Those 4 people who upvoted allpower need to understand what permafrost actually is.
Because really, it's just ice without water. Bring it to a warmer place and it thaws. Not melts, thaws.
"Permafrost is soil that has been frozen year round for at least two years. (...) The upper layer of permafrost, or the active layer, sometimes thaws in the summer. Recently, the active layer of permafrost has been observed to be getting larger with time, which means more permafrost is melting each summer."
https://www.wunderground.com/resources/climate/melting_permafrost.asp
This is a scientific article and they call it both melting and thawing.
"The increase in the active layer doesn't mean there's more ground being frozen, it means more permafrost is melting seasonally, losing its distinction as "permanent." Permafrost is not only affected by climate change, but eventually will affect climate change itself by releasing the greenhouse gases it stores. (immense amounts of carbon and methane) (...) Researchers at the National Snow and Ice Data Center estimate that by 2200, 60% of the Northern Hemisphere's permafrost will probably be melted, which could release around 190 billion tons of carbon into the atmosphere. "
Because really, it's just ice without water. Bring it to a warmer place and it thaws. Not melts, thaws.
https://www.wunderground.com/resources/climate/melting_permafrost.asp
This is a scientific article and they call it both melting and thawing.