That earthquakes release vast quantities of energy is widely accepted; however, the most commonly experienced component, radiated seismic energy, is a minor contribution to the total energy budget. The elastic rebound model for earthquakes recognizes that elastic strain energy does work displacing, deforming, and accelerating the crust, as well as causing frictional heating. In this paper we present an energy budget for dip-slip fault rupture in an extensional tectonic regime. A computational model of an elastic-plastic-viscous crust hosting a single fault, modeled as two surfaces in frictional contact, demonstrates contrasting energy flows between the hanging-wall and footwall fault blocks. Our analysis suggests that in the period leading up to an earthquake, the total strain energy contained within the crust decreases, although a local increase within the footwall at mid-crustal depths is observed. During an earthquake, the footwall is subject to an elastic rebound, whereupon uplift of the fault scarp is caused by a mid-crustal stress drop and elastic expansion that releases strain energy. In contrast, gravitational potential energy released from a subsiding hanging wall does work compressing the wider crust, particularly in the mid-crust at the fault tip. This has the unusual consequence of increasing strain energy throughout much of the upper crust during an earthquake. These counterintuitive energy flows suggest that extensional deformation is caused by stored gravitational potential and elastic strain energy, and not by the external tectonic forcing.