Dust flux, Vostok ice core

Dust flux, Vostok ice core
Two dimensional phase space reconstruction of dust flux from the Vostok core over the period 186-4 ka using the time derivative method. Dust flux on the x-axis, rate of change is on the y-axis. From Gipp (2001).

Monday, May 18, 2015

'Fracking' breaks Oklahoma

Well, maybe not yet.

This story in Zerohedge has attracted the interest of the Centre for World Complexity (that's me). So I've decided to take a little break from my ongoing travelogue of China to talk about some real geology for a change (possibly the first time this year).

Our topic today is intraplate earthquakes.But rather than reiterate what Wiki has already collated, let's apply our limited understanding of earthquakes to the situation in Oklahoma, while noting that our conclusions may also be applicable in other areas where fracking is being pursued (North Dakota? Saskatchewan? Ontario?).

Most large earthquakes happen at the edges of tectonic plates where the grind slowly past one another, but there are large stresses within plates as well. For one reason, the continental plates are all composed of small bits of tectonic material that have all become stuck together due to innumerable collisions of smaller pieces of material which couldn't be subducted. Because the plates are so large, the forces that drive them are dispersed over a large area, and stresses accumulate not only near the edges, but along any fracture that may exist within the plates interior.

Sometimes these intraplate stresses cause really big earthquakes. Perhaps the most famous such earthquakes happened in New Madrid, Missouri in the early 19th century. With a magnitude up to 8, they were the largest known earthquakes not directly related to a subduction zone in America's (known) history.

Seismic hazard map for the United States, from here.

Seismic hazard can be assessed in a couple of ways. By far the most significant approach is based on a study of the historical record of earthquakes. Hence, the two big red spots in the eastern US come from the large series of earthquakes in New Madrid in 1811 to 1812, and the Charleston earthquake in 1886. Several earthquakes have also occurred in the St. Lawrence valley (NE US) as well.

The historical record in the US is short, but the geology is long. The scale invariant nature of earthquakes allows us to estimate the recurrence interval of very large earthquakes in other places of the map (devoid of historical large earthquakes). Such recurrence intervals may be greater than a thousand years In such areas, stresses do build, albeit slowly--thus the likelihood of a large earthquake may be much greater than estimated purely on the basis of the historical record because it is so short.

Pumping liquids under pressure deep into the rocks has been correlated with small earthquakes since at least the 1960s. Our understanding of why this happens is more recent. It seems the fluids act much like a lubricant, allowing stresses that are already present in the rocks to be released. As far back as the 1960s, there were proposals for using such methods to control the build-up of stresses in the rocks and so prevent large earthquakes from occurring--however the approach has not, to my knowledge, been undertaken as a deliberate policy, probably due to liability concerns.

Now we see a series of reports and presentations (all pdf) by the Oklahoma Geological Survey showing a relationship between small earthquakes and fracking activity (the most important activity appears to be disposing of waste water in deep wells). Naturally, some people are concerned about liability.

Although many small earthquakes can be tied to oil and gas activity, no one has ever tied a large earthquake to such activity. And it is likely that no one ever will. Although the O&G activity is increasing the likelihood of small earthquakes, it is difficult to say what the impact on the likelihood of a large earthquake will be. The earth, even smaller parts of it, is a complex system, and part of what makes them interesting is that their response to stimulus is at least partially a function of the entire past history of the system. Our knowledge of earth history (especially around Oklahoma) is incomplete.

Suppose that in the absence of fracking, the recurrence interval for a magnitude 7.5 earthquake in a certain part of Oklahoma is on the order of 10,000 years (I have no idea if this is reasonable). Increasing the likelihood of small earthquakes may make a large earthquake more likely. If the last major earthquake in the area occurred in the 16th century, then probably there wouldn't have been enough time for the stresses in the system to build for a large earthquake to be triggered by fracking. But if the last big earthquake was ca. 10,000 BC, then there might be a problem.

There are costs and profits to be made from all kinds of human endeavours. Drilling for oil is one of them. I don't think we can allow the risks of induced earthquakes dissuade us from searching for oil, as it is a key determinant for economic progress. My concern is--are the people making the profits from oil exploitation going to be the ones paying the inevitable costs? 

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