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The relationship between fracking and low strength, shallow depth earthquakes

April 16, 2012
The Seismologic Society of America is having their annual convention in San Diego this week.  Among the various papers that will be presented is an intriguing one from Austin Holland of the Oklahoma Geological Survey titled “Earthquakes possibly triggered by hydraulic fracturing in southeastern Oklahoma.”  The link is to the paper abstract, which I am also pasting below.  This is by no means a definitive paper, but the research seems solid.  While I am a strong supporter of fracking for both economic and national strategic reasons, I am also a supporter of serious state monitoring and regulation of fracking for environmental reasons.  The only way that we can go forward and maximize this resource is to be careful about potential problems.  I don’t think the results of the present study are any reason to call for a moratorium on fracking, but I would support a requirement that seismic monitoring stations be present at fracking locations in order for researchers to obtain more and better data (the distance of monitoring devices from the locations of the earthquakes is a methodological limiting factor in Holland’s study).  Even if a direct link between fracking and these low strength, shallow depth earthquakes is established, it will not necessarily be a fatal blow to the fracking industry.  There would have to be a compelling reason to believe that these seismic events – which do not cause structural damage above ground – could lead to larger and more damaging earthquakes.  We are a long way from finding such evidence, but we should definitely do what we can to more closely monitor fracking sites.

ABSTRACT

Earthquakes were identified by cross correlating template events from a few manually identified earthquakes and cross correlating these templates through the entire operation period of the Earthscope Transportable Array (TA) station X34A. This produced a series of 116 earthquakes, which occurred from January 17, 2011 19:06 through January 23 03:13 UTC. The earthquakes ranged in magnitude from 0.6 to 2.9 with an associated b-value of 0.98. No earthquakes were identified outside of this time period for entire TA occupation of X34A. Correlation coefficients for the events ranged from 0.43 to 1.0 with a mean event correlation coefficient of 0.74. These earthquakes only occurred during and shortly after hydraulic fracturing stages of a nearby well, Picket Unit B Well 4-18. This well is located in a tectonically complex region of south central Oklahoma with many mapped faults and the well penetrated overturned formations at depth associated with an overturned syncline. The area is also located near the greatest concentration of historical seismicity within Oklahoma. Because the nearest station was about 35 km away from the earthquake sequence, obtaining accurate absolute locations for the earthquakes is difficult. Several velocity models were used to assess the uncertainty in location due to the choice of velocity model with a total uncertainty attributed to velocity models of about 330 meters. Earthquakes were then relocated using HypoDD and cross correlation of phase picks, with a total mean 2σ uncertainty of 200 m. The earthquake locations strike roughly north, sub-parallel to the mapped minor fault sets in the area. The earthquakes appear to have occurred at shallow depths from about 2 to 3 km, and occurred within 3 km horizontally of the well. The strong temporal and spatial correlation of earthquakes to hydraulic fracturing in a nearby well suggests the possibility that these earthquakes may have been triggered.
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