Kia ora, today we are in Victoria University in Wellington
New Zealand, and I am with Jamie Howarth, my ex-colleague and good friend. Kia ora kotou,
my name's Jamie Howarth, I'm a paleo seismologist and a senior lecturer here at Victoria University
and I'm excited to be here and talk to you a little bit about the work
we've been doing on the Alpine Fault and in particular how that work informs the likelihood
that we're going to see a large earthquake on that fault in the future.
Now my understanding of the Alpine Fault is it's a globally significant faultline that generates very
large earthquakes potentially, and so of course it's really important for scientists to understand
how often does that happen, when were the last earthquakes, how big were they? You know and
that that research has occupied many people for a long time in a very difficult environment because
the Alpine Fault crosses an alpine region in New Zealand it's very inaccessible, there's lots of
dense forests, wild remote areas in new zealand, it's quite hard to get information.Aand that's
what's so interesting about what Jamie's been doing because he's been able to sort of in
spite of those conditions find out some really important information about the Alpine Fault.
So Jamie I'm going to hand over to you and you're going to take us through some of the ways you've
done your research first of all, and then what you've found out over time because of your methods
and your approach and what that means for us. Fantastic so from space the Alpine Fault is a very
obvious and really straight feature that basically cuts through the South Island. It's a plate
boundary fault so it forms the boundary between the Pacific Plate in the east and the Australian
Plate in the west. And it's a particular type of plate boundary - it's a transform plate
boundary - and that basically describes the sense of motion between these plates. The Pacific Plate
is actually moving southwest relative to the Australian Plate which is moving northeast. Now
that plate motion is accommodated elastically so that means that the fault is stuck
and it's building up the stress associated with that plate motion that's continuous.
So it's building up then suddenly it's released in a big big earthquake. Okay and we can see
the Alpine Fault runs from offshore at Milford Sound north along the base of the Southern Alps,
through the central upper South Island where it does a little bit of a dog leg before running
offshore at Cloudy Bay near Blenheim. Okay so there is a little bit of upward movement then
on the fault to produce the mountains, but it's not all upward. There is a component of vertical
motion or compression and it's that compression which has built the topography that we see.
Okay let's start with what was previously known before you did your work here about the fault and
where that was studied. So a really famous record of past earthquakes on a fault actually anywhere
in the world, comes from the very southern onshore part of the Alpine Fault from these sites called
John O'Groats and Hokuri Creek. And if we zoom in to John O'Groats I can explain why
these are such good sites for recording earthquakes. So we can see the stream here
that is running across the Alpine Fault. And what happens when we have a large earthquake
is that this stream is offset and essentially dammed, and as a result of that damning we have
water and sediment spilling out over this wetland and earthquakes are recorded then as interbeded
sediment layers in the stratigraphy of this wetland that alternate between sediments produced
by the river and sediments produced by the wetland itself. Okay so some of the kind of rocky
material, gravel, sand that sort of thing? Yep, so that would be an earthquake interbedded with
very organic material formed by the little plants that grow on the wetland. By using sediment cores
that basically act as a time machine going back through time, we're able to identify those layers,
date them with radiocarbon dating and produce a history of large earthquakes on the fault.
And we've been able to do that at these two sites John O'Groats and Hokuri Creek
slightly to the north, over a time frame that spans the last eight thousand years
or 24 Alpine Fault earthquakes. So here we have a plot of the timing of the last
24 earthquakes on the Alpine Fault. So on the x-axis here we have time in calendar years so
the present day is all the way to the right of the screen, and then we go back to around about eight
and a half thousand years ago and then each one of these distributions you see here is our estimate
for the timing of an earthquake. And one of the really neat things about this record is
that the historic record spans this tiny little area right up at the right hand end of the plot,
so we're able to actually look at the recurrence of these earthquakes over a really long time frame
and that gives us a lot of confidence that we actually understand how this fault behaves. There
is around about 300 years between each of these events and that timing between events is pretty
constant as we go back over the record. So 300 years, and the last earthquake was? In 1717 around
about 300 years ago. Okay yeah what does that mean just from this information? What does that mean
about the probability of an earthquake let's say in the next 50 years? Well so because we've got
a really long record of earthquakes from this point at the southern end of the fault, we can
use it to forecast the likelihood that we will see an earthquake at some time in the future.
