So the question is, what was it like for me on the 5th of March? So like most people in New Zealand I
got shaken awake, and actually for me it was also you know some upset family members okay I'm being
awake. So my first focus was kind of really on the family and I looked at what the event was
saw that it was relatively far, so I knew that the tsunami panel would be doing their job and
and working out whether there was any threat, and so I got onto my computer and then went
into calculating the earthquake forecast. That's basically like an equation where you put in what
time window you are interested in say we wanted the first car forecast to start at
10 o'clock for 24 hours, and then usually at the beginning we do like within the next 24 hours,
within the next seven days, then the next 30 days to give people an idea of what to expect.
And so i i use that model, and on top of that um just because it was derived from parameters
in New Zealand, you know like but we also wanted to throw in another to just see what
the variability would be, we used the model from a global study of subduction zones.
We calculated probabilities from that as well as ranges of earthquakes to be expected.
And then i checked those numbers with my colleagues and worked with the communication teams
to get that published as a story on the geonet website. From the general public point of view the
feedback that we get is that it is really useful to know that this is expected, and also that the
number of aftershocks over time decay, so it's quite nice to be able to track that the numbers
going down it gives you know reassurance that in a way this is normal and it's also passing.
Were the three earthquakes of March 5th related and what caused them. What caused them is a very
basic question it goes back to our model of the seismic cycle. In the seismic cycle we know we
have tectonic plates that are shifting around the surface of the earth. The earth's crust is moving
the places where these plates come together we're accumulating elastic strain. So if you think of a
chopstick being bent, if you don't bend it so far that it breaks it's got elastic strain in
it and it'll go back to where it was. If you put a little bit more strain on it then it can snap
and then the two pieces will go straight and then you have an elastic rebound and you give off
energy and that's an earthquake. So the same thing happens when two tectonic plates come together,
you can have a release of that elastic energy and a big earthquake so that's why earthquakes happen
at plate boundaries. Um the link between the earthquakes is a little bit more difficult we
know that if you're very close to two earthquakes or if you're very close to another earthquake,
you're a lot more likely to have a future earthquake and the mechanisms for that are
relatively easy to explain or we have models that fit in a lot of situations for how that happens.
It gets a little bit more tenuous when you think about earthquakes triggering earthquakes at great
distance with a bit of time in between them. So if you think of the um the first and second and
third earthquakes that we had where we might have a few hundred kilometers in between them,
then we have theories for why this happened these theories are hotly debated in science and the the
answers aren't conclusive. We've got ideas about how this happens we we know it happens we see it
in reality, but um definitively saying why that happened on the day is really hard thing to do.
Every time there's a earthquake specifically large earthquakes, you get this big disruption
to the stress that's in the crust around that earthquake. So say with the east cape earthquake
that rupture will have pushed the stress from the colliding tectonic plates, will have pushed some
of the stress that was released in that earthquake to the surrounding area. And because of that the
stress goes up in some areas goes down other areas and then the earth has to sort of settle
that stress out. So the continuing earthquakes we see after the large earthquake are essentially
just that stress being being worked out by by the earth crust and the tectonic plates there.
So can we predict what might come after a large earthquake? Now we certainly can't predict,
um so what we tend to say is is we can was forecasting. And the difference is a prediction is
we'd say deterministic and we know this is going to happen in this time frame in this window in
this location and we just don't have the knowledge and the information to be able to do that.
But we we can forecast which in earthquakes the way we use forecast is that's something is more
or less likely to to happen in some time frame or space. We talk about clustering so after an
earthquake happens we know after a big earthquake happens we know there are going to be a lot more
earthquakes to come after that, and we can we can forecast that pretty well and they follow
some pretty basic laws and then there's one called the Omori law that's been around since
the 1890s it's one of the oldest laws in seismology and it's held up really well,
and we use that kind of as just some of the basic information we do we add a lot more to that
but we can say roughly how many aftershocks will occur where they're more likely to be.
But an interesting thing is that one of the confusions with using the term aftershock is
that aftershocks can actually be larger than the main shock. So there's always a probability that
something bigger can occur and we we account for that in the information that we put out so after
8.1 there is some probability that something larger than an 8.1 can occur. So we can't
predict anything specifically we can say here are the range of possible things that can occur
and this is how relatively likely each one of those those things are.
Okay so the question was where are the Kermadecs and why do we have earthquakes there.
So the Kermadec are a group of four volcanic islands located about eight hundred to a thousand
kilometers northeast of Whakatane, they're joined to New Zealand by about 30 underwater volcanoes
which effectively form a continuous volcanic ridge extending right the way down to Whakatane.
Tectonically, the Kurmadecs form a subduction zone, which is where the pacific plate is being
subducted beneath the Australian plate. This subduction zone extends beneath the north
island of New Zealand along the Hikarangi margin, and then also extends as far north
past Tonga and within a hundred kilometers of Samoa. So collectively it's over three and a
half thousand kilometers long. Why do we have earthquakes there? In the broader sense they
they result from the interaction and collision of the Australian and pacific tectonic plates.
Before the plates were even in contact the pacific plate starts to produce earthquakes
as it starts to bend flex and crack before it's subducted beneath the Australian plate,
when the plates are in contact they continue to produce earthquakes but the largest events
are occurred on the interface which forms between them. This these earthquakes result
from friction which temporarily causes the pacific plate to drag or push the pacific plate
in the direction of its motion when the stress accumulated exceeds the strength of the fault,
the Australian plate rebounds and the fault slips producing an earthquake. So
this scenario is effectively what happened in the magnitude 8.1 earthquake last week.
So it's very difficult to say how many aftershocks there are following
a large earthquake like the ones that have happened on last Friday,
because the way we observe these earthquakes we don't do it directly we do it from
recordings of the seismic waves. And when we're looking at those sizing wave recordings
we're looking for sort of distinct bits of energy coming in the kind of pulses that you feel as an
earthquake goes on. When you have lots and lots of earthquakes happening in a short period of
time like we get at the beginning of an aftershock sequence, it's hard to say which packet of energy
that we're seeing comes from which earthquake. And so when you flip that around you say okay well how
many aftershocks have there been, you're looking at this very very murky picture going oh is that
a wave coming in? Is that a wave coming in? Are they from different earthquakes?
No that's really hard to say especially during the immediate period of a response.
The question is how does GNS monitor earthquakes and tsunamis, and then how do we turn that into
forecasts that we can use to keep New Zealand safe and resilient. The answer is a series of answers,
first we monitor the earthquake waves generated before the tsunami hits. We know that about 80 of
tsunamis are generated by earthquakes, so it makes sense to use the first information we have about
the earthquake to tell us something about the tsunami. The way we measure the earthquake is to
look at ground shaking we have our own instruments across New Zealand the geonet instruments,
we also have access to regional instruments in the southwest pacific that measure ground shaking
from these big earthquakes. When we received that ground shaking we determined whether or
not we think that came from an earthquake that was big enough and shallow enough to have generated a
tsunami, and if we think it has that triggers a series of other steps. So the first thing we do
is we generate um based on that early information what we think an approximate land threat is in New
Zealand, and we push that information to the appropriate authorities as quickly as we can.
Once we've done that, then we start looking at the water wave measurements so after we receive the
ground shaking information then we start to record things like tight gauge measurements when the
tsunamis hit coastlines, or dart measurements when the tsunamis travel over some ocean bottom sensors
we have we've recently installed in the southwest pacific. When this happens we measure the wave
that travels over it and then we use these waves these measurements to improve our forecasts about
what's actually happened and what will happen as the tsunami travels to our coastlines.