I'm John Ristau, I'm a seismologist with GNS Science and I work as part of the GeoNet team.
I'm one of the duty seismologists for GeoNet.
And I'm going to give you a demonstration of how we locate an earthquake,
how we delegate the magnitude of it and how we put this information out there for everyone.
This is an example of what an earthquake looks like when we record it.
These top six traces here are all coming from one station
and when we have an earthquake we get numerous traces coming from all across the country.
This is an animation that's going to show how we actually record an earthquake.
We have an example of three seismic stations
and these are linked to the data centre by satellite
and this is going to show the traces, the seismic records
as they come in when the earthquake happens.
When the earthquake happens, these two different lines
are going to represent the P waves which is the yellow line,
it's always the fastest wave, it's always the first one to arrive,
and the S wave which is the orange line, that's always the second wave to arrive.
You can see when we're close to the epicenter of the earthquake, where it originated,
the P waves and the S waves are very close to one another.
As they travel along, the separation between the the P and the S waves gets greater and greater,
so there's a clear distinction between the P waves and the S waves.
We now see the S waves have started to arrive at the second station
and there is a greater time separation between the P and the S waves
so you tell that the second station is further away from the earthquake than the first station was.
If we continue this long a little further
we now see the P waves starting to come in at the furthest station
and then finally we now see the S waves arriving at the furthest station.
You can also see the amplitudes of the waves is getting less and less with each station
because the the seismic energy spreads out the further you get from the epicentre of the earthquake
and therefore the amplitudes get smaller and smaller.
This animation is going to show how we can get the location of the earthquake.
Once we've determined how far the earthquake is from a particular station,
we can basically draw a circle around the station
because we know how far it is from the station but we don't know in what direction.
and it could be anywhere on that circle.
Now if you get that same information from a second station you draw a circle around it,
and these two circles will intersect at two points
and so we know that the earthquake has to be located at one of those two spots.
So if we get that same information from a third station
you can see that the three circles will intersect at only one location.
And that's where the earthquake is.
In actual practice there's going to be a bit of error on getting the distances from each station,
and so the circles won't nicely overlap at one single point.
But if you do this with ten or twelve stations
you can get a very well-defined location for the earthquake
within maybe two or three kilometres.
We can then extend this to three dimensions, so not only can we get the location on the surface
but we can actually tell how deep the earthquake was,
whether it was five kilometres deep, 20 kilometres deep, 100 kilometres deep.
The very last step then, is to get the magnitude of the earthquake
and the magnitude depends on the height of the signals that we receive at the different stations.
There's an equation that factors in both the height, or amplitude of the signal
and the distance from the station.
We do that for a number of different stations,
and then average the results to get the magnitude of the earthquake.