I recently listened to an interview on Radio NZ with Chris Goldfinger, professor from Oregon State University, on super-earthquakes. You can listen to it online, but here is the gist of the discussion.
Historically, geologists have thought that mega-earthquakes (Mag. 9), like the ones that occurred in Japan in 2011 and Sumatra in 2004, were thought to only occur in Chile and Alaska because this is where they have happened in the recent past. But human lifetimes and memories are extremely short when compared with time on a geological scale. When the Japan and Sumatra earthquakes shook the world seismologists had to rethink their theory as most of them did not expect anything so big in either place.
It’s worth noting that a couple of Japanese scientists did forecast something big for Japan. Goldfinger recalls a meeting in 2005, in which the last slide of a particular talk from a Japanese geologist said we should expect a Mag. 9 earthquake in North-East Japan sometime soon. There was silence, then polite applause then the next speaker. No-one thought much about it.
So where else might a giant earthquake strike? The world’s biggest earthquakes will always come from a subduction zone. A subduction zone is one of three major plate boundaries where one plate, usually an oceanic plate, dives under another, usually a continental plate. The Pacific Rim, which includes New Zealand, is a continuous subduction zone, also known as the ring of fire.
In New Zealand, the subduction zone is the Hikurangi Trench which runs from about 100km offshore of the eastern part of the top of the North Island, all the way down to the coast off Kaikoura in the South Island. According to Goldfinger, there is no subduction zone in the world that can’t produce a Magnitude 9 earthquake. So Hikurangi is a serious contender.
The answer is therefore, yes. New Zealand could get a Magnitude 9 earthquake and not only that, there’ll be a tsunami to go with it. All subduction zones are submarine, so anytime you’ve got a major earthquake on a subduction zone, there’s going to be a tsunami.