the earthquakes? The researchers focused on a very small area that contained the vast majority of the recent earthquakes (red box in the image at the top of this story). The area also happens to contain most of the active injection wells, and the earthquakes did indeed occur close to active wells, sealing the study’s conclusion that the quakes are human-caused.

There is one interesting exception, however. There is a small cluster of wells just outside the area the researchers focused on that actually handled about 40 percent of the total volume of injected wastewater—but almost no earthquakes occurred there. The researchers think this highlights an important fact: the geologic details of the injection sites are critical.

In order for these injections to trigger earthquakes, there must be faults loaded with strain in the area where fluid pressure is increased. The increased fluid pressure relieves the friction keeping the fault locked, allowing the blocks to slide along the fault and release their pent-up energy as seismic waves.

In Kansas and Oklahoma, the faults are in deep igneous and metamorphic rocks just beneath the sandstone layer used by the injection wells. It’s quite possible that there either isn’t a suitable fault near these quiet wells, or the pressure change they’re causing simply isn’t reaching a fault. Perhaps easy flow paths cause the pressure to spread out horizontally rather than downward, for example.

As in Oklahoma, it’s still possible that southern Kansas could see a significant earthquake at reduced injection levels, but it’s encouraging to see that seismic activity has already dropped. The geologic complexities illustrate why it’s so difficult to answer the obvious question that remains: how can wastewater injections be adjusted to avoid triggering earthquakes?

Bulletin of the Seismological Society of America, 2018. DOI: 10.1785/0120170209 (About DOIs).