Question: How many earthquakes (approx) has there been in History and how do the tectonic plates move back into position?

Tim Craig answered on 11 Jun 2018:

Two great questions here.

‘How many’ earthquake there have been is a difficult one to answer – our observational record is improving all the time, which allows us to detect smaller and smaller earthquakes. Now, if there is a M4 earthquake pretty much anywhere in the world, we know about it. But if we go back even 30 years, that wasn’t the case, and 100 or more years ago, there could have been even quite large earthquakes that might not have been recorded.

In general, across the entire world, there is usually about one ‘Great earthquake’, which is above a magnitude 8, every year or so, roughly one magnitude 7+ every month, and a magnitude 6 every other day or so. But earthquakes also don’t necessarily occur at a constant rate through time – sometimes we have more, sometimes we have less – so these numbers are only guidelines.

Earthquakes occur as a result of tectonic plates moving past each other. A magnitude 6ish earthquake typically takes up a few metres of motion between plates. But the plates don’t move back afterwards – they just keep on moving along (very slowly – millimetres per year, usually). This is the process that results in the surface of our planet changing, and landmasses moving around, and in the formation of features like mountain belts.