The slip that brought rocks crashing down on the main road into New Zealand's capital city last week was on a slope modified in the 1950s and that's got scientists thinking.
There hadn't been much rain or an earthquake when the slip in Ngauranga Gorge on Tuesday disrupted rush-hour traffic and reminded everyone of the fragility of infrastructure.
Anthropogenic, or man-made slopes, modified by cutting and filling, began as soon as people started settling in New Zealand, and they got bigger from the 1950s onwards when increasing demand for homes and infrastructure demanded much larger scale earthworks, GeoNet's scientists say.
The scale of the "landslide problem" and the level of risk to critical infrastructure from man-made slopes is still not well known in New Zealand though research is ongoing, they say.
The Ngauranga slip closed five of the six lanes on the main road north of Wellington, including all three southbound lanes.
Between 90 and 100 cubic metres of debris hurtled down the hillside at about 1.8m/sec.
About 400 slope failures occur on Wellington City Council's road network per year and many are on man-made slopes.
The Ngauranga slip is the second high-profile landslide that has happened on modified slopes in Wellington the past three months, GeoNet says.
The other is the fill slope failure that occurred adjacent to Halifax Street in the suburb of Kingston on April 6.
No modified Wellington slopes have been tested under strong earthquake ground shaking, as the ground motions experienced since their construction have been too low to trigger failure.
It is known that during the magnitude 7.8 Kaikoura earthquake last year many cut and fill slopes along State Highway 1 and the railway line failed in the Upper South Island, GeoNet said.
GeoNet says landslides do not always need a trigger, such as heavy rain or an earthquake to occur.
Landslides are also complex and a lot depends on location, gradient and base geology.