Hikurangi Subduction zone

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tdgeek

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Topic # 230636 6-Mar-2018 09:03

Interesting piece, a potential 9.0 with a tsunami that is the real danger

https://www.stuff.co.nz/national/nz-earthquake/101989915/megathurst-quake-could-bring-12mhigh-waves-to-nzs-east-coast

and

https://www.eqc.govt.nz/sites/public_files/1600-tsunami-hazard-Hikurangi-subduction-zone-interface.pdf

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Reply # 1968697 6-Mar-2018 09:19

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Here you go:

Paleotsunami database map

As well as the Hikurangi, there's also the Kermadec Trench.

There's also some scary Paleotsunami evidence with large run up on the SE coast of Australia. Problem with all this is that unless there's some other evidence dating an event (ie quake) that might have been responsible, there's no way of knowing where or what the event was that set off the tsunami - local or distant quake, volcanic, or even asteroid strike etc.

DarthKermit

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Reply # 1968698 6-Mar-2018 09:19

To all my friends on the east coast: goodbye, it was nice knowing you.

tdgeek

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Reply # 1968738 6-Mar-2018 09:27

DarthKermit:

To all my friends on the east coast: goodbye, it was nice knowing you.

Might well be true, that. The wall of water in Japan was horrendous, it went a few km inland I think. If the coastal towns were near hills I assume it will just bank up for some time

MikeAqua

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Reply # 1968775 6-Mar-2018 10:22

This an interesting one for us. We are in a 10th floor apartment in Welly, on the flat a couple of blocks from the harbour.

I tried it one day walking down all the stairs and up the street to a safe height and it took me just over 10 minutes. In a real evacuation with stair and street congestion I think it would take a little over 15 minutes to exit the building and get to a safe elevation. I know that exiting our 7th floor office in a fire drills takes about 5 minutes.

That puts me in a real dilemma. A tsunami could arrive during that time and catch us on the flat.

Assuming the building is still intact: Should I stay or go?

Mike

tdgeek

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Reply # 1968778 6-Mar-2018 10:29

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MikeAqua:

This an interesting one for us. We are in a 10th floor apartment in Welly, on the flat a couple of blocks from the harbour.

I tried it one day walking down all the stairs and up the street to a safe height and it took me just over 10 minutes. In a real evacuation with stair and street congestion I think it would take a little over 15 minutes to exit the building and get to a safe elevation. I know that exiting our 7th floor office in a fire drills takes about 5 minutes.

That puts me in a real dilemma. A tsunami could arrive during that time and catch us on the flat.

Assuming the building is still intact: Should I stay or go?

Stay. The wall of water once it hits land is a flowing wall, not even flowing fast. In Banda Aceh, it was literally at jogging speed, and just kept coming. In japan it was like that at times as it spreads out, probably would not be more then a solid flowing river. It would speed up in town as the land mass is reduced. So it should not be a massive disaster movie type wave.

MikeAqua

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Reply # 1968781 6-Mar-2018 10:38

tdgeek:

MikeAqua:

This an interesting one for us. We are in a 10th floor apartment in Welly, on the flat a couple of blocks from the harbour.

[blah blah blah]

That puts me in a real dilemma. A tsunami could arrive during that time and catch us on the flat.

Assuming the building is still intact: Should I stay or go?

Stay. The wall of water once it hits land is a flowing wall, not even flowing fast. In Banda Aceh, it was literally at jogging speed, and just kept coming. In japan it was like that at times as it spreads out, probably would not be more then a solid flowing river. It would speed up in town as the land mass is reduced. So it should not be a massive disaster movie type wave.

That's what we had decided on before Kaikoura

Unfortunately during the Kaikoura quake, the evac alarm was triggered, which may or may not indicate a fire.

Makes the decision a little trickier, although our building is a better case than most for fire (steel/concrete with exterior breeze-ways instead of halls and also multiple sets of exterior emergency stairs (I really should learn where the additional stairs are).

Mike

tdgeek

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Reply # 1968784 6-Mar-2018 10:41

MikeAqua:

tdgeek:

MikeAqua:

This an interesting one for us. We are in a 10th floor apartment in Welly, on the flat a couple of blocks from the harbour.

[blah blah blah]

That puts me in a real dilemma. A tsunami could arrive during that time and catch us on the flat.

Assuming the building is still intact: Should I stay or go?

Stay. The wall of water once it hits land is a flowing wall, not even flowing fast. In Banda Aceh, it was literally at jogging speed, and just kept coming. In japan it was like that at times as it spreads out, probably would not be more then a solid flowing river. It would speed up in town as the land mass is reduced. So it should not be a massive disaster movie type wave.

That's what we had decided on before Kaikoura

Unfortunately during the Kaikoura quake, the evac alarm was triggered, which may or may not indicate a fire.

Makes the decision a little trickier, although our building is a better case than most for fire (steel/concrete with exterior breeze-ways instead of halls and also multiple sets of exterior emergency stairs (I really should learn where the additional stairs are).

Actually it would be M8 to M9, so it might depend how far away the epi centre was as to the EQ risk

frankv

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Reply # 1968829 6-Mar-2018 12:06

Very interesting.

