Schematic diagram of a starquake driven by spin-down, with time t on the horizontal axis and rotation rate Ω on the vertical axis. In state “a”, the star is relaxed. From “a” to “b” it spins down, with its crust deforming and building up stress. At “b”, the crust reaches a critical stress and fractures, the moment of inertia decreases, and the star suddenly spins up to a new relaxed state “c”.