Yesterday civil-defense sent out a warning that we may see some strong sea surges
following a massive earthquake off the coast of Alaska,
but then Civil Defense downplayed the threat of an actual tsunami,
so exactly where in the world does an earthuake have to strike
for us to be really worried about a giant wave
and how long would we have to get to safety?
Joining us is Graham Leonard, an expertin hazard mapping at GNS Science. Tena koe, good morning.
Good morning Jack. We want to work through three main areas of concern this morning,
and I thought you might be able to give us a bit of
an insider's explanation on exactly how this works
for when we should and shouldn't be concerned about tsunamis
and we might start with the outer regions this morning.
We consider the whole rim of the Pacific as a potential
source for tsunami
but we break it down into distant, regional, and local sources
so starting with distant sources, that sources more than three hours
of tsunami travel time away from New Zealand.
We're worried about tsunami potentially from earthquakes along South America,
Central America, off of Canada, Alaska and Japan for that source,
and in that case it's usually 12 or more hours of warning time, definitely more than 3 hours.
in that case we have an official warning system with Civil Defense in place
to tell people when they need to evacuate.
and so for a distance source such as the earthquake areas you just mentioned,
what is the threat of tsunami? it's pretty minimal right?
Well it depends on the place. The the size of the tsunami we might get depends on three things:
how much of the sea floor is deformed by the earthquake,
how far away it is, and what the orientation of that seafloor is to New Zealand.
So the Alaskan earthquake a couple of nights ago, it's a long way away,
it was less than magnitude 8, and the orientation wasn't quite right,
so we didn't get much of a tsunami from it
but a different earthquake up there - any one of those things a bit different,
we could get a bigger tsunami, and somewhere a bit closer with a different orientation
like Chile or especially Peru, we could get quite a substantial tsunami.
Okay, that's interesting I wonder with the Alaskan situation too, if you can say
there's been a big earthquake off the coast of Alaska,
well between us and Alaska there's a little bit,
we'll wait and see if anything happens to Fiji, if nothing happens to Fiji we should be alright?
Yeah well we've got access to deep ocean dart buoys which the tsunami travels over.
The weight of the water activates this pressure
sensor and we can actually see the signal coming across it
so for those distant sources that's one of those early signs.
We can calibrate how big the tsunami might be based on what
the signal coming from those dart buoys is.
Right, let's move a little closer to New Zealand. And you call this the regional area.
Yeah so we consider the area within 1-3 hours
of travel time to New Zealand as the regional source area.
and most of the concern there is the area up the Kermadec Islands, up through to Tonga and Samoa.
So if we've got an earthquake up in that area, it might take 1-3 hours to get here.
Again we've got enhanced warning arrangements in place with Civil Defense.
So you might feel a long the earthquake in that case
but the goal is to give official warnings through official channels for that one.
So you should start getting a little more concerned if we hear of a really big
earthquake off the coast of Tonga or somewhere like that.
Let's talk local sources now,
these are the earthquakes that really have the potential to cause tsunami.
It's the same sort of source it's this plate boundary all the way around the Pacific,
but the bit of it that's offshore from Kaikoura, past Wellington, past Napier, up through East Cape
Tsunami from there are less than an hour away,
they could just be minutes away depending on where you are.
So in that case it's all about 'Long, strong, get gone'.
So if the earthquakes longer than a minute or it's strong enough that it's hard to stand up
you need to evacuate immediately to high ground or inland.
Don't wait for any official warning. The fastest and best warning is that earthquake itself.
and because it's so close, that's also the source where we could
get the biggest wave.
Right, an earthquake lasting more than a minute.
Now for people who went through the Christchurch earthquakes,
they said they seemed to last for ages
but it's really hard to have a perspective on time when you're in the moment.
Yeah so in Wellington where I am now, we felt the Kaikoura earthquake for at least a minute
whereas actually in Christchurch those quakes we're less than a minute
But yeah they feel like forever, for sure.
Okay hey that is so interesting, thanks for your time and your help this morning.