Brian,

You could be correct that momentum of a rupture could cause a quake to rupture through the creeping section. But that section is pretty long. I don't know whether the process is "momentum", or if, say, a 20 m slip event would cause stress changes along the tip of the rupture.But, "momentum" works for me to communicate a rupture powering its way through.

I don't know enough about this; too bad we don't have anyone posting on this who works on these problems (Vidale would be the closest, but he has not posted a science commment here for a long time).

Chris