So my name is Charlotte Pizer. I'm a PhD student at Victoria University of Wellington, and
I am studying evidence of past earthquakes, mainly based just north of Napier, along the
Hikurangi subduction margin. So the Hikurangi subduction margin is the interface between
two plates and that is located in New Zealand just offshore of the east coast of the North
Island. And we're interested in looking for evidence of these earthquakes because they
are the biggest threat to New Zealand. So they'll cause the largest earthquakes and
potentially the largest tsunamis as well. So the interface is offshore but then it dips
beneath the North Island as well. And that's why there's the great potential of large tsunamis
because that displacement will occur offshore and displace the water causing a large tsunami
to propagate towards the coast. So to start with we have to go and seek out this evidence
of earthquakes. So to do that we go into coastal areas and take long cores. So these are usually
in big pipes and we push the pipes down into the ground and then winch them back out, chop
them up into meter lengths and that's what we have here. And then we open them up and
we see what's inside and hopefully we have these big changes in sediment type that represent
earthquakes. So this is one of my cores and just so we're aware of what this actually
represents, this is the top of the core so this represents the surface and as we go through
the lengths here everything gradually gets older. So these sediments have been placed
on top of each other with this one being the oldest and then getting gradually younger
towards the present sediment that's at the top. Okay so the way that we kind of distinguish
where evidence of earthquakes might be is we look for changes in the sediments. So what
I'm doing here is just scraping the surface of the sediment to expose all of these contacts
between the different units so that we can see whether the changes are really sharp like
they are here or whether they're more gradual. So I'll take you through this core from the
bottom to the top. So starting at the base here we have this estuarine environment that
we can tell from the shells and then it goes slowly up getting more fine sediment into
this silt but still estuary because we have lots of shells in here. Then we have the evidence
of the Waimahia volcanic ash. We can already get a handle on how old the sediment is because
we know the age of this eruption. So this eruption was around three and a half thousand
years ago, so everything above this is younger than three and a half thousand years. And
then it is changing to a much more terrestrial environment that we think represents a forest
peat. And then down to the base of the next section it continues with this forest peat.
We see the more gray silts coming in representing a more marine environment, and then this sudden
change here to this really coarse material that has fragments of shells within it as
well, so we know that the change happened here from something more terrestrial below
to something more marine above. So this must have represented subsidence, so the land going
down, because it's closer to the ocean here than it is below. We go up here through more
marine sediments and we know this because it's sandy and it has shells in and then to
the base of the next section, getting more terrestrial here so maybe this
section is slowly uplifting. Some more organic material here so maybe going back towards
the forest environment that we saw earlier on in the core, and further up we see this
really sharp contact here where it changes from this organic silt to sand, so this could
represent another earthquake here where the land suddenly went down. So then this marine
environment transitions this could be another event here but we're not quite sure, it's
less obvious, and then we go towards the top of the core and the next section and then
the top of the core which just represents the paddock that we cored in, so we're not
really interested in this much.
So something that's really important for us to work out is if we see evidence of earthquakes
in these cores we need to figure out when they occurred. So I mentioned before we know
the age of this volcanic ash, so everything must be younger than it, but what we can also
do is pick out things like fragments of shells like this one, and things from the forest
peat like seeds and then we can use radiocarbon dating to get an age on these sediments, to
work out when the earthquakes occurred. So in order to get a good story of the earthquakes
in this region we need lots of cores to make sure that the things that we're seeing are
actually represented firstly within this valley and then broader scale we look in other valleys
close by and then even further away so we can try and decide how big the earthquakes
are that we can see in these cores. So we really want to get a good handle on how often
and how big Hikurangi subduction earthquakes are because that type of earthquake new zealanders
haven't experienced before within historic time. So the earthquake record that we've
got from Pakuratahi is really important because it shows that the land has gone up and down
quite a few times within the last 3000 years. And we don't necessarily know the sources
of those earthquakes yet so what we need to do now is date them, find out when they occurred,
and then try and compare them to earthquake records from elsewhere along the margin. So
the way we can do this is to look at earthquake records from onshore along the margin but
also what we're trying to do is look at offshore cores that also contain records of earthquakes
so that we can stitch them together and try and get a better picture on the timing, the
location of these big earthquakes, and the size of them as well.