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When is an earthquake…

…not an earthquake?

A long time ago, I spent several years working in the field of earthquake seismology. It’s perhaps strange then, that I never really gave much thought to what might properly be called a earthquake, and what not. (My defence is that the earthquakes I was studying were, well, obviously earthquakes – nowhere near any grey area).

I actually came to think about this after a visit to the Darby Houses in Coalbrookdale (these are the houses where the famous Darby family iron masters lived). One of the exhibits there referred to the Buildwas “earthquake” of 27 May 1773.

What actually happened?

The effects of the Buildwas earthquake are probably best defined by the plan drawn up by George Young (see picture – note that north on his plan is not oriented to the top of the page as we would conventionally expect on modern maps). He shows how great gashes up to 30 feet (9m) deep have opened up in the hillside on the north bank of the river, with about 300 yards along the hillside affected; how field boundaries and the road have been pushed southwards towards the river; and how even the river itself has been forced onto a more southerly course.

It seems that, for a time, the river was effectively dammed, preventing navigation through the affected area and leaving boats below stranded on the river bed. The river, remember, was a primary route for transporting goods from the industrial areas around the Severn Gorge in the days before the railways.

But was it an earthquake?

Here’s what my dictionary says about the word: “A shaking of the ground; usually spec. a convulsion of the earth’s surface produced by volcanic or similar forces within the crust”. As an aside, given our modern understanding of plate tectonics, this definition seems a bit dated, but leave that aside for the moment.

Was there a “shaking of the ground”, a “convulsion of the earth’s surface”? From the information in Young’s plan, I should say so.

Was it “produced by volcanic or similar forces within the earth’s crust”? The nearest active volcanoes are in Italy and Iceland, so not that, although England does experience a few genuine earthquakes arising from long standing stresses on geological faults, but contemporary records say that there had been heavy rain for several days prior to the “earthquake”, and that the river had been running high. We also know that the whole of the Severn Gorge is¬†subject to instability, with many small landslips recorded. Indeed, as I write, a huge project to prevent further landslips in the Jackfield ¬†area has recently been completed.

So it would seem most likely that what happened was a slippage of relatively superficial earth and rock, not related to deeper or more wide-scale geological processes and I think, now, we would rather use the term “landslip” to describe this event. But that does seem to lose a little romance, and I for one will probably put aside scientific rigour and continue to refer to the Buildwas earthquake.

As a postscript, the map snippet below shows the area as it is today; you’ll see that the site is still marked as “Landslip” on the modern map!

Note: I am indebted to the University of Nottingham’s Weather Extremes blog article for much of the historical information contained in this piece.

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