The Fiordland Earthquake and Tsunami, New Zealand, 21 August 2003

Abstract

The M_W7.2 earthquake at 12:12 UT August 21 2003 was the largest shallow earthquake in New Zealand for many years. GPS and seismological data are consistent with the earthquake rupturing the plate interface near the coast of Fiordland, a mountainous and largely unpopulated area in the southwest corner of the South Island. Fortunately, because of the remote location, the earthquake resulted in few injuries and relatively minor damage to property. However the earthquake triggered many landslides on the steep slopes that typify Fiordland. The fall of one landslide into Charles Sound caused a small local tsunami that reached a maximum of 4–5 m above high tide mark on the opposite shore. A small tsunami was also generated on the Tasman Sea coast, which was recorded on tide gauges at Jackson Bay (300 mm peak-to-trough) 190 km northeast of the epicentre, and at Port Kembla (170 mm peak-to-trough) on the east coast of Australia. Spectrogram analyses are used to demonstrate that the observed tide-gauge signals are best explained by the excitation of local resonances by the trans-Tasman tsunami. Modelling of the tsunami indicates that the wave amplitudes are compatible with estimates of offshore coseismic deformation of close to 0.5 m vertically as indicated by preliminary modelling of post-earthquake GPS data, and that the tsunami arrival times are compatible with the timing of the mainshock.