That's one of the big questions. How could such a low coefficient of friction build up so much strain?

I'd have to re-read the article, but I think they said things got so slippery AFTER the fault started to move. There is another parameter called stiction, which is the amount of energy to get something moving. Perhaps the clay is sticky, but once moving is slippery.

Reminds me of non-newtonian fluids.

http://en.wikipedia.org/wiki/Non-Newtonian_fluid
http://www.youtube.com/results?search_query=non-newtonian+fluid&sm=3

Brian