  Magnetar activity is generated by shear motions of the neutron star surface,
which relieve internal magnetic stresses. An analogy with earthquakes and
faults is problematic, as the crust is permeated by strong magnetic fields,
which greatly constrain crustal displacements. We describe a new deformation
mechanism that is specific to strongly magnetized neutron stars. The
magnetically stressed crust begins to move because of a thermoplastic
instability, which launches a wave that shears the crust and burns its magnetic
energy. The propagating wave front resembles the deflagration front in
combustion physics. We describe the conditions for the instability, the front
structure and velocity, and discuss implications for observed magnetar
activity.
