The Canterbury earthquake sequence in New Zealand was characterized by high-energy earthquakes(1), a complex pattern of faulting and an extended series of aftershocks(2). The mainshock occurred in September 2010, yet the largest aftershock occurred 172 days later. Beneath the Canterbury region, the Hikurangi Plateau-a large igneous province that was subducted about 100 Myr ago(3)-remains attached to the crust, welded to greywacke rocks that were fractured during the earthquakes. Here we use three-dimensional seismic tomographic data to investigate the role this crustal structure played in the unusual Canterbury earthquake sequence. We identify a broad, 5-km-wide region centred beneath the surface break and coincident with the depth of maximum fault slip during the mainshock that is characterized by unusually low seismic P-to S-wave velocity ratios of 1.60. Yet before the mainshock this region exhibited ratios(4) of 1.71. We interpret the reduced velocity ratios as the signature of greywacke rocks that have been weakened by the fresh rupture front producing widespread cracking around the fault zone. We suggest that recovery of rock strength between the mainshock and largest aftershock could explain the long delay between the two events. In contrast to a common assumption in aftershock forecasts that crustal strength is constant, we conclude that energetic earthquakes can lead to widespread changes in the strength of Earth's crust.