Are earthquakes tidal?

Are earthquakes worse when there's a full moon? It may sound like an old wives' tale, but new research suggests there could indeed be a link. To find out more, Laura Brooks spoke to Cambridge University Earth scientist Alex Copley...

Alex - An earthquake occurs because as Earth's tectonic plates move relative to each other, the forces gradually build up on the surfaces that cut through the crust that we call 'active faults.' When these forces get big enough, then the rocks break and move in an earthquake. These sort of stresses build up over an awfully long time, sort of millions and thousands of years.

There are another kind of stresses which affect the earth, which are the stresses due to the gravitational pull of the sun and the moon acting on the Earth. This is both acting on the rocks themselves and also making the oceans move around and the ocean tides, which pushes against the continents and changes the stress in the rocks.  These stresses are very small compared to those in relation to the motions of the tectonic plates but they fluctuate very rapidly, sort of multiple times a day.

So the idea that people have had for a long time is that you're more likely to have an earthquake when these tidal stresses are promoting slip on a fault rather than when they're inhibiting it.

Laura - It sounds like scientists have been speculating about this for a little while but there's now been a study published this week which looks a bit more closely at this link. Can you tell us what they found in that work?

Alex - So this recent paper, what they did was they looked at some very large earthquakes that happened recently and they saw that these happened at the times when these tidal stresses were sort of promoting motion on the fault.

However, when they looked at all the earthquakes, not just these big ones, they found no relationship. So there are two ways you could interpret this result: one is that when the tidal stresses are in the correct orientation, you're more likely to turn a small earthquake into a big earthquake, or the other way you could interpret this is to say that because they're only looking at a small number of big earthquakes, that you just don't have enough for the patterns to be representative. And the jury is still out as to which of these two options is the correct one.

Laura - And earthquakes of this scale are quite rare, aren't; they - how often do they occur?

Alex - So the three main earthquakes that they looked at in this paper were high in the magnitude eight and in the magnitude nines, and these happen very rarely. So we've had three in the last twelve years but that's unusual. The most recent ones before this recent phase were in the 1950s and 1960s. So this is the problem that because you don't have many earthquakes of this size, because they don't come along very often, we've just not seen very many of them since we've had the instruments available to monitor them.

Laura - OK. And by understanding this link a bit better, will it help us to predict earthquakes do you think?

Alex - Earthquake prediction is a slightly sticky subject. So we're very good at working out where earthquakes will happen and how big they will be, but we're not very good at all at working out when they will happen. In terms of trying to sort of reduce earthquake hazard this isn't a problem because if we can work out where the earthquakes will be and how big they will be, then that's all you need to be do to be able to build the buildings that won't fall down and kill people.