18 January 2021

Gravity always wins: in New Zealand landslides are more deadly than earthquakes

Posted by Dave Petley

Gravity always wins: in New Zealand landslides are more deadly than earthquakes

New Zealand is a country with an abundance of natural hazards – earthquakes, volcanoes, floods, tsunami, cyclonic storms and landslides, amongst others.  Of the geophysical hazards, it is earthquakes and volcanoes that attract most of the attention, driven by large events such as the Christchurch earthquake sequence, the Kaikoura earthquake and the Whakaari /White Island eruption.  Landslides are generally not considered to be such a major issue around the world, and are often dismissed as a secondary hazard.

Interestingly though, my colleagues at GNS Science have been looking the comparative impact of landslides and earthquakes through time in New Zealand.  The online news service Stuff has a good article about this work today.  The simple conclusion is startling:

Landslides are significantly more dangerous than earthquakes, according to an analysis by GNS Science.​

Interestingly the article provides some detail to this through some words from Jo Horrocks, the chief resilience and research officer of the Earthquake Commission (EQC), the national insurer against earthquakes (and landslides).  She noted that landslides cost the country an average of NZ$250 to 300 million (about £125-150 million) and that over the last 160 years they have claimed about 1800 lives.  This is considerably higher than the toll from earthquakes.

The aftermath of landslides in Christchurch, New Zealand

Gravity always wins – the aftermath of landslides in Christchurch, New Zealand

The reasons that they are under appreciated is that landslides tend to happen as frequent events from which the losses accumulate.  Huge clusters of landslides do occur – the Kaikoura earthquake was a fine example – but of course these are usually triggered by another event, such as an earthquake or a cyclone.  Very often it is the landslides that cause the losses.

I am sure that this effect is seen in many other countries as well.  The article in Stuff has a nice quote, taken from an article in the Te Ara Encyclopaedia of NZ :

Gravity always wins