On Boxing Day, 26 Dec 2004, a large earthquake (M9.0) occurred off the west coast of Northern Sumatra at 00:59 UTC (12:59 NZST) at a shallow depth in the earth’s crust of only 10 km. The epicentre is shown in the seismic setting (see EMSC map) where subduction of the India Plate occurs beneath the Burma Plate. The release of energy produced a sea-bed surface rupture of over 1000 km in length north of the epicentre up to the Andaman Islands.
Movements of the seabed from models and measurements suggest the seafloor on the western side was uplifted by up to 5 m and shifted horizontally up to 11 m (USGS site), while on the eastern side of the rupture (facing Thailand and Sumatra), the seafloor dropped about 1-2 m. It is these vertical movements that gave rise to the tsunami waves that devastated several countries around the Indian Ocean with the tragic loss of over 280,000 people. Tsunami run-up heights of over 30 m were recorded in Banda Aceh (Sumatra) and up to 12 m in Khao Lak (Thailand). See the interview with one of the team members from NIWA on a Thailand reconnaissance survey in January 2005. The full nzsee_sumatra_tsunami_report.pdf on the Thailand survey describes the findings of the team and the implications for New Zealand.
The tsunami waves travelled around the world including the Pacific and Atlantic Oceans before dissipating. In New Zealand, the tsunami waves first arrived in the South Island about 16.5-17 hrs after the earthquake, having bent around Australia via the Southern Ocean.
The plots of the tsunami waves that reached New Zealand are shown in the two plots on the left; one for sites where the sea level is recorded at 1-minute intervals while a larger plot shows the tsunami for sites where sea level is recorded at 5-minute intervals.
The sites in the New Zealand open-coast sea-level network are shown on the map.
The processing behind the tsunami plots started by stripping out the local tides that dominate the sea-level record. Then a wavelet filter was used on the de-tided data to bring out the time-varying tsunami signatures between 2 and 64 minute periods. The tsunami waves measured at other port sites in New Zealand are available from the Mulgor Consulting website.
Acknowledgements: Sea-level data is supplied on a regular basis by various agencies: PrimePort (Timaru); Port of Lyttelton, Otago Regional Council, Northland Regional Council, National Tidal Centre (Adelaide), Tasman District Council, Westgate Transport (Port Taranaki); Hawkes Bay Regional Council, Environment Waikato, Antarctica NZ. Derek Goring developed the initial wavelet routines while at NIWA.