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The Canterbury Plains earthquake

Updated Friday, 8th October 2010

A 7.0 magnitude earthquake hit near the Canterbury Plains in New Zealand on September 4th, 2010. David Rothery answers some of the questions raised by the quake.

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David Rothery

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 Canterbury earthquake happened at a depth of about 5km which counts as shallow, so it did do a lot of shaking of the ground so it had the potential to kill a lot of people.  In fact it was very, very similar in size and shallow depth to the magnitude 7 earthquake that hit Haiti earlier in the same year which killed 200,000 people.  So, it’s quite remarkable if you think about it that in the New Zealand case nobody was actually killed. 

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 Canterbury earthquake of September 2010 and you’ll see that there are many other earthquakes on this map.  The orange dots are the shallow ones like the Canterbury earthquake.  The green and blue dots nearer the top are deeper earthquakes and the nature of the plate boundary changes up at the north end of the South Island and this line here is the plate boundary and most of the earthquakes quite naturally are close to a plate boundary.  There are some earthquakes out to sea here further from the plate boundary and the Canterbury earthquake is on the fringe of the main earthquake zone of New Zealand but it’s not unknown for there to be earthquakes there.  And the reason you get earthquakes away from the main plate boundaries, there are minor faults which split off the plate boundary, in continental crust a plate boundary is never a single line, there is a minor fault splitting off here that’s mapped.  There is a much smaller minor fault underneath the Canterbury area where the earthquake happened that let rip just a few weeks ago.

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 New Zealand dollars to repair the damage, that’s to repair the buildings, to renew the infrastructure, sewage supplies have been damaged, water supplies have been damaged.  There are some spectacular pictures of railway lines being bent completely out of shape.  It will cost a lot of money to put this right but it would have been far, far worse if they’d not spent enough money making the buildings safe so people at least didn’t die when the buildings were damaged.

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 Pacific Ocean, and cause a lot of damage on distant shorelines.  Now, for a tsunami to happen, the earthquake really has to happen underneath the sea because you need to displace seawater to cause the tsunami waves.  Now, the epicentre for this earthquake was a fair way in land, it was 30, 40 kilometres away from the coast so although it shook the ground very violently it didn’t displace seawater and no tsunami ensued.

Title: Why can’t we predict earthquakes?

Nobody knew that this earthquake was going to happen.  They knew in New Zealand they were at risk of earthquakes.  They’d designed buildings and other structures to withstand earthquakes but we can’t tell still where and when an earthquake is going to happen.  You can tell that if an earthquake has happened on this part of the fault then maybe the strain is now concentrated further along the fault and that’s where the next earthquake is going to happen, but you can’t tell when it’s going to happen.  But at least we can prepare and make sure that people know how to behave in an earthquake which they do in New Zealand and at least we can build buildings that will withstand earthquakes and not kill people during an earthquake, and I think the New Zealanders have handled this fairly well.

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.

7’10”

 

Find out more

BBC News on the earthquake

Want to understand the forces at the heart of our planet? Consider the Volcanoes, Earthquakes & Tsunamis course.

 

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