Gareth Pike asked:
Is it plausible to draw any kind of like between nuclear testing in countries like North Korea and earthquakes?
Connie Orbach pulled in Cambridge University's Dr Alex Copley for an answer to this shaky question...
Alex - In earthquakes, two big bits of rock are sliding past each other along a surface that we call an active fault. In an explosion whatís happening is the explosion is pushing against all the rocks around it. So both of these set up vibrations that travel through the earth. But because they involve different kinds of motion, in one case the sliding of rocks past each other, in the other case this pushing of rocks outwards, the vibrations that they set up are different - they have different characteristics.
Connie - Hmm - not completely the same. But still, could one cause the other?
Alex - Thatís a similar question to the sort of well known question of ďcan fracking generate earthquakes.Ē Because youíre doing a similar thing in both cases - youíre sort of changing the forces in the earth. Both of them can generate little earthquakes on their own, as to whether they can generate a big earthquake, the kind of size of earthquake which is going to damage building and things, that depends on whether there is already a prestressed and ready to go fault plain nearby
So if you set off a bomb next to a big fault which is already primed and ready to go, then the little bit of energy youíre putting in can tip it over the edge and you can generate a very big earthquake. However, the energy youíre putting in is not enough to generate a big earthquake on itís own. Unfortunately, we donít know much about where the big faults are and what the stress state on them is - how close and primed and ready to go they are. So this means itís difficult to know before it happens whether bombs or fracking are actually going to generate a big earthquake or not.
Connie - This isnít making me particularly secure. What about South Korea - is there any chance that their recent earthquakes could be linked to bomb testing in North Korea?
Alex - Thatís almost certainly a coincidence. There are earthquakes going off all over the place all the time. So, if you say youíre going to look at everything within a hundreds of kilometres radius, then itís not a surprise that there's an earthquake going on there - itís just a coincidence.
Connie - Well, there you have it. Nuclear bomb testing could lead to an earthquake but itís likely to be very, very small.