‘Megathrust’ quake risk is real, if remote

Just when you thought it was safe to go back to the water, an American geophysicist has built a computer model of a "megathrust" earthquake which would send tsunami waves 12 metres high crashing against the east coast from Hawke’s Bay to Christchurch.

Steven Ward from the University of California has been studying the Hikurangi subduction zone, running offshore north-east from Marlborough, where the Pacific and Australian tectonic plates collide.

Ward’s computer simulation of a very large offshore earthquake demonstrates how tsunami could engulf the eastern shorelines of both the North and South Islands, with very little warning – from 10 minutes to an hour.

Reporting of Ward’s work has been decried on social media as alarmist and scaremongering. Essentially, it is also old news.

Surveys by the underwater research vessel Tangaroa from 2011 revealed evidence of more than 150 quake-induced submarine landslides and many active faults in a canyon in Cook Strait, just 70km off the Marlborough Sounds

Geologists since the 1990s have been telling us that a series of concentric former beachline terraces at Turakirae Head, southeast of Wellington, also provide evidence of previous "uplift" earthquakes, strong enough to lift land out of the sea.

Radiocarbon methods suggests that there have been at least four such events in the last 7000 years. That may seem like a reassuringly long time, except that the most recent two happened in only the last 600 years.

One of them is well documented – the magnitude 8.2 Wairarapa earthquake of 1855 which dashed buildings to the ground in the young settlement of Wellington, raised land out of the sea along the its waterfront, and sent tidal waves over Miramar Peninsula.

The story of Hao-whenua was told to the early 20th century Pakeha ethnologist Elsdon Best by Wairarapa Maori who informed him there were formerly two entrances to Wellington Harbour – the current entrance and one where the suburb of Kilbirnie now stands.

Best was sceptical of the story because he could not fathom how an earthquake which lifted the seabed could be called a ‘‘land swallower’’. He apparently did not consider that the name might refer not to the quake, but to the subsequent tidal waves which swept in from the sea.

In modern times, while there is no immediate reason to be alarmed, no-one should turn a blind eye to the deadly potential of an offshore quake which, at possibly magnitude 9, could be much stronger than the Kaiko¯ ura earthquake of 2016.

And no-one should blame scientists, or accuse them of being alarmist, when they simply report the evidence before them. If we choose to live in this dynamic and unpredictable environment, we must be aware of its history and what it is capable of doing to us.