Summarising the May 2008 NZ science: A big earthquake (mag 8-9) + tsunami is probably about due in the Wairarapa/Hawkes Bay/Poverty Bay/East Cape area (1880, 1947 => ~70 year cycle). The tsunami is unlikely to significantly affect NZ cities outside that region. It could be very bad (5+m tsunami = 10+m inundation) for Napier, Hawkes Bay, or Gisborne. Or the whole S-E coast if the whole lot goes in one bang.

The Stuff article is (predictably) full of emotive words like "devastating", "breeding ground for tsunamis", "massive waves", "disaster". It says " waves ranging from five to 12 metres could strike almost the entire east coast of New Zealand" and "Ward said the the sites predicted to see the largest tsunami waves would be Christchurch, Wellington and Hawke's Bay", which seems to be at odds with the NZ science. In the worst case scenario, the NZ model shows a maximum 1-2m sea level rise around most of the coast from Stewart Island to Auckland and the Chatham Islands, corresponding to about 2-4m inundation height. Whilst it's not going to be pretty in Chch or Wn, they don't get anywhere near the "largest waves".

Presumably there's been more recent science to change the prediction from 10 years ago.

tdgeek

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Reply # 1968836 6-Mar-2018 12:15

frankv:

Very interesting.

Summarising the May 2008 NZ science: A big earthquake (mag 8-9) + tsunami is probably about due in the Wairarapa/Hawkes Bay/Poverty Bay/East Cape area (1880, 1947 => ~70 year cycle). The tsunami is unlikely to significantly affect NZ cities outside that region. It could be very bad (5+m tsunami = 10+m inundation) for Napier, Hawkes Bay, or Gisborne. Or the whole S-E coast if the whole lot goes in one bang.

The Stuff article is (predictably) full of emotive words like "devastating", "breeding ground for tsunamis", "massive waves", "disaster". It says " waves ranging from five to 12 metres could strike almost the entire east coast of New Zealand" and "Ward said the the sites predicted to see the largest tsunami waves would be Christchurch, Wellington and Hawke's Bay", which seems to be at odds with the NZ science. In the worst case scenario, the NZ model shows a maximum 1-2m sea level rise around most of the coast from Stewart Island to Auckland and the Chatham Islands, corresponding to about 2-4m inundation height. Whilst it's not going to be pretty in Chch or Wn, they don't get anywhere near the "largest waves".

Presumably there's been more recent science to change the prediction from 10 years ago.

When Banda Aceh hit (I think it was that one not Japan) we in ChCh got a small ripple wave from it, furthert up north it was more, but still small. Id have thought if its local and a genuine uplift, we would get it quite bad. But, as its local maybe not that much of a wall? Or at leaat a very long wall that Japan had, that seemed to never end

Fred99

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Reply # 1968838 6-Mar-2018 12:19

frankv:

Presumably there's been more recent science to change the prediction from 10 years ago.

First the 2004 Tsunami, then Japan heightened awareness about the probability of megathrust quakes in subduction zones, and the likely impact.

When "where next?" is the question, then off the E coast of NZ and the Cascadia region off the West coast of N America fit the model.

frankv

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Reply # 1968847 6-Mar-2018 12:29

MikeAqua:

This an interesting one for us. We are in a 10th floor apartment in Welly, on the flat a couple of blocks from the harbour.

I tried it one day walking down all the stairs and up the street to a safe height and it took me just over 10 minutes. In a real evacuation with stair and street congestion I think it would take a little over 15 minutes to exit the building and get to a safe elevation. I know that exiting our 7th floor office in a fire drills takes about 5 minutes.

That puts me in a real dilemma. A tsunami could arrive during that time and catch us on the flat.

Assuming the building is still intact: Should I stay or go?

Bear in mind that the predicitions are for earthquakes centred somewhere between Cape Turnagain and East Cape... lots of Wairarapa coast would be flooded long before the tsunami reached Wellington.

As well, Wellington CBD and Hutt Valley is a special case, being situated on a harbour with a narrow entrance. The rate at which the water level in the harbour will rise will be limited by how fast water can get in through the harbour entrance. My wild-ass guess would that you would have more than 15 minutes after Seatoun was inundated before the CBD was endangered.

surfisup1000

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Reply # 1968851 6-Mar-2018 12:36

Talking in geological time scales these big events occur all the time.

In human timespan, it could fine for the next 10,000 years or we could get hit tomorrow.

It is of interest how the wave goes around the east cape and hits tauranga, although 4 meter wave isn't really going to do a huge amount of damage .

surfisup1000

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Reply # 1968852 6-Mar-2018 12:36

Talking in geological time scales these big events occur all the time.

In human timespan, it could fine for the next 10,000 years or we could get hit tomorrow.

It is of interest how the wave goes around the east cape and hits tauranga, although 4 meter wave isn't really going to do a huge amount of damage .

tdgeek

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Reply # 1968853 6-Mar-2018 12:36

Without being alarmist, the Gisborne quake at 70 year recurrence is due, was last in 1947. The Alpine Fault at every 300 years is also due. With all the recent activity from Chch, Seddon, Kaikoura and others, who knows what single big quake may do to the many faults we have. An uplift will be M8 to M9, Alpine Fault they reckon 8 to 8.5

Fred99

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Reply # 1968855 6-Mar-2018 12:38

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Niwa simulation from 2008, based on what would happen in the event of another "1854 quake" (so not a megathrust on the Hikurangi)

Wellington will be cut off - the airport has been flooded, all roads in and out probably impassable. That's extra reason to have an emergency kit - help won't be coming immediately.

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