So the probability from this record of seeing an earthquake at this site on the
Alpine Fault is around about 29% in 50 years. So we can be reasonably confident that we can
forecast when the next event is likely to occur here, but what we really want to know is how much
of the fault breaks during those earthquakes, and whether or not the forecast from here is
applicable along the length of the fault, Right, because there could be a place way up north where
there's a different record because the earthquakes were not the same. Exactly! As it turns out
lakes adjacent to the Alpine Fault provide really excellent archives of past earthquakes.
So the cool thing about these lakes is that they were formed when glaciers retreated
at the end of the last glacial maximum sometime around about 17 000 years ago,
and they've been archiving the processes that occur in this environment ever since then,
in the sediments that accumulate at the bottom. Okay so the ice retreats, there's an empty
basin - fills with water and then sediments slowly pile up at the bottom of those lakes for the last
17 000. Yeah yeah, and what's really interesting when the Alpine Fault ruptures in a large
earthquake, we generate strong ground motions, they disturb the sediments or the the natural
pattern of accumulation in a way that allows us to recognize that a shaking event has occurred.
And they do that continuously. So unlike that fault-bounded swamp that we were looking at
at John O'Groats and Hokuri Creek, the sediment archive or record that we're after, is some 80
meters under the surface of this lake and the way we actually sample it is using a Mackereth
Corer which is a system that is lowered to the bottom of the lake and we use compressed air to
push a tube into the sediment that samples many thousands of years of accumulation, and then the
whole thing lifts off. You can see in this video the Mackereth Corer dramatically breaching the
surface of the lake as it comes to the surface with our sediment archive or sediment record
attached to it. Once we have that core we take it back into the lab, split it and then study it to
identify these earthquake layers. The neat thing about lakes for reconstructing the spatial extent
of earthquakes they almost act like nature's seismographs - they're continuously recording
when there's an earthquake they change the way the sediments are being laid down, and we can
identify that and then they continue recording and one of the really powerful things about that is
it means that they're able to record earthquakes that occur very closely spaced in time.
And that's important because it means we can distinguish events that rupture one part of the
fault from an earthquake that occurs in another part of the fault, but really closely spaced
in time. So that's useful for starting to piece together this picture of rupture extent along the
fault. Great! so just a question: you know, let's say you found a layer in the bottom of the lake,
how do you date it? How do you know? So we use radio carbon dating
of leaf material from the vegetation that grows around the lake, and we can determine
the timing of an earthquake within a decade or so using these lake sediments. Can you tell me a
little bit more about the different lakes? They're distributed along the entire length of the fault
and we've investigated four of them here. So here's that site that we talked about right
at the southern end of the fault and we've got really nice earthquake records from Lake Ellery,
Lake Puringa, Mapourika - you've seen, and Lake Kaniere up by Hokitika.
And this allows us to put a picture together of earthquakes over the last 4 000 years
along this 350 kilometer stretch of the Alpine Fault. But one thing I just want to bring to your
attention is that while the Alpine Fault looks like this very straight linear structure, there
are actually really important, albeit subtle, changes in the fault that occur along its length.
And those changes are predominantly related to the strike of the fault - or the direction of it -
and the dip of the fault - so the angle at which it's diving away into earth's crust. For example
on the South Westland section here, which is a really important section with respect to
some of the earthquake records we'll talk about, the fault has a sub-vertical dip,so it's almost
straight up and down. And when we transition into the Central section which is also a really
important section which is the yellow section, here we have a much shallower dip and it's dipping
away to the east underneath the Southern Alps. And indeed that change in the strike of the fault
and the dip is what distinguishes the South Westland from the Central section and that will be
important for the earthquake behavior that we're about to talk about. Here we have a space and time
diagram. So it's complicated, but I'll take you through it. And essentially what it's showing us
is: unlike the last diagram we just saw the timing of earthquakes going back through the last 8000
years, this is showing us the timing but also how much of the fault broke in each earthquake. At
the bottom of the screen here we have the Alpine Fault laying on its side such that north is to the
left and south is to the right, and then going vertically on the screen here we've got time.
