00:00Let's talk next to Amy Gilligan, a seismologist at the University of Aberdeen.
00:06Amy, welcome. Why have these two earthquakes been so damaging?
00:11Is it their depth or their shallowness, their location, or what?
00:17The combination of all of these things, they're very powerful earthquakes over magnitude 7,
00:22which means a lot of energy has been released, but they're also very shallow.
00:25So the magnitude 7.5, the depth of that has been estimated to only be 10 kilometres,
00:30as that means that energy is getting released close to the surface and means that it can cause lots of
00:35destruction.
00:36It obviously depends also on where you are in relation to population centres.
00:40A same magnitude earthquake somewhere where it's very remotely populated wouldn't have the same impact.
00:47So it's a combination of its location in terms of where people live and the depth
00:52and just the power of the energy that's being released from this size of earthquake.
00:57These two earthquakes are 39 seconds apart.
01:01How unusual is it to see two earthquakes of this size almost back to back?
01:08Yes, it's a very unusual thing.
01:10It's what we call a doublet, which means it's two earthquakes of a similar magnitude
01:14where they occur quite close in time and in space.
01:17And it probably means that they are interacting, the faults inside of the Earth are interacting with one another.
01:23So probably the earlier one, we can describe that as a foreshock,
01:28that then the stresses inside of the Earth really are causing the 7.5 to have happened.
01:36And so they're probably related to one another.
01:38So it's quite unusual, although in Venezuela there was a doublet of a magnitude 6.2 and a 6.3
01:46occurring in September last year.
01:47So it's not unknown.
01:48And as we saw in Turkey a few years ago,
01:51we can get other magnitude 7 earthquakes occurring quite close after each other as well.
01:56So it's unusual and it's obviously particularly devastating to have two such large earthquakes occurring so close together.
02:03What is happening beneath the Earth's surface to produce two earthquakes like this?
02:10So in this setting is what we call a strike-slip earthquake.
02:15So these are where two tectonic plates are rubbing against each other vertically.
02:20So it's moving side by side.
02:23The stresses will build up over time and suddenly the stresses will be so big that they will slip.
02:30And so when one fault has slipped, that might then mean that the stresses on another fault are so big
02:37that that also causes that to slip and move.
02:41So we can tell that this earthquake was one of those kind of fault movements where they're moving side by
02:47side.
02:49Amy, I don't know if you could hear our correspondent Alistair D'Souza talking about the U.S. geological survey,
02:56suggesting there is this very high probability now of at least one magnitude 5 aftershock within the coming week.
03:05Is that always a given? Does that always happen?
03:08Yeah, it's a very likely thing.
03:10After earthquakes happen, we expect to see aftershocks.
03:13And with an earthquake this big, we would expect to see a magnitude 5.
03:17I think the USGS are estimating there's maybe a 25% probability of magnitude 6 and over 80% probability
03:26of a magnitude 5
03:27and smaller earthquakes that might happen.
03:29And obviously for the people living in Venezuela who are already dealing with the damage that have been caused by
03:35this earthquake,
03:36that's going to be something they need to be mindful of in the coming days and weeks.
03:42These earthquakes happened along the boundary between the Caribbean and South American tectonic plates.
03:50Can you explain to us why this region is particularly prone to powerful earthquakes?
03:58Globally, we mainly see earthquakes happening on tectonic plate boundaries.
04:02And as you said, this is where we've got the Caribbean plate moving alongside the South American plate.
04:08So here we see the Caribbean plate moving eastward relative to the South American plate at about 20 millimetres per
04:16year.
04:17And it's that movement over time that's going to cause the stresses to build up and mean that there are
04:23big earthquakes.
04:25So this is one of the biggest that we've seen in Venezuela for a number of for a long time.
04:30It's probably the most devastating, perhaps since the one in 1967, which is a magnitude 6.5.
04:36And there haven't been that many of this size of earthquakes in Venezuela for when we've had instrumental records.
04:44However, it's not unknown.
04:45And because it is on that plate boundary, we would expect there to be large earthquakes like we have seen,
04:51but rarely.
04:52And it's, yeah, it's something that we do expect because of that closeness to a tectonic plate boundary here.
05:00I wonder, can any human endeavour ever have prepared for anything like this in terms of building better, building more
05:09resilient buildings?
05:10Or is this just so enormous there's nothing that could have been done?
05:15So we can't predict earthquakes.
05:17We don't know when they're going to happen.
05:19Steps, like you say, building buildings that are able to withstand certain amounts of shaking is something that governments can
05:27mandate globally.
05:30Making sure you've got the infrastructure in place for populations when an earthquake does happen.
05:35So things like making sure you have emergency response systems in place, adequate hospital, water and emergency preparedness facilities, and
05:44also people knowing what to do in the event of an earthquake.
05:48So earthquake drills are something that happen around the world, a bit like fire drills, so that people know what
05:53to do when an earthquake happens.
05:55And so that preparedness is something that we can try and do, as well as making sure we've got buildings
06:01that are, as to the best as we can, try and withstand some of the shaking that occurs when a
06:07big earthquake happens.
06:09Amy, good to talk to you, and thank you so much for your time and coming on the programme.
06:12Amy Gilligan, the seismologist from the University of Aberdeen.
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