There were two reports last week exploring whether earthquakes in small towns were linked to hydraulic fracturing, a process used by oil and gas produces to extract natural gas from hard rock.

The first report out of Lancanshire, UK was from the British Geological Survey and found that it was “highly probable” that the quakes were caused by drilling. It said that the geological formation at that part of the country had led to the events, which were unlikely to be repeated.

The second report came out of Oklahoma Geological Survey and can be downloaded here:

An examination of the paper suggests that there is doubt — not surprising considering that earthquake reconstruction is a shaky science. While there were only 2 quakes in Lancanshire, there have many in south-central Oklahoma, and it is one of the most active regions.

While models suggest that the quakes occurred close to where drilling happened, and around the same time — there is no way to say for sure.

The strong correlation in time and space as well as a reasonable fit to a physical model suggest that there is a possibility these earthquakes were induced by hydraulic-­‐fracturing. However, the uncertainties in the data make it impossible to say with a high degree of certainty whether or not these earthquakes were triggered by natural means or by the nearby hydraulic-­‐fracturing operation.

The hypothesis that the two events could be linked comes from a body of research from the 70’s onwards where scientists purposely created earthquakes by injecting fluid into the ground. Hydraulic fracturing is nothing more than injections of chemicals (whether benign or otherwise) into rock:

Reservoir and fluid-injection induced seismicity (e.g. Raleigh et al., 1972)

An experiment in an oil field at Rangely, Colorado, has demonstrated the feasibility of earthquake control. Variations in seismicity were produced by controlled variations in the fluid pressure in a seismically active zone. Precise earthquake locations revealed that the earthquakes clustered about a fault trending through a zone of high pore pressure produced by secondary recovery operations.

A Reservoir Analysis of the Denver Earthquakes: A Case of Induced Seismicity

Deep injection and closely monitored induced seismicity at Paradox Valley, Colorado

Source parameters of injection-induced microearthquakes at 9 km depth at the KTB Deep Drilling site, Germany

A checklist

But uncertainties aside, scientists have devised a checklist to say whether it is probable that earthquakes and human activity are linked:

Question 1: Are these events the first known earthquakes of this character in the region?

Question 2: Is there a clear correlation between injection and seismicity?  

Question 3: Are epicenters near wells (within 5 km)?  

Question 4: Do some earthquakes occur at or near injection depths?  

Question 5: If not, are there known geologic structures, that may channel flow to sites of earthquakes?

Question 6: Are changes in fluid pressure at well bottoms sufficient to encourage seismicity?

Question 7: Are changes in fluid pressure at hypocentral locations sufficient to encourage seismicity?  

But even if the answer is ‘yes’ to all of these, it is impossible to say with absolute certainty that the events are linked.

This uncertainty arises both because the underlying physical mechanisms of earthquakes are poorly understood, and because in nearly every specific situation there is inadequate or incomplete information about regional stresses, fluid migration, historical seismicity, etc. Clearly, a series of seven or ten yes or no questions oversimplifies many of these issues.”

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