Alpine Fault rupture will be ‘largest earthquake in recorded history’

When the Alpine Fault ruptures Timaru may shake for at least two minutes, but wide-spread devastation in the district is not expected, councillors have been told.

Speaking to the Timaru District Council’s environmental services’ committee on Tuesday morning, Dr Tom Robinson, a senior lecturer from the University of Canterbury in the disaster risk and resilience team, showed modelling in the event of the fault rupturing – which could happen at any time.

While the confronting presentation showed the shaking was expected to be “damaging” to “heavily damaging’’ in the district, Robinson said the good news was, as Timaru is a long way from the fault, it “is not going to be the focus of an Alpine Fault event”.

“Chimneys will collapse, there will be liquefaction around Washdyke and damage, but we’re not going to see the same levels of damage as there will be on the West Coast,’’ he told the meeting.

READ MORE:
* Earthquake roadshow: Being prepared for when the big one strikes
* Higher risk of Alpine Fault quake highlights need for preparation on West Coast
* Alpine Fault quake unlikely to trigger tsunami in Timaru

Explaining the Pacific Ring of Fire and the plate boundary, Robinson said New Zealand was “incredibly complex tectonically speaking’’.

“We’re lucky in some respects – we’re one of only two places in the world where you can see the fault on land.’’

Robinson said research showed the last significant quake on the Alpine Fault was in 1717. The next severe earthquake on the Alpine Fault was likely to occur within the lifetime of “most of us, or our children’’.

Over the past 8000 years the fault had 28 earthquakes, and there was no reason to suspect that was suddenly going to stop, he said.

Patterns showed the fault suffered a major quake on average every 300 years.

Using this modelling, it should have ruptured in 2017, Robinson said.

“That leads us to believe there is a 75% chance it will rupture in the next 50 years.’’

The vast earthquakes on the fault in the past were about magnitude 8, he said.

“To put that in perspective a magnitude 8 earthquake would be vertical displacement, and the ground moving two to three metres.”

The intensity of shaking damage would be at MM07, meaning the entire South Island would feel it.

“A MM5 and above will mean every single New Zealander will feel the Alpine Fault rupture.

“In Sydney they will feel it. This will be the largest earthquake in recorded history.’’

It would be the kind of shaking “where you are very, very scared’’ and desks would jump, and loose roof tiles would fall, he said.

The direction of its rupture was also important, Robinson said.

“We assumed the epicentre will be somewhere around [Aoraki] Mt Cook.

“If it were to occur near Milford Sound it would be our worst case scenario.’’

This means all the quake’s energy will rupture north for about 400 metres, he said.

Councillors were showed an animation of how the earthquake would propagate up the coast showing waves.

It showed Timaru shaking for at least two minutes after the earthquake began.

“Two things we really worry about – the intensity and shaking,” Robinson said.

“Two minutes is a long time. A very, very long time.’’

The quake would also trigger secondary hazards and could trigger landslides which could block the river, form a quake lake, causing overspills, eroding channels, and eventually it would fail causing a dam break flood, he said.

“The secondary hazards are often far worse.’’

Roads throughout the South Island would also be blocked and there would be a loss of electricity, Robinson said.

However, Timaru, should fare well as the east coast had a “reasonably robust’’ network.

Councillor Allan Booth asked if Timaru being built on basalt rock was an advantage.

Robinson said while that meant the chances of liquefaction were “very low’’, the “bad news” was the hard rock meant the shaking would be felt more strongly.

“The shaking doesn’t last as long, but it will be stronger.’’

In Washdyke, he expected there would be liquefaction.

The council’s team leader and emergency management advisor Phill Mackay​ said six workshops around planning for the Alpine Fault quake had been held, and he expected a good framework to work on would be developed by the end of the year.

Councillor Owen Jackson moved that the council receive and note the data and Booth seconded it.

All councillors were in favour of the data.