  Crustquake events may be connected with both rapid spin-up `glitches' within
the regular slowdown of neutron stars, and high-energy magnetar flares. We
argue that magnetic field decay builds up stresses in a neutron star's crust,
as the elastic shear force resists the Lorentz force's desire to rearrange the
global magnetic-field equilibrium. We derive a criterion for crust-breaking
induced by a changing magnetic-field configuration, and use this to investigate
strain patterns in a neutron star's crust for a variety of different
magnetic-field models. Universally, we find that the crust is most liable to
break if the magnetic field has a strong toroidal component, in which case the
epicentre of the crustquake is around the equator. We calculate the energy
released in a crustquake as a function of the fracture depth, finding that it
is independent of field strength. Crust-breaking is, however, associated with a
characteristic local field strength of $2.4\times 10^{14}$ G for a breaking
strain of $0.001$, or $2.4\times 10^{15}$ G at a breaking strain of $0.1$. We
find that even the most luminous magnetar giant flare could have been powered
by crustal energy release alone.
