An Earthquake through the Night

Got woken just after 3am by a 5.3M earthquake last night. It woke Daniel also and he started crying. Knowing I wouldn’t fall asleep again straightaway (heart thumping), I went online to Geonet to see what size it was. They took ages and ages to upload this time so I checked out twitter and discovered most of Christchurch was also online and waiting for the magnitude to be broadcast. Something interesting though, most of the tweeters were guessing it was around magnitude 5 and on the Greendale fault. I thought the same.

So how do we know?

Well where earthquakes are concerned, proximity to epicentre is everything and you can get an idea of this by how long the shaking lasts. Big earthquakes last longer.

From my own experience here:
4M < 10 seconds
5M 10 – 20 seconds
6-7M 20 – 40 seconds

I read that Japan’s 9M earthquake on March 11 2011 lasted 3 – 5 minutes. I hope never to feel anything like that.

The 5.3M last night was of reasonable duration and was a strong rolling motion but it didn’t shake things off shelves which suggested to me that it was further away. The 4M we had last week was short and caused the glasses in the kitchen to clink and sloshed some water out of the dog’s bowl but the 5M last night did neither, yet I still knew it was bigger because it lasted for longer.

The richter magnitude scale is logarithmic so it may sound like there’s little difference between a 5M and 6M, but all other things being equal, you can expect 10 times more shaking in a 6M than a 5M and 100 times more shaking in a 7M compared to a 5M.

The worst combination for an earthquake is big, shallow and with an epicentre close to a populated area. Wellington straddles the plate boundary of the Pacific and Australian tectonic plates. I read that the Wellington fault, which runs right beneath the city, last produced an earthquake of magnitude 7.6. When it next ruptures it is expected to cause 4-6m of sideways movement as well as some vertical displacement. How do Wellingtonians sleep at night!

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