Be Prepared Month: What are the odds?

Our ‘Be Prepared Month’ project is all about helping you get yourselves ready for anything. It might be that you’ll never need your emergency kit or plan; alternatively it might be the smartest thing you ever did.

So what are the chances of us needing to break out the emergency kits? It’s a question that might have been considered hypothetical just a couple of years ago; of course since then we’ve learned that big disasters are not just events that happen to other people.

This particular blog is running the risk of becoming novel-sized, so your author is breaking this up into two parts. Today, we take a close look at our most recent nemesis and ask, is the earth really becoming more violent?


We know too well the damage earthquakes inflict. The 2011 Canterbury quake claimed 185 lives, trashed our second largest city and, for a time, brought the entire country to a standstill. And yet it wasn’t the deadliest earthquake we’ve had; 256 people were lost in the magnitude 7.8 that destroyed Napier and Hastings in 1931.

RiskScape, a joint venture between GNS Science and NIWA, rates New Zealand’s seismic activity as varying “regionally from moderate to very high”. It certainly feels like our corner of the earth is getting more peeved recently, but what do the numbers say?

The GeoNet website lets you construct your own graphs and maps based on the info you plug in. It’s interesting fun: tell their programme what you want to know and let it do the hard work. I decided to have a play with it on 17 July, hence the cutting off of each year on the 16th.  (If we can have midwinter Christmas, why not midwinter New Year too?)

EQNZ earthquakes

So nothing out of the ordinary really, other than that big spike in 2010-11 and… wait a minute, why is the figure so low for the year just gone?? This time I asked a real person at GeoNet and her prompt response was they’re still processing all those smaller quakes, so the number should rise a bit. Fair enough.

She also sent this useful link. If you’re looking for an earthquake rule of thumb, it goes like this: expect a magnitude 4-4.9 every day, two 5-5.9 a month, two 6-6.9 a year and one 7-7.9 every three years.

So we’re not necessarily having more quakes – in fact after last year we’re probably having fewer. Are they getting any more violent? Well, just a little.

The median magnitude rose from 2.31 three years ago to 2.65 last year – a number still barely noticeable to most. (Quick stats note: I consider the median more representative because, unlike the average/mean, it is less vulnerable to being influenced by wacky outliers.)

So what about all those recent biggies? The 7.1 that hit Christchurch in September 2010 wasn’t actually the biggest we’ve seen in the past decade:

What made it, and February’s 6.3, more destructive was their location. We have seen six quakes measuring seven or more since 2001 – but they were usually out at sea, very deep, or in sparsely populated areas. In this regard, as bad as things have been, they could have been even worse. After all, that 7.8 that hit Fiordland in 2009 was the same magnitude as the Hawkes Bay quake of 1931.

It would be fair to say the earth is about as angry round these parts as it’s ever been – maybe just a little more so in the last few years. Earthquakes, in short, are what they’ve always been – a very real possibility, very hard to predict (even by harnessing the power of the moon and the stars), and definitely something to be prepared for.

Having nearly exhausted my quota of pretty pictures and graphs for today, we’ll explore the next few sections next week. Try not to have any tsunamis, floods, or volcano eruptions in the meantime ok? Or if you do, be prepared first!

UPDATE: Of course: there was one very important factor in Christchurch’s February quake your author forgot. As Andrew commented:

You’re forgetting ground acceleration. It wasn’t just the proximity to the centre of Christchurch of the Feb 22nd quake, it was the ground acceleration.

The 2.2 Gs of vertical earth acceleration EQUALED THE WORLD RECORD for recorded violent ground movement during earthquakes.

Yes indeed. The vertical acceleration far exceeded the horizontal, which caused, from accounts, buildings and people to be ‘tossed up’ into the air.

The 2.2g figure, by the way, means the ground moved at 2.2 times the acceleration of gravity. For perspective compare this to the September 2010 quake, which peaked at 1.26g, and the 7.0 that devastated Haiti, which was estimated at 0.5g, and you can see this was clearly no ordinary 6.3. Thanks Andrew for steering us in the right direction!

UPDATE 2: Here’s a cheap plug for part two, which is online now.

5 thoughts on “Be Prepared Month: What are the odds?

  1. You’re forgetting ground acceleration. It wasn’t just the proximity to the centre of Christchurch of the Feb 22nd quake, it was the ground acceleration.

    The 2.2 Gs of vertical earth acceleration EQUALED THE WORLD RECORD for recorded violent ground movement during earthquakes.

    Talk to Dr. Quigley at Canterbury University. He will probably give you more information to help your understanding of the Canterbury sequence of quakes as the research continues.

    Nice post, all the same 😉

  2. Pingback: Be Prepared Month: What are the odds part two (or “More crystal ball gazing”) | Open2view

  3. Pingback: Be Prepared Month: That’s a wrap (up) | Open2view

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