Earthquake upgraded to 7.8

Gerry Brownlee has informed media that Geonet Science have upgraded Monday’s initial earthquake from 7.5 to 7.8, making it officially a whopper.

This makes it a similar size to the largest shocks recorded in New Zealand:

  • M 7.8, Dusky Sound, 15 July 2009The earthquake which struck the Fiordland region in the evening of Wednesday 15 July was the biggest since the Buller and Hawke’s Bay earthquakes of 1929 and 1931.
  • M 7.8, Hawke’s Bay, 3 February 1931The 1931 Hawke’s Bay earthquake caused the largest loss of life and most extensive damage of any quake in New Zealand’s recorded history.

That last one, from 1855, caused extensive damage and ground upheaval around Wellington.

This upgrading has followed recalculations of what was a complex combination of shocks. It will lead to upgrading of probability predictions of more and larger after shocks.

USGS had always rated it at that: M7.8 – 53km NNE of Amberley, New Zealand

The November 13, 2016 M 7.8 earthquake in North Canterbury, New Zealand, occurred as the result of shallow oblique-reverse faulting on or near the boundary between the Pacific and Australia plates in South Island, New Zealand. 

The complexity of the event, involving a main energy release delayed by about 40 s, combined with an early aftershock distribution extending about 150 km to the north-northeast of the mainshock, suggests the potential for triggered slip on the Pacific:Australia subduction zone interface.

This matches current Geonet explanations that the initial quake, which last for 30-40 seconds probably triggered and was followed by a chain reaction moving north east affecting multiple fault lines, now thought to be at least 6.

This map shows revised understanding of that fault system in relation to the shocks.

kaikoura-earthquake-faults-e1479265716143

The Kaikoura earthquake

Commonly large earthquakes break a single fault in the earth’s crust and aftershocks occur in a focused area around that fault as the crust adjusts to the movement. Monday’s event was much more complicated. The location of aftershocks and field observations of where the crust has broken (see map) indicate that earthquakes occurred on more than one fault.

Already, it looks as though parts of well-known faults such as the Hundalee, Hope and Kekerengu Faults have ruptured (broken) and also parts of less well-known or unknown faults around Emu Plains, The Humps Fault zone and Waipapa Bay have also ruptured. Such a sequence of earthquakes occurring on different faults in such a short time-frame has not been witnessed before in New Zealand. There will be many types of data and days of collection and analysis required to make sense of what happened and what the implications of this earthquake sequence are for the future.

The Spinoff has more explained by GNS Science geologists Nicola Litchfield in This stunning map shows that six faults – at least six – ruptured in the big Kaikoura quake

That’s a lot of faults.

Yes. Usually you expect one big earthquake on one big fault, but like in the Christchurch earthquakes, and in fact we saw this in Edgecumbe in 1987, we often get these complex ruptures, where multiple faults rupture in the one earthquake. But this was a bit of a surprise, as to how many and which ones have ruptured.

Do we know which one went first?

The earthquake started in the south. So the big red dot, the M7.5, that’s the epicentre of the first earthquake, so we’re pretty certain that the faults just to the north of that – the Emu Plains, the Humps fault, the Hundalee fault – went first, then it continued up the coast, up to, a little bit on the Hope fault but particularly the Waipapa Bay and the Kekerengu fault went second.

I certainly felt that in Wellington: when I woke up I felt two parts to the earthquake, and we think that’s what happened, we had a southern part and then a more northern part.

I think the first part, the southern part, was about 30 seconds, and then followed immediately afterwards by the ruptures up to the north. Most people would consider that just one big long earthquake.

Does each one of those ruptures cause its own aftershocks?

Yes, you can see the cloud of aftershocks that have happened and they’ll be focused around all those faults.

More to come on this no doubt, for those who are interested in the details.

In the meantime a road from the south to Kaikoura has been opened, but is open to ‘army grade 4 wheel drive vehicles’ so it is not open to the public.