EARTHQUAKE!!!

There was an earthquake last night, centered down south about 100km out of Tuatapere. It was apparently felt as far away as the Taranaki region, which isn't surprising considering it measured 7.8 on the Richter scale. We definitely felt it here in Christchurch, it was similar to most earthquakes I've felt in recent times, a rolling one like a wave rather than a violent shaking. The main difference was how long it lasted - it went on for ages! Apparently it was shaking for around 2 minutes.

Lucky for you I did a geology paper last semester so I know a little bit about earthquakes. I'll share some details I know.

Plate tectonics
New Zealand is situated on the boundary where the australian plate and the pacific plate meet. They are constantly moving (to the tune of a few mm every year). What makes it more interesting is the fact that above the north island the pacific plate is subducting under the australian plate, where as below the south island the australian is subducting under the pacific. Add to this the Strike Slip characteristics of the Alpine fault (plates moving past each other, one moving north, other moving south) and you have a country prone to earthquakes.

Magnitude vs Intensity
Last night's earthquake measured 7.8 on the Richter scale. But what does that mean? The main thing to consider is the difference between magnitude and intensity.
The Richter scale measures the magnitude, that is how much energy is released. However, this doesn't necessarily correlate to what is felt.

The modified Mercalli scale measures intensity, that is how damaging a quake is (such as the shaking, or destructiveness of it... pretty much what you feel). It has a scale from 1-12, where 1 is barely felt and 12 is large scale damage and destruction. So what people in the deep south felt may have been about 8 on the modified Mercalli scale, where as here in Chch it might have been like 5 or 6 and up north maybe it was barely felt at a 2 or 3... It will depend on the depth of the quake, geology of the area, distance from the epicenter, to some extent the magnitude, etc, etc.

Final facts?
An earthquake like last night occurs when the plates which have been grinding and pushing against each other, building up energy, then all of a sudden something gives way, releasing the built up energy in an earthquake. This happens all the time with energy generally being released in small earthquakes that we often don't notice. However, with energy building up on the plate boundaries, if the movement is stopped (ie the plates get caught or wedged or something and they can't move and release energy a bit at a time), the energy keeps building and building until something gives and the built up energy is released in a bigger earthquake.

I hope you have learned a lot from this 3 minutes of earthquake facts. I also hope I got an A for Geology. Still waiting to hear my results...