Did the Earth Move for You?

AT 18H38 ON 16TH JUNE 2023 AN EARTHQUAKE OCCURRED. IT WAS FOLLOWED A FEW HOURS LATER, AT 04H27 AND 09H13 ON 17TH JUNE, BY TWO AFTERSHOCKS

Don’t worry; this was not an event that laid low a teeming city and killed thousands. In fact, no one was killed, or even seriously hurt, and the damage to property was (thank goodness) restricted to a few walls partially demolished. Some damage was done, of course, to ornaments and loose items around the houses in the area, but it could have been very much worse.

The epicentre of the earthquake was the village of La Laigne in Charente-Maritime, France, roughly midway between La Rochelle and Niort. The intensity of the earthquake was between 4.8 and 4.9, which is moderate but not alarming. The tremor was reportedly felt over much of north-central France, from Bordeaux in the South to Caen in the North, and as far east as Clermont-Ferrand. I have to say that, in my home near Ruffec, I didn’t notice a thing, but this is a rural area, and we are used to unexpected vibrations from passing farm equipment. That would have been the sort of vibration one would have felt.

But how can it be that France has suffered an earthquake? Surely they happen in distant lands where there are mountains and volcanoes, not in peaceful, rural France? What causes them, anyway?

A Short Explanation of Earthquakes

Until about 200 years ago, an earthquake was the Wrath of God, pure and simple. Wages of Sin. Goodnight, Sodom and Gomorrah. Bye, bye Lisbon - don’t know what you’ve been doing, but it was obviously bad!

Then people began noticing that earthquakes seemed to be a fact of history, even before there were humans to earn a bit of wrath. The fledgling science of geology found evidence of huge upheavals in ancient rocks that would have put many modern earthquakes to shame.

An earthquake happens when two sections of rock slide past each other, releasing some stored energy. If there is a lot of stored energy, you get a big, devastating earthquake; if it is a relatively minor energy release, the effect is usually small. The details can depend upon local geological conditions, but generally that is the case. But why should rocks have stored energy? That is because of the way the Earth’s crust behaves. Our Earth has a hot, mostly solid, core, surrounded by a mantle of

ByMikeGeorge

Mike George is our regular contributor on wildlife and the countryside in France. He is a geologist and naturalist, living in the Jurassic area of the Charente plastic semi-rock that has the ability to flow slowly under the influence of convective heating. The whole lot is covered by a thin layer of solid rock called the crust. That is the bit we live on. The movement of the slowly-flowing mantle causes the crust to deform and to float about in sections, rather like the skin on a custard that is being heated. These sections can move apart, slide past each other, or even collide. This is the mechanism by which oceans open (as sections part) and mountains rise (where sections collide).

This gives plenty of chance for stress to build up in the rocks of the crust. At the margins of the sections (often referred to as “plates”) there is plenty of stress, and large earthquakes and volcanoes are common. But we are not in that situation here, are we?

Not now, we aren’t, but the sites of activity have changed over the thousands of millennia the Earth has existed. At one time, 300 million years ago, we were at the south edge of a collision zone that pushed up the mountains of Armorica and the Massif Central, implanted the granite from

Dartmoor to the Scilly Isles, and crumpled up the Mendip and Cotswold Hills in England. That left a huge legacy of stress and cracking in the existing rocks below us. Later the rocks of the Jurassic and Cretaceous covered them, but the stress was still there, and this has been causing movement in the old rocks, which also affected the later rocks, ever since. Most of this is now low-level stress, but it still makes itself felt when the crust twitches and releases a bit more energy.

Also, the Pyrenees Mountains, caused by the movement of the Iberian Peninsula relative to France, started forming some 80 million years ago and have not yet finished growing; there will be a lot of stress release to happen there for a long while.

Stress release is what happened in June in La Laigne. A small vertical movement along a line of weakness about 3 km below the surface released enough energy to be felt, and to do some minor damage. The northern part of New Aquitaine is in fact classed as an area of “moderate” earthquake risk – check the deeds of your home! And blame the Armorican

Mountain-building Event 300 million years ago!

Energy vs Effect

The energy an earthquake releases is directly calculable from the seismograms recorded for it. This is what the Richter Scale records. The energy levels are written in Arabic numerals. This can give a reasonable indication of the effect of an earthquake at low energy; high-energy earthquake effects are less predictable, and depend on the local geology. Incidentally, the scale is not linear, it is logarithmic. Thus, an earthquake at level 4 does not release twice as much energy as one at level 3; it releases 10 times as much.

To relate the effect of an earthquake, several scales have been adopted. These are made by actually observing the damage done and correlating it. They tend to follow the energy scales to some extent, especially at low levels, but the levels are written in Roman numerals to distinguish them.

Further study

If you want to know more, either about this particular event or about earthquakes in general, there is a very good web address that my colleague at BRGM sent me, it is rather scientific, but has excellent full-colour maps and charts to explain matters: www.brgm.fr/en/news/news/earthquakevillage-laigne-charente-maritimeinitial-analyses