The science of shaking

What happens under the ground in an earthquake?
23 March 2021

Interview with 

Lisa McNeill, University of Southampton


Dry, cracked soil


When the ground starts to tremble, what’s going on below? Adam Murphy’s been finding out from the University of Southampton's Lisa McNeill…

Adam - Earthquakes can be absolutely terrifying events, showcasing that we are very small parts of a very big planet. The ground under our feet may seem solid, but it is split into loads of plates that gradually move around like the American plate and the Eurasian plate. But the planet isn't getting any bigger, so there's not a lot of room, and that means plates can crash into each other or scrape past one another along fault lines, but they don't move quickly. And a lot of stress can build up.

Lisa - The stresses build up on that fault plane and they build up over hundreds of thousands of years. And then when they build up a certain amount of stress that will get released rapidly over seconds and minutes. And that's what is the earthquake. It's that movement on the fault.

Adam - That's Lisa McNeill, professor of tectonics from the University of Southampton. Some earthquakes are bigger than others, and in a lot of places you would measure it with the Richter scale - where an increase of one on the Richter scale means an earthquake that is 10 times stronger. The 2011 Tohoku earthquake was a nine on the Richter scale, but it's not the only scale that's used.

Lisa - But in science, we actually use a different scale called the moment magnitude scale. And that's more scientific, it's better at measuring particularly the earthquakes at the larger end of the magnitude scale as well. And what that actually measures is the area of the fault that moves. And then the amount of slip that occurs during that fault movement as well.

Adam - And while the magnitude is important, it's not the only thing to consider.

Lisa - If you've got an earthquake that's being generated or fault under the sea, you potentially might generate a tsunami. The depth of the earthquake makes a difference. Usually the shallower the earthquake, the more impact it has on the surface and the human population. And also there are different types of faults that move in different ways. They tend to sort of generate some different scenarios as well. So there's lots of different types of earthquake. The primary factors are going to be how big a piece of fault moved and how much did it move? So that's what constitutes the magnitude. And the larger the earthquake, generally the larger the impact it's going to have.

Adam - When we talk about that 2011 Tohoku earthquake, you can't forget about the tsunami that followed. A giant wave crashing into the coast. It's what caused the problems with the Fukushima reactors. So where did they come from?

Lisa - A tsunami is where you're basically moving the sea floor in a certain way so that the water column above it moves. And that's what generates the tidal wave, the tsunami. So for a fault moving in an earthquake, it has to be above a certain magnitude to have a likelihood of generating a tsunami. And usually people sort of quote around a magnitude seven or seven and a half, something like that. And you do have to sort of have some kind of vertical motion of the sea floor. And that ground, that sea floor will have built up strain before the earthquake. And then it all gets released. And the whole sea floor moves, which basically moves the entire water column above that area. So in some of these very large earthquakes that tend to happen on subduction zones, which is where one tectonic plate goes underneath another, so these are where the largest earthquakes occur. So Tohoku-oki in Japan in 2011, is an example of that. In some of these cases, the area of the fault that's moving is hundreds by thousands of kilometres, or maybe sort of a hundred by hundreds by a thousand kilometres, something of that sort. So it's a really big area. So not surprisingly, you know, that does have an impact in terms of movement of the water column.


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