Historical reports of an earthquake in Tonga in 1865 November identify it as the only event from that subduction zone which generated a far-field tsunami observable without instruments.
Run-up heights reached 2 m in Rarotonga and 80 cm in the Marquesas Islands. Hydrodynamic simulations require a moment of 4 x 1028 dyn cm, a value significantly larger than previous
estimates of the maximum size of earthquake to be expected at the Tonga subduction zone. This warrants an upwards re evaluation of the tsunami risk from Tonga to the Cook Islands
and the various Polynesian chains, which had hitherto been regarded as minor.
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