By Zoë Corbyn of Nature magazine

Small earthquakes are a recognized risk of hydraulic fracturing, or "fracking," a procedure in which companies unlock energy reserves by pumping millions of liters of water underground to fracture shale rock and release the natural gas trapped inside. Researchers now say that they can calculate the highest magnitude earthquake that such an operation could induce--though it won't determine the likelihood of a quake occurring.

The model is crude, but it should be "good enough" to use in the field, says Arthur McGarr, a geologist at the US Geological Survey in Menlo Park, California, who presented the work yesterday at the annual meeting of the American Geophysical Union in San Francisco. "The method could help engineers trying to plan for future earthquakes know what to plan for," he adds.

McGarr's team says that the quantitative method is applicable to any operation that involves injecting fluid deep underground. Apart from fracking, such activities include the disposal of fracking fluids--in which companies get rid of their drilling water by pumping it into disposal wells--as well as geothermal-power generation and carbon dioxide sequestration.

It is rare, but not unheard of, for fluid-injection operations to cause detectable earthquakes. The number of such tremors has increased over the past decade as the amount of deep fluid injection has risen, says McGarr. Fracking itself is thought this year to have spurred quakes in Lancashire, UK--of magnitude 2.3 and 1.5--and Gavin County, Okla., of up to magnitude 2.8 (see "Fracking caused British quakes").

Straightforward relationship

McGarr and his team studied seven cases of quakes induced by fluid injection. They included the Oklahoma fracking site where 8,900 cubic meters were injected; a scientific bore hole in Germany, where an injection of 200 cubic meters of salt water caused a magnitude 1.4 earthquake; a geothermal-energy project on the outskirts of Basel, Switzerland, that was terminated after an injection of 11,600 cubic meters of water triggered a series of quakes of magnitude up to 3.4; another in Cooper Basin, Australia, where a 20,000-cubic-meter injection resulted in a magnitude 3.7 quake; and a liquid-waste-disposal project in Colorado in the 1960s, where an injection of 631,000 cubic meters triggered earthquakes of magnitude up to 5, the largest yet seen as a result of fluid injection.

The researchers found a proportional relationship between the volume of fluid injected and the magnitude of the earthquake.

"If you inject about 10,000 cubic meters, then the maximum sized earthquake would be about a magnitude 3.3," says McGarr. Every time the volume of water doubles, the maximum magnitude of any quake rises by roughly 0.4. "The earthquakes may end up being much smaller, but you want to be prepared for the worst-case scenario," says McGarr. The relationship is straightforward, but it is the first time that anyone has quantified it, he adds.

McGarr has also developed a similar method--now in standard industrial use--to work out the maximum size of earthquake that can be triggered as ore is extracted in underground mining. This work "builds on the same concept", he says.

He notes that the latest work doesn't give the probability of an earthquake actually occurring: that depends on other factors, such as the strength and permeability of the rock.

Max Wyss, director of the World Agency for Planetary Monitoring and Earthquake Risk Reduction in Geneva, Switzerland, said that the study was a "good review" of the situation but did not achieve what is really needed--working out the maximum induced earthquake that can be tolerated at any given location. "We need to do better at estimating the risk for society from the earthquakes," he says.

McGarr's team hopes to submit the work for publication soon, with further case studies.

This article is reproduced with permission from the magazine Nature. The article was first published on December 9, 2011.

1 Comments

1. 1. Darth_Loki 08:03 PM 12/9/11

   Their calculations are probably relevant to known, stable, geological conditions. The earthquakes in Oklahoma appear to be a perfect storm of conditions including the possible existence of two big asteroid impact craters. It does not seem possible that the buried craters escaped the intense scrutiny of the nearly 480,000 oil and gas wells that have been drilled in the state. I don't think that all the companies missed it, I think some were able to keep it as a lucrative business secret.
   
   The asteroid impact crater causing most of the problems is about 13.5 miles across centered at approximately 35.557N, -96.871W. The smaller one I have (tentatively) named the Meeker Crater since the town of Meeker is about 3500 feet above the crater. The larger is the Choctaw (tentative name) Crater and it is approximately 66.0+ miles across located at 35.060N, -97.315W. I followed some earthquakes above Choctaw Oklahoma to their source. I am guessing that they are between 250 and 300 million years old.
   
   The 1952 previous record 5.5 Earthquake was on the Northwest edge of the Choctaw Crater.
   
   
   What is visible on the surface does not appear to be the actual impact structure. What is visible are the long term effects of the faulting of the sedimentary rock layers that had formed above the structure. As the new rock were being lain down, they would develop faults as the impact structure beneath them continued to settle. These faults layers would tend to erode more quickly then adjacent areas. We are probably just seeing the surface shadow of the real crater, which is still 3500 feet below in the East and more(?) than 8000 feet down in the West.
   
   The oil well logs also show a structure. An old well log 1929 T. B. Slick has mirror image strata for 1400 ft on either side of the shale at 4956-4981. This happens when a big section of rock gets flipped onto the rock next to it. 1000's of tons of rock. The well location is Section 13 Township 13N Range 01W. Total depth 6971 Ft. Oklahoma County Oklahoma
   
   Other well logs show shale debris on the edges. There is usually about a 2000 to 4000 ft section of messed up layers and fractured shale inside the crater. Outside the smaller Meeker crater I found references to boulders, iron pyrite, and possibly metallic iron. If it were a Nickel-Iron asteroid which created the 66+ mile diameter Choctaw Crater it would have been 4800 Km or nearly 3 miles wide.
   
   http://impact.ese.ic.ac.uk/ImpactEffects/
   The above link is how I got the estimate for the size of the impactor

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