Landslides deadlier than earthquakes
Landslides are significantly more dangerous than earthquakes, according to an analysis by GNS Science. It had identified 1800 fatalities over the past 160 years caused by landslides or slips, which is significantly more than earthquake casualties over the same period, said Jo Horrocks, the Earthquake Commission’s chief resilience and research officer.
The two deadliest earthquakes since 1860 – the Christchurch and Napier quakes – killed about 440 people between them.
Landslides cost the country an average of $250 million to $300m each year, she said.
The cost of landslides and slips often get rolled into damage caused by storms or floods. A weather event in Napier on November 9-10 last year for example, is listed by the Insurance Council as a ‘‘flood’’, even though several homes were hit by landslides.
‘‘The whole bank just came down,’’ said Napier artist Freeman White. The landslide stoved in a wall of his house, shunted his woodburner and damaged a bathroom wall.
He was on his roof clearing gutters and had to flee. Then a second slip came down. ‘‘If I’d been in the wrong place, I’d have been dead,’’ he said. ‘‘It was so touch and go. My wife thought I was dead.’’
‘‘The landslides in Napier demonstrated again that landslips are a major risk to people and property in New Zealand, which we need to understand and manage,’’ said Horrocks.
And not all of them are triggered by floods or earthquakes. A January 23, 2019, ‘‘debris avalanche’’ on the coastal cliffs at Cape Kidnappers, Hawke’s Bay, seriously injured two tourists and was witnessed by many people.
‘‘I could see a big part of the cliff start to part … and start sliding down and I thought, ‘Oh, s..., it’s really collapsing,’’’ said Jungho Son of South Korea.
‘‘In the ocean, I still could hear the rumbling sound, because it was still going . . . I literally thought I couldn’t make it . . . because I was [getting hit] by rocks . . . but it stopped.’’
‘‘There was no discernible trigger for the debris avalanche, with no seismicity and limited rainfall recorded in the week prior,’’ concluded a GNS report on the incident. Blame was laid on ‘‘cliff material weakening through time’’.
The Department of Conservation now actively discourages people from walking near the sea cliffs.
‘‘Gravity always wins,’’ warns a 2006 Te Ara Encyclopaedia of NZ article on landslides. ‘‘Compared to many other countries, New Zealand has a high number of landslides.’’
Mountain ranges are still being uplifted and feature steep slopes. Rock is weakened by folding and faulting. Elsewhere, much of the land is hill country formed by rivers cutting into soft and clay rock. The soils are weak because they are derived from volcanic ash or loess. Throw in high rainfall and earthquakes and the risks increase.
To address these dangers, a National Landslide Database is being created to ‘‘capture all current and future landslide information from local and regional councils, Crown entities and geotechnical consultants’’.
Many organisations –including GNS, EQC, NZ Transport Agency, KiwiRail and local councils – hold valuable information on landslides, but no single entity has had overall responsibility for managing this information.
It will become an ‘‘asset for any organisation involved in planning housing and infrastructure in New Zealand’’, said EQC’s Horrocks.