So here's the year 2000 and we go back about 4000 years in this record here. Each one of these
red marks here is an Alpine Fault earthquake that has occurred at this particular site
and what we can do is using statistics we can correlate between sites, allowing us to
map the spatial extent of rupture along this fault. The important take-home message to
see from this is that where we have these red horizontal lines we have these large magnitude
eight earthquakes that break the majority of the south western - central section of the fault,
in excess of 320 kilometers. But what's really interesting is that there are other periods in
the past such as this one here, demarcated by these yellow earthquakes, where earthquakes
rupturing the southwestern section or the central section actually stop at this location,
or the boundary between these two segments. And they have a much smaller magnitude
in the order of mid-sevens as opposed to in excess of eight. So it's still a large earthquake but
the ground motions associated with a magnitude eight that breaks in excess of 300 kilometers of
the fault is going to be significantly greater and the spatial extent of those significantly larger
than magnitude 7. So distinguishing between the two is a really important thing to do. So based
on this paleoseismic record alone we are able to say that the central part of the Alpine Fault
has a conditional probability- so a forecast - of producing an earthquake in
the next 50 years of about 75 percent. So that's dramatically different from 29percent! - That is
dramatically different from 29 percent. Okay so i'll just get this right: you've discovered that
the likelihood of an earthquake on the Alpine Fault central section has been, if you like,
increased with this new information to 75% in the next 50 years. So it's pretty hig!
Very high! On a global level, what does that look like in terms of: is that big? high?
I would say that that's very high! There are some faults that have higher conditional probabilities,
but not many. Okay yeah what else can you tell me? Well what we can actually do is tell you
what the magnitude or the amount of the fault that's going to break in the next earthquake, and
we do that by combining our empirical observations of how the fault has behaved in the past
with a computer model that's based on the physics of how earthquakes behave.
And by bringing those two sources of information together we can actually forecast not just the
likelihood that we'll see an earthquake, but what its magnitude will be and what part of the fault
will actually rupture. Okay and what we see when we do that is there's about an 82 percent chance
that the next earthquake is also going to be a magnitude 8 through-going rupture that breaks
both the central and the southwestern section. Okay um it feels very sobering what you're saying,
because what I'm hearing is that our understanding of the Alpine Fault now, is telling us that the
chances - all we can talk about is chances or statistics or probabilities - the probabilities
or chances of a future earthquake on the Alpine Fault are higher that it will happen in the next
50 years than we thought, and that the fact that it's most likely going to be a magnitude 8 plus.
I think you've summarized it perfectly. The one thing that I will add is that I think knowledge
is power in this instance Julian. At the end of the day the fault for all intensive purposes
is no more hazardous today than what it was yesterday before we knew about this information.
But what we're able to do knowing that there's a really high likelihood of
a magnitude 8 earthquake on the Alpine Fault is actually start to plan for it.
And if we plan for it we can build resilience into our communities and into our infrastructure.
Does it change anything in terms of - I'm a person living in a house somewhere on the South Island
potentially quite near to the Alpine Fault, do I do anything different? What do I do about this?
I think as an individual it doesn't really change that much at all. You know we've had a spate of
large earthquakes in New Zealand over the last decade starting with the Christchurch earthquake
sequence and then with Kaikoūra. We all know what we need to do as individuals with respect
to having our earthquake response plan for the household in place, making sure we have supplies
in the house in the event that we are cut off from our supply lines, and really I think that all this
new information does is sharpen the point a little bit. It makes us just a little bit more aware,
a little bit more conscious that we need to have those plans in place. And if we haven't done it
maybe it provides us with the encouragement we need to go out tomorrow or on the weekend
and make sure that those supplies are stocked up, those water bottles have been refreshed,
and that you sit down and have a korero with your family about what your response plan is
going to be in the event that you experience a large earthquake on the Alpine Fault or indeed
on any one of the other 500 or more active faults that we have in New Zealand.
So Jamie I'd like to really thank you. This has been an eye-opening discussion with you today.
I think your work has been phenomenal - I've seen you in action - we've been in the field together,
I've seen you working on getting those cores. It sounds easy it's incredibly technical,
it's really problem solving at its best in the most incredibly wild environments with
all the weather and all of the wilderness around you. So fantastic! Thanks so much
for this opportunity to hear about your research and so I hope people at home have enjoyed -or
if that's the right word - have felt enlightened, Something I'd really like to say too about it is:
this is what science is. Science is value for society. We now have some insight which guides
us and gives us really good information. So that's great thanks so much. Yeah thanks
for coming along Julian. As always it's a pleasure discussing science with you, and yeah
I look forward to taking our message out to the New Zealanders.