I love having a problem that perhaps no one's ever solved before and bringing together different
people with different expertise, and to try to go forward rather than stagnating. Lucy from Auckland
asks why do we commonly see the sea receding prior to large waves happening with a tsunami. That's a
great question Lucy and it's a question we often get asked. If you think about a tsunami as being
a wave like you might see on a string that you move up and down or you might see on a calm pond
when ripples propagate out from throwing a rock. You know that the wave has crafts and troughs,
crests and troughs and crests and troughs. So when you see the water go out first it means
that trough is arriving at the coastline first. Why would this happen when we make a tsunami from
an earthquake where when a tsunami is made from an earthquake, it's made because you have places
on the sea floor that either rise creating a big column of water that then smashes down and
spreads out, or drops creating a trough in the water that then spreads out and goes out like a
propagating tsunami or moving tsunami. And a lot of tsunamis are caused by both of these motions
where you get uplift and subsidence from the same earthquake some are caused purely from uplift,
some are caused purely from subsidence. So you can imagine if the part of the the way
that hits you first is caused by that dam dropping portion, you'll get a receding of
the ocean. Great question Lucy, and I looked at it answers your question and I hope now you
understand that sometimes we get some a wave rise a wave drop before the big tsunami waves come on.
But what's absolutely crucial is that anyone, if they do see some sort of abnormal activity
in the ocean like this if you see the water recede for a long way out you need to treat
that as a possible tsunami coming in and you need to evacuate it accordingly. Thank you very much.
So this question from Marion in Palmerston North why do we always worry about tsunamis when they
only cause really small waves in New Zealand. This is a really good question. So first of all
tsunamis come in a range of sizes from centimeters to many meters, and like a lot of hazards we see
the small events much more frequently than we see the very large events. But you're right and most
of our lifetimes we've only seen quite small waves from tsunamis in New Zealand. But our historical
record and our geological record tell us that New Zealand is exposed and vulnerable to much
larger tsunamis, and they could happen again in the future. So in our written period of history
we've had about 50 tsunamis that have impacted the shores of Aoteraroa, most of them have been
quite small but one interesting one is the 1947 earthquake up near Gisborne, this was a magnitude
7.2 earthquake. It triggered a tsunami that was up to 30 metres high in some places along the
coastline north and south of Gisborne. At the time it was a very sparsely populated coastline
so there were no fatalities, but if the same event happened today it would probably have a
much greater impact because the coastline has a lot more development on it. The geological
record is really interesting too, so we can use and when when large tsunamis occur they pick up
marine sediment like rock and sand and shells and deposit it inland we can use these sediments to
date the timing and size of past earthquakes. And these sediments are present at many places
around the New Zealand coastline, and they tell us that in the past we've had tsunamis that have
been many meters high and have gone hundreds of meters to kilometers inland. All right I
hope that helps answer your question Marion. I go out to sea a lot on ships to study things
like earthquakes and and tsunami working together with a big team of people just super super fun.
Had a question from Bill in Wellington, and he's asking how can a tsunami be so huge. He also says
I think I find it difficult to understand how the sheer volume of water that is displaced from the
original area can be such a large amount and how far inland it seems to be able to go. So tsunami
are basically generated large due to a sort of a sudden displacement of the seafloor, sudden
displacement of the seabed, most commonly this happens in earthquakes that's where most of our
tsunami are generated, but this can also happen we have submarine landslides and volcanic activity.
Those can also displace large portions of the seabed and generate tsunami. I'll focus a little
bit on earthquakes and earthquakes you can have hundreds of kilometers long area maybe even up to
a thousand kilometer long area displaced. And so in the really big earthquakes you might be getting
up up to maybe a hundred cubic kilometers of water of water continents being displaced. And out in
the deep ocean where so many of these tsunamis are generated you know in deep water, you know the the
waves might be very small and maybe less than a foot but the wavelength of the wave can actually
be very wide like more like 100 kilometers or so. So if you're out and boat in the ocean you
wouldn't even know this wave probably was passing underneath you however things really start to get
real when we get closer to the coastline and get into shallow water. And so as that wave
comes into the shallower water everything kind of scrunches up together and piles up and you get a
much much higher wave that then ends up you know inundating your coastline, and you know as many
people have seen photos and videos of this this type of thing happening. So what really controls
how far inland is tsunami goes it's gonna be very dependent on the shape of the seabed beneath the
ocean near the coastline that's gonna really influence how that that tsunami wave builds up
and comes in. Also the shape of the coastline and also the topography. So if you have a long
flat land area going inland from the coast then your water can potentially travel further inland
as opposed to where if you have a very steep coastline it's not no going to be able to
travel as far inland. So I hope that answers your question, thanks for asking such a good question.
Mojo from Wellington asking an interesting in question does the Wellington harbour protect
the city from a tsunami. The answer is rather complex can be both yes and no. It's really
depends on how tsunami will impact the city and aware of that will come from. The Wellington city
can be impact by tsunami from different sources, including those outside the harbour and also those
inside the harbour. If a tsunami is coming from outside the harbour, for example South America,
across the Pacific Ocean, the narrow harbour entrance will be able to limit the amount of
the tsunami energy entering the harbour. Then the tsunami impact will be less severe inside
harbour than outside. But in a large Hikurangi zone earthquake when tsunami may also find its
way through the low-lying southern suburbs such as Lyall Bay or Kilbernie. If their tsunami
is generated from inside the harbor such as the Wellington fault then the narrow harbour entrance
channel limits the amount of energy escaping. Additionally the particular shape of Wellington
Harbour means it's natural water oscillation patterns may likely be mobilized by either
ground motion or the incoming tsunami waves. This is kind of like me holding a bowl of water,
we shake the ball the water will oscillate and make a higher and higher appearance field. It may
also cause much longer duration of tsunami inside the harbour. Hopefully this answers your question.
Okay. So the question is if there was a big earthquake and it collects dams in the South
Waikato area would as soon I may go down the Waikato River and wipe out Hamilton. Thanks
for this question. So first of all dams in New Zealand are designed for certain seismic safety
standards. So they are engineered to withstand certain amounts of ground shaking. Now the
good news is is that the South Waikato area is relatively low seismicity area of New Zealand,
so there are very few active faults in their area, and the likelihood of very high
intensity shaking is quite low. And the other thing is is that about every ten years or so
this information on seismic hazard is reviewed and incorporated into dam safety assessments,
so they're continually using new knowledge about seismic safety to monitor those dams.
I guess in the very unlikely circumstance that a dam did collapse in the South Waikato area,
the lake cables and the revenue was very well monitored so you would be able to see
these changes quickly and hopefully have time to evacuate downstream if there was a large pulse
of water coming down. The other thing I wanted to mention was that if you are near and one of
these large reservoirs or lakes during an a big earthquake along with strong earthquake
it's still a good idea to just evacuate that area and get away from the water anyway. This
is because large earthquakes can trigger seiches which slosh the water back and forth sometimes for
hours after an earthquake. And large earthquakes can also trigger landslides into the lake, and a
landslide going suddenly into a lake can trigger a tsunami there as well. All right hope that helps.