Could this slow movement actually be due to movement of the upper plate over a plastic state of the lower crustal material? Could this be due to movement of vicious magmatic material at depth? Suppose there was a change of modulus of elasticity? There is a recent study that showed that ketchup flowed suddenly from bottles due to a change in viscosity. The study indicated that "shear thinning" was the change that caused the behavior to be so different. Could the same thing be happening here, that something is changing the properties of shear for a period of time, allowing slippage to occur?

• Shear Mystery