I'm a seismologist at GNS Science. I use computer models to
simulate seismic waves from an earthquake
that propagate through that crust of New Zealand.
When an earthquake occurs, GeoNet's seismometer stations record its wave forms.
I use this information to characterize the earthquake source
in my computer model.
Using 100 computers working together, it takes about one hour to make this calculation.
I'm going to show you one of my models
on the computer screen here, these are the active faults
in New Zealand, and this is a Plate Boundary Fault.
I'm going to show you a movie of seismic wave propagation
that originated from an Eketahuna earthquake
that's the epicentre of the main shock.
So I'm going to stop here first,
and colour corresponds to up-and-down motion -
how fast the ground moves vertically.
And the outer circle corresponds to
the compressional waves or P waves, and this circle corresponds to
what is known as S waves or shear waves
and you see this stronger ring that is emerging right now
and that's the surface waves. surface waves are guided along the surface of the Earth,
they die out less quickly
than the body waves or P and S waves.
so you see that the amplitude of body waves or P and S waves
are already small, as shown in the lighter colours,
whereas, the amplitude surface waves is really dominant at this a snapshot.
I'm going to run the movie a bit further now.
At this stage, the wave captures this the complex three-dimensional geology
of the North Island of New Zealand, but in particular
in this region, the wave motion
gets amplified and so you see a longer duration
of the motions here and there.
These are caused by the presence of shallow sedimentary rocks.
When the wave encounters shallow sedimentary rocks, the motion gets amplified,
and so it is represented by this stronger color
as opposed to the surrounding region.
I'm going to just let it run for a while now.
so you see that even after 3 minutes after an earthquake
there are still prolonged vertical ground motions that
are appearing in the North East of the North Island.
So this is a different number two, this is a Cook Strait earthquake.
It occured about 50km south of Wellington.
I'm going to run a movie again for this earthquake.
I will stop here first.
Again this outer circle corresponds to P waves,
and S waves begin to emerge, so this inner strong circle is an S wave,
but the surface wave hasn't emerged yet
because it takes some distance for surface waves to develop.
So this P wave
first reached Wellington in about 0.3 minutes,
which is about about 20 seconds after the beginning of this earthquake.
I'm going to run the movie a little bit more.
P waves travel faster, and S waves travel slower
and so this is why there's a gap between these two.
The surface waves propagate even slower than the S waves,
so that is why it emerges at the end
so at this instance, surface waves amplify
due to the presence of shallow sedimentary rocks.
You also see a little bit of amplification
in the Taupo volcanic zone. This is because the volcanic rocks
have a slower wave speed, just like the sedimentary rocks.
What this shows is - the ground shaking
from an earthquake will be different
in different parts of New Zealand, according to a different
underlying geology and different types of rocks.
Finally I will show the third example
off the coast of Gisborne in 2007.
This was a magnitude 6.6 event.
I'm going to just let the model run.