THE Earth's most active zone of mantle seismicity arises from the subduction of the Pacific plate at the Tonga trench¹. It is not known why this slab generates so many more earthquakes than other subducting slabs worldwide. Above the subduction zone the active Tofua (Tonga) volcanic arc is separated by the V-shaped Lau basin from a remnant arc, the Lau ridge, located at the eastern edge of the Australian plate². The irregular and discontinuous magnetic lineations within the basin have proven difficult to interpret^3,4, and so the regional kinematic framework has been obscure. We report geodetic measurements of crustal motion within the Tonga-Lau system, which reveal the fastest crustal motions yet observed. The Lau basin is opening at a rate which increases northwards to a maximum of ∼160 mm yr⁻¹ No straining is observed within the northern Tonga ridge, suggesting that it comprises part of a rigid microplate. Convergence rates across the Tonga trench increase northwards to a maximum of ∼240 mm yr⁻¹. The extraordinary seismic activity of the subducting slab is probably related to this unusually rapid subduction.