Can humans cause earthquakes?

We put Moosa's question to planetary expert David Rothery...

David - Yes, with qualifications. It depends what Moosa has in mind when he says an earthquake. I mean, the vibrations within the Earth have different degrees of intensity and people will have heard of the Richter scale. There are 100 or so earthquakes of magnitude 6 or above somewhere in the world every year like the one in Italy a couple of weeks ago. There are millions of magnitude 2 earthquakes. How small can you go and still call it an earthquake? Coal mines collapsing cause vibrations which can be detected, sometimes felt by people really close by. Is that an earthquake?

Kat - I mean presumably, something like a massive bomb going off as well would cause the Earth to shake.

David - That cause vibrations through the Earth which you can measure at a certain distance and workout the energy at the source. Fracking is supposed to cause earthquakes but there's a debate about whether it's causing an earthquake that wouldn't have happened anyway. If you frack close to a fault, you may give the impetus to let that fault move and the earthquake will happen now rather in a hundred years' time. Is that caused by humans or just triggered by humans and is there a difference there? It depends on what size of earthquake Moosa has in mind and whether he means cause something which would never have happened or just trigger a natural event which was going to happen anyway if you'd waited long enough. Like many questions in science, a great question but very complicated to answer.