Tuesday, 20 August 2013

Some other shaking...

I'm gonna take this "opportunity" to shift my focus for this post. Well in line with my blog title we're still talking about shaking, but this time it's not volcano-related. Instead, we're going back to one of my awesome previous homes: Wellington! You Wellingtonians out there will, of course, immediately know what I'm talking about. And those of you outside the coolest little capital in the world should have heard about it by now: Wellington is shaking A LOT!
It all started on Jul 21, when a magnitude 6.5 hit the city late in the afternoon. Centred in the Cook Strait at 13 km depth it thankfully did not result in any casualties or major damage. From GeoNet's shake map below we know that it must have felt most intense in the region around Blenheim, which is a bit closer to the epicentre (the point at the surface of the Earth that is closest to the source of the quake) than Wellington.
Source: GeoNet
The earthquake was followed by a bunch of aftershocks, which is normal after an event like this. According to the Gutenberg-Richter law for every earthquake of magnitude X in one region we expect roughly 10 earthquakes of magnitude X-1, 100 of magnitude X-2 and so on. In other words, and to make things a bit easier to understand,  say the Cook Strait earthquake was a magnitude 6, we expect 10 magnitude 5, 100 magnitude 4, 1000 magnitude 3 earthquakes following the main shock. This is what we call a mainshock-aftershock sequence. The timing of these aftershocks depends, but in general there is a lot of aftershocks in the first few hours and days, and they will become fewer and fewer over time. Here's an animation showing the first few hours of earthquake activity before and after the main shock.
Now if everything was perfectly simple we could stop right here. But things are slightly more complicated: There are also sequences of earthquakes where a bunch of smaller events lead up to a bigger one. The smaller ones are sometimes referred to as foreshocks, because they happen before a larger event. This is exactly what happened in the Italian town of L'Aquila in 2009, when a magnitude 6.3 struck after a few weeks of smaller earthquakes. There was a lot of media coverage on this tragic event and the trial that some of involved scientists had to go through, but to talk about this would be a new post. Instead I will sum up the essence: We can (and should!) advise people to act more cautious under certain circumstances. However, we cannot predict whether a larger earthquake will follow a sequence of smaller ones. There are probabilities for earthquake sequences to develop in one way or another. These probabilities are e.g. based on records of previous earthquake sequences for any given region. But again, how these probabilities should be communicated with the local people is not really that clear. There is a definite need for improvement! (see also my previous post about communication)

To get back on topic: Wellington. So why are there earthquakes in the first place? Well, the situation is again rather complicated. In New Zealand we're really lucky (or unlucky, from a non-geoscience perspective...): North and east of the North Island, the Pacific Plate in the East dives (or subducts) under the Australian Plate in the West. Southwest of the South Island, on the other hand, it's the other way round: The Australian Plate dives under the Pacific Plate. In between those two subduction zones, there is a large transform fault, which is called the "Alpine Fault". In this area, the two plates slide past each other sideways. Here is an image showing a transform fault:

Source: http://supercronopio.es.ucl.ac.uk/~crlb/
 How exactly does the transition from one to the other work? Nobody really knows... that's why people like me love New Zealand, it's like a natural laboratory with lots of things that are yet to be discovered! :) So in Wellington we're just north of the region where we go from subduction to sliding sideways. The Pacific Plate is sinking into the mantle, and we sit on the Australian Plate riding on top of that process. The Cook Strait earthquake, however, was in the sideways sliding zone, an area with lots of little transform faults that eventually all merge into the Alpine Fault in the South. Such an earthquake with side-by-side motion is called strike-slip earthquake. These earthquakes are (usually) not as strong as the subduction zone earthquakes. Examples for large subduction zone earthquakes would be the Japanese "Tohoku" earthquake in 2011, or the Sumatra-Andaman "Boxing Day" earthquake in 2004. Yet, strike-slip earthquakes can be quite damaging! A recent example for an earthquake that was partly strike-slip would be the 2011 Christchurch earthquake ("only" magnitude 6.3, but a lot of damage and many casualties!). Wellington was a bit luckier than Christchurch, partly because it was further away from the epicentre, partly because the event in Wellington was deeper (13 km vs. 5 km for Christchurch), and for a number of other potential reasons. 
But... this doesn't mean that Wellingtonians get to lean back and rest assured that nothing will ever happen to them! First of all, based on the aftershock law that I mentioned above, Wellington - just as Christchurch and any other place that has experienced large earthquakes - will continue to get aftershocks for the next year or more. Second, there is always the possibility of similarly large or even larger earthquakes following, on the same fault, or on other faults nearby. Third, there is still the subduction zone that wasn't too much involved with this particular event, so it's going to be quite interesting to see some studies on the probability of subduction earthquakes based on this most recent event. So, as harsh as it sounds, if you're a bit faint hearted or are simply looking for a quiet place to live then you might want to consider a place different from Wellington! Bear in mind though, a lot of places including my lovely current home Vancouver may also have a high chance of getting potentially large and destructive earthquakes. I strongly suggest that everyone living in places like this have an emergency kit packed and ready to go. The NZ campaign "Get ready - Get thru" has some good advice on what to put in such a kit, how to prepare for an earthquake, and how to act during and after. In Canada, the BC government for example provides similar information. Both websites also have a lot of info on other natural disaster that might happen in the area. To end - in all seriousness - a little line from Pineapple Express:

Well be careful, man. Be careful. Wear shoes in the house. Safety. Safety first, then teamwork.

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