There’s recently been a large earthquake on the South Island of New Zealand near the city of Christchurch but west of it on a place called the Canterbury Plains, and people seem to be referring to it as the Canterbury earthquake so that’s what I’ll do. Now, it was magnitude 7 on the Richter scale and there are about 20 earthquakes in an average year of magnitude 7 somewhere in the world, so not very often in New Zealand though they are used to earthquakes in New Zealand, and 7 is quite big. Magnitude 8 is big, there’s only one of those every year and a magnitude 8 earthquake really is bad. Now for a magnitude 7 earthquake to do a lot of damage it has to be quite close to the surface. Now, the
If you live in New Zealand you’ll have felt many earthquakes in your life as like as not and the one that happened recently on the Canterbury Plains was where there was a four metre displacement along about a 20 kilometre length of fault that moved, and this fault was associated with being very close to the boundary between two of the earth’s tectonic plates, where one plate in this case is moving sideways against another plate. Now, plate movement doesn’t happen gradually, it happens in jerks. The strain builds up and up and up until it suddenly gives way and that’s what happened, that a fault gave way, there was a jerk and it shook the ground quite violently for about a minute.
Here’s a close up map of the South Island of New Zealand. Here is the site of the
Title: Is it only along the rupture that damage is done?
Damage doesn’t just happen directly above where the earthquake begins or directly along the line of the fault that gives way. The ground shakes for a radius of many tens of kilometres away from the epicentre and the shaking gets less the further away you go.
Title: What can be done to make buildings safe?
It’s well known what you have to do to make buildings safe so that they don’t fall down and kill people during an earthquake. It costs a bit more, maybe ten or so percent extra on the cost of building something that will stand up to an earthquake, and the most important thing is that when the walls shake during an earthquake, they shake in the same direction. If they shake in opposite directions the roof can fall down, and that’s the most common way of people dying in an earthquake, the structure that they’re inside just collapses on them. It happened in Haiti, a big earthquake in January 2010, lots of buildings were shaken down and just completely telescoped down upon themselves in an earthquake that was no bigger than the one that’s recently happened in New Zealand, but in New Zealand they’d spent a bit extra, they’d obeyed the seismic building codes and the walls and the roofs swayed in the same direction and the walls and roofs were tied together so that the roofs couldn’t shake loose and just telescope down. And although we see a lot of masonry that’s fallen out onto the streets, the frameworks of the buildings survived; the people inside survived. If there had been lots of passers-by outside on the street, the falling masonry, the bricks and so on that fell down would have injured quite a few people, but it happened at 4:30 in the morning, people were in bed, there weren’t people on the streets by and large, so fortunately nobody was actually killed in this earthquake.
Title: So no one was killed and few were badly injured, how much damage was done?
Now, although no buildings fell down a lot were quite severely damaged and it’s not yet clear whether they are so badly damaged they will need to be demolished and rebuilt or whether they can be reconstructed. I’ve seen estimates of about two billion
Title: We hear a lot about earthquakes causing tsunamis. Why didn’t this one?
Now, when this earthquake happened people’s immediate fear was, is it going to cause a tsunami, because a tsunami can travel all the way across the
Title: Why can’t we predict earthquakes?
Nobody knew that this earthquake was going to happen. They knew in
Well, I’ve been talking about one earthquake in particular because it’s a recent headline-making event. If you want to find out more about earthquakes or indeed volcanoes and tsunamis because they’re all related, we have an Open University short course which I chair at an introductory level which is a great way to find out more about this amazing natural phenomenon. It’s called, unsurprisingly, Volcanoes, Earthquakes and Tsunamis.
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Want to understand the forces at the heart of our planet? Consider the Volcanoes, Earthquakes & Tsunamis course.