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The Scale of the Seismicity “Problem”

Andrew Montford, GWPF

Towards the end of last week, shale gas company Cuadrilla Resources posted a graph of recent seismic activity in the UK, showing how small the tremors at their site outside Blackpool are far far smaller than the naturally occurring tremors that have occurred elsewhere in the UK in recent months. The graph also highlighted 0.5ML cutoff level, above which they are required to switch off the frac’ pumps temporarily under the government’s “traffic light” system.

The graph was criticised in some quarters because, instead of showing each tremor separately,  Cuadrilla had shown the average tremor size in each location. This tended to obscure the larger tremors induced by Cuadrilla’s activities. “It’s the largest tremor we should be worried about”, the critics observed, not unreasonably.

With this in mind, I thought I would update the graph so as to show each tremor separately. At the same time, I have also corrected two other features of the original graph that might potentially mislead. Firstly, Cuadrilla had included tremors located offshore, at least one of which was, at 4.8ML, rather large. I don’t think it’s reasonable to include these, since offshore tremors will not affect anyone living in the UK (unless they are large enough to launch a tsunami). Secondly, the scale on the y-axis is logarithmic. This is fine for scientists, but for the general public it tends to give a false sense of the relative size of the events involved. I’ve therefore also changed the presentation so that the sizes are on a linear scale.

In my new graph of recent UK seismic activity (see below), the tremors are sorted left to right by size.  The tremors induced by Cuadrilla are shown in red at the left-hand side. Although I have stretched the graph a lot, you will still need to zoom a lot to see them, and if you are on a mobile phone, it’s probably not even worth trying. The rest of the tremors – mostly natural – are in blue. The largest, on the right-hand end, is the 3.1ML tremor in County Durham, which approaches the level at which you might expect such events to be felt at the surface (although the British Geological Survey has no reports that anyone noticed this particular one).

Recent UK Seismic Activity. Cuadrilla red, others blue. y-axis is 10^(published ML value). Data per BGS .

Of course, if I had included the 4.8ML Norwegian sea tremor, the graph would have had to have be 50 times as tall to give the same size bars for the Cuadrilla-induced tremors. And even without it, it’s simply not worth showing the 0.5ML red traffic light level – it would be indistinguishable from the x-axis.

Is anyone worried yet?