In September 2010, we had a magnitude 7.1 earthquake to the west of Christchurch City,
and since that time there's been a number of aftershocks
spreading both to the West and to the East
and we've had yet another one, but a very significant one,
a magnitude 6.3 in the Port Hills south of Christchurch.
We're estimating that about 10 kilometres of ground has broken
under the surface of the Port Hills and that's resulted in very strong shaking.
As you can imagine, its proximity to the city has caused major devastation.
What our instruments are picking up is where this rupture started,
and as it unfolds over the ensuing seconds,
our instruments recording those vibrations.
Because that ground is disturbed,
over the next few hours, days, and weeks
there's lots of smaller earthquakes along that rupture.
And that helps us to map out that line
that has broken in this particular earthquake.
When the fault is ruptured, shock waves spread out from that zone
and we detect two main sorts of energy waves,
the P (Primary) wave and the S (Secondary) wave.
The P wave itself has characterised as a jolt
to those who are living near the earthquake,
and the S waves are more side-to-side swaying motions.
Often the P wave is the one wakes people in the night,
and they subsequently feel the S wave.
the further you are away from an earthquake,
the greater the time between you feeling the P and then the S wave arriving.
In general it's the S wave, the side-to-side motion,
that causes damage to buildings,
they're not designed to be shaken from side to side.
Here's an example of one of our instruments,
this one is located at Oxford, northwest of Christchurch.
It measures three directions of shaking,
the top one is the vertical shaking,
and the bottom two are two perpendicular horizontal directions.
You can see the p-wave arriving here, just after the flat line,
and distinctly you can see the S wave arriving on either of these two traces here.
The energy changes quite markedly in character at this point.
If we zoom in on the P wave,
you can see clearly that the ground was at rest,
and then the energy started to roll in.
I can do the same for the S wave
which shows strong sideways shaking compared with what had gone on before.
There was some shaking from the P wave,
the S waves has now arrived and is shaking the ground sideways.
This is a map showing how much Christchurch shook.
We have many instruments throughout the central city,
and we've colour-coded the shaking at those instruments
to reflect the sort of damage we might expect.
Anything that's coloured red, we can expect structural damage,
and certainly structures that were really weakened by previous earthquakes have collapsed.
I'm now going to show you P and S waves
arriving at three different distances for this earthquake.
I'll start with the station that was closest to the epicenter
located in Heathcote Valley, just south of Christchurch city centre.
If I reduce the traces down a bit,
we can see a P wave arrival there,
an S wave arrival there,
and the time interval between those two is about one second.
Let's move south to Ashburton.
P there, and S there,
and that time distance is about 12 seconds.
And even further afield, we'll go to Wellington,
We have a P arrival there, a clear S arriving there
and that's about 35 seconds.
If you're right in Christchurch city centre,
we saw on the screen there was about one second,
it might as well be instantaneous to anybody who's feeling that earthquake,
you won't notice the difference between a P and an S.
We try and measure magnitude at each of the seismograph stations
and you can see the numbers here, bouncing around over six.
We average those over all of the stations to give you the final magnitude,
which for this is earthquake we estimated at 6.3.
The GeoNet homepage shows shaking intensity in the last minute anywhere in New Zealand.
So if you felt an earthquake, even before we've actually located it,
you can see the sort of impact it will have had on the country.
The Christchurch area lit up like a Christmas tree.
We could see instantaneously that there was likelihood of damage having occurred from that.
If we now take a wider view
of how this earthquake has impacted the rest of Canterbury,
I can zoom out of this map,
and as we show more of the South Island,
the yellow zone largely reflects an area
where people might have had contents damage, things come off shelves.
And the blue and the green colours are where people have felt the earthquake,
but we're not expecting any real impact at all.
This earthquake has occurred at about five kilometres below the surface,
and that's meant the energy waves have not had very far to travel
before they strike the surface and cause devastation.
As you can see from this map here, the zone of severe damage is quite localised,
but within that zone, it was intense.