About 150 houses on both sides of Christchurch's Port Hills will have to be abandoned, a University of Canterbury geologist says.
David Bell told The Press yesterday that 100 houses would definitely not be reoccupied, including some at the top and bottom of collapsed cliffs in places such as Sumner and Redcliffs.
"It'll probably ... be 150 [all up]."
The city council has declared almost 600 houses on the Port Hills off limits while engineering consultants, including two international experts, assess the rockfall danger.
The final number of houses to be condemned will be decided by the Government and will depend on what remediation work can be done.
The consultants' report, which will go to the Government for a decision, is thought to be weeks away.
Bell, who gave a public lecture on earthquake-related rockfalls and slope stability last night, said a "goodly portion" of hillside suburbs could be at significant risk from rockfall.
Engineering geology lecturer Dr Marlene Villeneuve, who also spoke last night, said the cliff failure in some places was too great for remediation work to be viable.
"Trying to design something to keep that cliff up, to keep those houses up there, is just not [feasible]."
Large-scale engineering work, including rock barriers or earth embankments, could protect houses "but then we come to the question of who's going to pay."
Violent ground shaking in the February 22 earthquake hit the Port Hills harder than experts expected, collapsing 6000-year-old cliff faces and creating fissures hundreds of metres long, which tore houses apart. The June 13 quakes caused further damage, hitting Sumner, Scarborough, Taylors Mistake and Lyttelton harder than February's shake.
Ground acceleration on the Port Hills in the September quake had been 25 per cent of gravity and afterwards many hillsides were declared safe.
However, scientists had no idea that February's quake would be so close and shallow. Ground acceleration on the Port Hills during February's quake was almost 10 times greater than September, at 2.25 times that of gravity.
Bell said the figure was mind-blowing.
"We didn't get it right – and I haven't met anybody yet who has said we did, or would have predicted this ... We did not expect 15 metres in one go to fall off a cliff face, when that cliff had been there for 6000 years and showed nothing apart from the odd rock falling off it."