One of the questions that have been asked a lot
is why the shaking from this recent February 22nd earthquake in Christchurch was so severe.
And an obvious answer is that compared with the 7.1 back in September,
this earthquake was actually much closer to Christchurch
and it was also at a shallower depth.
But then people say well we had a 6.3 in 1987 at Edgecumbe in the Bay of Plenty,
and there's ground there that is similar to Christchurch
that's great swamps and the like
why didn't we have the same ammount of liquefaction
and severe ground damage that we've seen in Christchurch?
The answer to this question is that this was actually a very punchy earthquake.
What I mean by 'punchy' is it radiated a lot of seismic energy.
When we talk about the magnitude of an earthquake,
what we are actually doing is measuring what happened on the fault
and what we do is we we multiply the rigidity of the Earth
times the slip that happened during the earthquake,
and in the case of the September earthquake it was about 5 meters
and then we multiply that by the area of fault which slipped, and that gives us the magnitude.
But what happens is you can have earthquakes of the same magnitude,
but that are actually radiating different amounts of seismic energy.
a good example is the Dusky Sound earthquake
which happened in July 2009 down here in Fiordland.
That earthquake had a magnitude of 7.8,
it was a really big earthquake in terms of its magnitude.
It had a lot of slip over a very big fault,
and in fact it was the interface between the Australian Plate
diving under the Pacific Plate.
But that actually caused very little landslides
and very little damage for the magnitude that it was,
and the reason is that this was a very smooth fault
which ruptures very often on the scale of 100 years or so,
and what we find for those is that for their magnitude,
they actually generate very little seismic energy.
We can measure how much energy an earthquake is radiating
by what we call an 'energy magnitude.'
now when we look back at the Darfield earthquake in September,
the magnitude of this was 7.1, but the energy magnitude was actually 7.4
so relative to its size, it was actually producing
abnormally large amounts of seismic energy
which then translates into shaking.
And the difference is even more extreme
for the earthquake which happened on the 22nd of February.
The magnitude was 6.3, but the energy magnitude was actually 6.7.
And the reason we have this big difference
is because the faults that moved, both in the 7.1 and in the 6.3,
have very high friction across them.
And this translates into abnormally high shaking from these events.
And the reason we have that high friction is because of
the types of rocks that these faults are surrounded by.
This really large amount of seismic energy
which was radiated from the 6.3 aftershock
is the reason we have all this damage in Christchurch.
This is reflected on our recordings of ground motion
on what we call 'strong motion recorders',
and here i'm showing two records
from very close to where the earthquake happened.
One at Heathcote Valley Primary School
and the other at Pages Road Pumping Station in Christchurch.
What you find with both these records,
and in particular the Heathcote Valley one,
is that the acceleration of the ground during this earthquake
was 2.2 times the acceleration due to gravity,
and in Pages Road it was 1.9 times the acceleration due to gravity.
So imagine if the ground is going up very fast,
and then it accelerates down faster than gravity.
So the deeper, stronger part of the crust is going down faster than gravity,
but what it doesn't leave the soil layers behind
because they can't go any faster than gravity,
and these soil layers are actually peeling off,
and this is what's called the slap down effect.
and it's only been seen very rarely around the world
because these are very extreme motions that we've recorded, on a world scale.
But of course what happens then,
is that the soil layers just lose their cohesion
because they're in tension, and all the water in those soils separates out.
But then what happens when the ground comes up again,
it meets these soil layers coming down,
and because water is highly incompressible,
it's essentially like a piston coming up and there's no gasket,
this ejects all these liquefaction products that everybody sees through Christchurch,
much more so than in the 7.1 earthquake in September,
because in that case, the ground was subjected to such extreme motions.
So the very strong shaking that we've had in Christchurch
is a culmination of a lot of effects, all of which have
contributed to this extremely strong shaking and this extreme damage.
We know that this type of earthquake doesn't happen very often,
we're talking about a cycle of maybe 10,000 years of that order,
but essentially what's happened in this case,
is Christchurch has been in the wrong place at the wrong time.