New Zealand’s megathrust earthquake problem


Scientists are drilling into the tectonic plate boundary off the coast of the North Island. They are trying to find out how close it is to rupturing and looking for clues to indicate levels of tectonic stress.

This is not something anyone should be surprised about. As a nation straddling an active tectonic plate boundary with both strike slip and dipping tectonic plate interface, we are subject to a range of future seismic hazards that at the moment are low risk, high consequence. Those unfortunately are drifting gradually towards high probability/high consequence – the longer it takes for one to happen the worse it will be when it happens.

This is why much research is underway both on land and at sea to understand the hazard posed by the tectonic plate boundary. Not only that, but to also understand how the tectonic interface works, whether there are geophysical or geochemical changes happening in the rock strata that might indicate how much time we have left.

A megathrust earthquake is one involving a large segment of dipping tectonic plate boundary that ruptures at once. Examples include the Tohoku magnitude 9.0 off the coast of Japan in 2011, the 2004 Sunda earthquake that unleashed a magnitude 9.3 earthquake. New Zealand has not had a megathrust earthquake in recent centuries.

This would be a devastating event if it happened in New Zealand. The energy released in the event would be immense – possibly 2000 times more than the It would involve the Hikurangi Trench rupturing from off the east coast of the South Island up past East Cape. The potential magnitude of the earthquake could be anywhere between M8.5 and M9.0. It would involve probably 5 minutes of sustained shaking – 3 minutes longer than the 2016 Kaikoura earthquake and comparable in length to the 2010 Chilean M8.8 event.

The earthquake would very likely trigger a tsunami. The major megathrust earthquakes of the last century have all triggered destructive tsunami – the worst in terms of casualties and overall damage without doubt being the 2004 Sunda event and the 2011 Tohoku event.

The message for coastal areas of New Zealand since the Kaikoura earthquake has not changed: LONG (lasting more than a minute) AND STRONG (can’t stand up)? GET GONE. And for all people caught in an earthquake that is not over in a few seconds and especially if at that point it seems to intensify, the message is “DROP (to your knees). COVER (under a doorway or desk). HOLD (on)”.

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