THE Earth’s most active zone of mantle seismicity arises from the subduction of the Pacific plate at the Tonga trench(1). 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 are is separated by the V-shaped Lau basin from a remnant are, the Lau ridge, located at the eastern edge of the Australian plate(2). 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 similar to 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 similar to 240 mm yr(-1). The extraordinary seismic activity of the subducting slab is probably related to this unusually rapid subduction.