So today we're in Mount Victoria which is a well-known lookout point in the middle of
Wellington City. So we've got a great view over the CBD, the harbour and a series of
hills on the other side of the harbour there. It's quite a straight line along that and
that's because at the foot of that is the Wellington Fault which has lifted up the hills
on the other side of the harbour and dropped the harbour down on this side. This is a very
obvious fault - we've known about it for a long time and as a result we've done quite
a lot of work to understand it. There are also a number of other faults in the Wellington
Region and you can see on this map what we're talking about. There's a lot of red lines
- these are all active faults and each of these could produce an earthquake. Today we're
going to look at four of these faults to go and see what they look like in the landscape
and then we're going to think about the research that we've done on each of these faults to
better understand the risk.
So here we are at Makara Beach where we're going to see our first fault and we're going
to go up here and have a look at these rocks up here.
Okay so we've come up to this outcrop now. What we can see is two different rock types.
You can see this one up here which is more layered and this one here which is more greeny.
And the line in between is the Shepherds Gully Fault. So you can see right along the fault
here is is eroded out and that's because the rocks are so crushed up, broken, they're quite
soft and so it's easy to erode and you can expand that to a larger scale. And we get
these valleys that occur along all of the faults that form these really typical landscape
features of the faults in this region. It goes down these valleys all the way to the
south coast and then it runs offshore and comes onshore again, and then goes off shore
at Pukerua Bay in the north. So we don't know a lot about the Shepherds Gully Fault, there
hasn't been a lot of work done on it, but there are some estimates of the recurrence
interval which is the average time between the earthquakes on the fault and that's estimated
to be around about 4 000 years.
So that's one of the lesser active faults in the Wellington Region.
So now we've driven up to the northeast to the Ohariu Valley. Like at the Shepherds Gully
Fault we're seeing erosion that's picking out that fault line along the valley. There
has been water that's pooled against this uplift and it's making this kind of swampy
depression here. Although we see this uplift on that side of the Ohariu Fault most of the
motion along this fault is strike slip. That means that the fault has mostly got sideways
movement. You can see from these drone images that this fault is a really straight and clear
feature of the landscape. So we know that the last earthquake on the Ohariu fault happened
about a thousand years ago. We're less certain of the recurrence interval on the fault but
we think that this is around 2200 years.
So we've continued on along the Ohariu Fault and now we're in a place called Paramata.
We've seen this fault in the landscape down in the Ohariu Valley and now we're at a spot
where the fault goes under this bridge behind me. So we've also seen that the ground on
either side of the fault moves upwards or side to side, so from that we know that if
there was an earthquake here this bridge would be severely damaged. This is why we need to
understand when these faults have had earthquakes in the past and how much they move so that
we can do what we can to protect ourselves from future earthquakes.
We've been over to the Ohariu Fault, we've been to the Shepherds Gully and I'm on the
Petone foreshore, and we're looking across Wellington Harbour and that forested bush-covered
hill is actually one side of the Wellington Fault which has been uplifted as well as it's
been moving sideways. And it's the sideways movement, but largely the up and down movement
on the Wellington Fault that has caused the harbour to be here. It's the controlling landscape
feature of the Wellington Region. One of the key - perhaps scary - features of the Wellington
Fault is it goes through our capital city.
We're now in Thorndon. I'm on the upthrown side of the Wellington Fault, we're looking
across the Wellington Fault to the Thorndon region and you can see a whole bunch of infrastructure
down there: the rail, the ferry, the motorway And a key aspect of the work that we do in
the geology world is to provide the information on what a Wellington Fault earthquake will
do. So the infrastructure here can be toughened and designed to accommodate that hazard. How
strong is it going to shake, how much displacement, where can the liquefaction happen? and all
these aspects have been the subject of studies over a number of decades now. In the Thorndon
region we've mapped the areas that will liquefy, and structures that are built on that have
been toughened to accommodate that, either having piles and foundations that go down
onto non-liquefiable stuff or in the case of the stadium before they built it there
they pounded the bajibas out of the ground to make it unliquifiable.
Now I'm in Upper Hutt, the Kaitoki region about 30 - 40 kilometers north and I'm standing
right on the Wellington Fault. These two manholes are one on either side of the fault measuring
strain that might be accumulating on the fault. And why we're here is I'm standing on the
fault and the Stuart Macaskill lakes are not and that was planned. So the Stuart Macaskill
lakes are the region's main water reservoir. It's real good that it's actually not on the
fault so when the fault moves these lakes are going to still be intact. And that's another
example of how previous knowledge of where the fault is allowed critical infrastructure
to be built away from the hazard.
We're at Turakirae Head which is about 20 kilometres east of Wellington. So to get here
we've walked along this gravel ridge which looks a bit like a track but it's not - it's
actually an old storm beach that was created by the sea in storms depositing gravel at
the back of the beach. It's not the modern storm beach though - the modern storm beach
is down there and the reason this one is back here is that in 1855 the whole of Turakirae
Head was lifted up out of the sea by six metres. That was a huge earthquake, around magnitude
8.2 and that caused impacts around the Wellington Region. You can see the whole staircase of
ridges just going up the hill and those represent uplift in a whole series of earthquakes in
the past. We sometimes find shells like this on the ridges and we can use radiocarbon dating
to find out how old these shells are and then that can tell us when the earthquakes happened.
The dating tells us that it's uplifted every one to two thousand years in an earthquake.
The fault that caused this uplift is not far away from here. The Wairarapa Fault runs along
this section of coast and then up past Lake Wairarapa. On the Wairarapa Fault itself is
a really cool location at a place called Pigeon Bush. In 1855 at Pigeon Bush one side of the
Wairarapa Fault moved something like 18 metres past the other one just in a few seconds.
And we know that because there are small stream channels that cut across the fault, and when
the earthquake happened it took the bottom half of the channel and just moved it sideways
so it doesn't really match up with the top half anymore. So this offset in one earthquake
of 18 meters is the single biggest documented offset of the Earth's surface in any earthquake
anywhere.
So we've now been and seen four of the many faults in the Wellington Region but one we
haven't been able to see is hidden beneath our feet. It's about 25 kilometres beneath
us and that's the Hikurangi Subduction Zone which is the interface between the Australian
and the Pacific plates. that is the region's biggest fault and can produce very large earthquakes,
so it's really important that we research all of these faults to understand the risk,
the impacts, how big they could be for the Wellington Region. And we're doing that as
part of the It's Our Fault programme. So the It's Our Fault programme here started in 2006.
We have a team of people who are working on the the active faults, on the Hikurangi Subduction
Zone, on tsunami modelling, on landslides and fire following earthquakes, and getting
all of this information into policy and planning to make the city more resilient. So we haven't
experienced a large earthquake in Wellington for a while, so this is a really good time
to get prepared for what could happen in the future. You can do your part by being informed,
going online finding out how you can get prepared at home, making your plans for what you can
do in an earthquake.