By Joshua Franklin and John Miller
ZURICH (Reuters) - A fresh mudslide swept into a remote mountain town in eastern Switzerland on Friday where an earlier huge landslip this week dumped rock, rubble and mud meters high and left eight people missing.
In the latest slide, a river of mud poured through tiny Bondo in the eastern Grisons canton, near the Swiss-Italian border, television images showed.
Rescuers continued their search for eight hikers from Switzerland, Austria and Germany reported missing since the initial landslide above the town on Wednesday.
Police in Grisons said the new slide hit in late afternoon and that some residents who had been allowed to return home had to be evacuated again. Nobody was injured, police said, but the slide smashed equipment that was being used to clear debris from the previous slide.
Melting permafrost due to warming temperatures is likely to be one of the factors behind this week's disaster, said Marcia Phillips, group leader for snow and permafrost at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL).
Such a cataclysmic event had probably been in the works for thousands of years as changes in the permafrost, rock structure, water content and pressure above Bondo conspired to destabilize the 3,369-meter (11,053 ft) Piz Cengalo mountain, Phillips said.
"Permafrost was one of several influencing factors, but not the only one influencing this kind of huge event," she told Reuters. "This week was a very large event, exceptionally large."
Smaller but still potentially dangerous slides may become more common as permafrost thaws with rising air temperatures, she said.
The average temperature in Switzerland has risen about two degrees Celsius since government measurements began in 1864.
"It's more likely that the smaller events get more common, not the larger ones," Phillips said.
Up to around -1.5 degrees Celsius (29.3°F), she said, permafrost had a stabilizing function.
"When the ice and the permafrost get warmer than that, it no longer stabilizes the rock," she said. "You then have a problem of cohesion between the rock and the ice. This can lead to instabilities."
After Wednesday's slide, mountain rubble and mud piled up as high as a 15-storey building in places. The initial rockfall kicked off tremors equivalent to a 3 magnitude earthquake, police said.
Helicopters equipped with heat sensors and technology to detect cell phone signals searched for the missing, as did dog teams and teams of 120 people.
The mountains surrounding Bondo have been hit by rock slides before, including a smaller one in 2011 that prompted a 4.5 million Swiss franc ($4.7 million) flood-protection system to be installed, Swiss broadcaster SRF reported.
Fatal landslides elsewhere in mountainous Switzerland are also rare but not unheard of. In 1806, a slide buried the village of Goldau south of Zurich, killing 457 people.
(Reporting by John Miller, Joshua Franklin and Michael Shields Editing by Richard Balmforth)