In several US states, fracking has become a standard method of extracting natural gas. The technique, which involves cracking open rock seams with pressurized fluids, is only just starting to be used in Europe. And, as it arrives, it's apparently shaking things up. Earlier this year, a fracking operation near England's northwest coast set off small tremors that were noticed by the citizens of nearby Blackpool. An analysis of the events has now been completed, and its authors conclude that, although fracking isn't likely to ever cause a significant earthquake, there are ways in which these could have been avoided.

Fracking is shorthand for hydraulic fracturing, a technique in which fluids are pumped under ground at high pressures. The fluids can expand existing fractures within the rock, creating passages for natural gas to escape. For the most part, concerns about the procedure have focused on its environmental impact, as the fluids involved are returned to the surface with a variety of contaminants that must be safely disposed of.

Prior to the events in the UK, only a single instance of fracking had been associated with tremors. These occurred in Oklahoma, and were quite minor events, with magnitudes of 2.8 and 1.9. Most events have a magnitude of less than zero, and can't even be detected with normal seismic equipment. But at the UK site, over 50 events have been detected, the largest reaching magnitudes 2.3 and 1.5. As a result, operations at the site have been shut down awaiting an expert evaluation of the problems. That report has now been released.

The UK is not noted for being seismically active, but it does have a number of faults, and many of them are currently under stress. (The report spends a lot of time describing the local geology in the area, which dates back to the "Late Devonian and Early Pennsylvanian times.") Although the amount of stress isn't sufficient to actually cause a fault to rupture, it lowers the amount of energy required to trigger one. This, according to the report, was the key factor in triggering the small tremors.

It appears the well used in the fracking wasn't actually in a fault, but it was near one. As a result, the tremors didn't occur during the point in the operation where the pressures were highest; instead, it happened significantly afterwards. The high-pressure fluids diffused from the drill site for hours afterward, which allowed them to spread to the fault site. The pressure was much lower by the time the fluid got there, but it was still sufficient to lubricate a fault under stress. Once they reached the fault, the pressurized fluids did exactly what they're meant to do: force open existing fractures.

All of the wave forms of the seismic events are similar in nature, so the report concludes that there's nothing especially complex or unusual going on. And, based on their geological models, the process is unlikely to produce anything much larger than the tremors the area has already experienced, which were caused by local slippage of a single slip plane.

That said, the quakes occurred near the surface, which is the reason that many people were aware of it. They also occurred close enough to the drill site to damage some of the equipment—the report refers to a casing in the site that was "ovalized" for several hundred feet during the incident. On the whole, its authors don't think that the problems represent any sort of safety issue.

Still, they think it's probably best that the events should be prevented where possible. The simplest way to do so would be to remove the pressure as quickly as possible once the fracking process is over. That would involve pulling the fluids back out of the site quickly, rather than allowing the pressure to dissipate.

More generally, they recommend that sites be equipped with seismic monitors, and be able to adjust activity if tremors are noted. The report refers to a traffic light system, where a green light for all activity is given when there is no sign of seismic activity. Any sign of tremors switches it to yellow, which involves heightened monitoring for two days after activity stops. Any event with a magnitude of greater than 1.7 would require that all fracking be stopped, and fluid immediately pumped back out of the well.

Given that the tremors occurred at the UK's first fracking site, there's a reasonable chance that these practices could be adopted nationwide there. In the US, however, fracking is already widespread. However, in the states where it's currently restricted or banned, this report may help motivate a consideration of seismic issues, at least on sites where the geology suggests similar problems could occur.