Seismology of the Christchurch earthquake

I’m sure that many of you, like me, have been shocked by yesterday’s tragedy in New Zealand. Christchurch is a beautiful city, which I’ve been fortunate enough to visit twice in the past year – my thoughts are with all those affected, especially those who’ve lost loved ones or are still trapped in the rubble.

Unfortunately, science still can’t predict when earthquakes are going to strike. But we do know why they happen, and we learn more each time.

Map of the Earth, showing the tectonic plates (click to embiggen)
Map of the Earth's tectonic plates. Notice New Zealand is right where the Pacific and Australian plates collide. (Image by USGS, via Wikimedia Commons)
As shown in the map above, New Zealand sits on the junction between the Pacific and Australian tectonic plates. The Pacific plate is actually subducted, meaning that is being pushed under the Australian plate, which is buckling as a result and forming New Zealand’s spectacular mountain ranges (for a good description of what’s happening here, see Greg Laden’s Blog).

Wednesday’s 6.3 magnitude earthquake, as has been widely reported, was technically an aftershock of the 7.1 magnitude quake last September. The fault line that’s caused these earthquakes was unknown before last September, due to be being buried under sediment and not having moved for thousands of years.

As described by Oregon State University’s Professor Robert Yeats, in this interview in Scientific American:

As plates move, they’re building up strain, and I estimate how much strain can build up before there is a rupture. New Zealanders have been good about trenching faults (digging trenches along fault lines to study previous seismic activity). The problem with this fault was that they didn’t even know it was there. That tells me it’s a pretty slow-moving fault but nonetheless when it builds up toward an earthquake of magnitude 7, then that’s going to continue to produce aftershocks for a long time. It’s not an exact science.

In this case, there have been nearly 5000 aftershocks since 4 September 2010. You can get a good idea of their extent from this animated earthquake map:

Screenshot of the Christchurch Quake Map (click to see animation)

If you prefer, here’s a video of a seismologist discussing the aftershocks.

To read more expert analysis, see the New Zealand Science Media Centre, or the excellent geology blogs Eruptions and Highly Allochthonous.

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