What impact did the Samoa tsunami have on New Zealand?

A powerful magnitude 8.0 earthquake ruptured the seafloor south of Samoa on 30 September 2009, unleashing a destructive tsunami on Samoa, American Samoa, and northern Tonga (Niuatoputapu). Its impact on distant New Zealand shores was picked up by a sea-level monitoring system operated by NIWA and other agencies. The system showed that peak waves arrived between 1–2 and 13 hours after the arrival of the first tsunami waves, depending on locality.

This plot shows sea level, with the tidal component removed, at 12 sea-level gauges around New Zealand in the 30 hours following the tsunami-generating earthquake (locations of gauges are shown in Table 2 below). The sea level is measured in millimetres at minute intervals and the lines track the peaks and troughs of the waves.

The tsunami was recorded along the entire east coast of New Zealand and on most of the west coast. It arrived at various stages of the day, from 4.2 hours after the earthquake at Kaingaroa on the northern side of the Chatham Islands, to about 8 hours in Westland at Jackson Bay and Charleston.

Samoa tsunami data (XLS 30 KB)

Peak waves

As New Zealand was not in the direct line of the rupture, and being over 2,600 km away, the waves had diminished by the time they reached our shores. The largest was at Kaingaroa, at nearly 0.9 m from crest-to-trough (but less than 0.4 m height above the predicted tide at the time). This peak wave arrived more than one hour after the first tsunami wave at Kaingaroa. Peak waves at Sumner (Christchurch) and Port Taranaki didn’t arrive until about 12-13 hours after the first tsunami waves.

Samoa tsunami data (XLS 30 KB)

As shown by these examples, waves from a far-off tsunami source can take some time to build up to a peak wave height. This is due to reflections off continental shelves, coastal headlands, and from much slower-travelling waves that hug the coast but diminish more slowly.

The delay in arrival of peak waves has important implications for managing the stand down of warnings and advisories. The heights of the peak waves experienced after the Samoa tsunami, while relatively small, can pose a hazard for navigation, causing strong currents in harbours and strong surging up and down beaches.

Local effects

By stripping out the dominant tide component, the sea-level plots reveal substantial differences in the way the tsunami waves behaved regionally and locally. Each locality has its own pattern of waveform, as incoming waves interact with the regional and local seabed contours and local coastline shape and orientation.

These local characteristics give each area its own resonant wave period (like the regular to-and-fro movements of water sloshing in a bath when paddled). This can increase the size of the incoming tsunami waves if the local resonant wave period coincides with the tsunami wave period. It’s important, therefore, to understand local conditions in order to predict what impacts a tsunami may have in different areas.

Sea-level monitoring system

The sea level gauges are sited on the shoreline around New Zealand’s coast and on some islands to collect accurate measurements of sea level and tides. These are used to monitor coastal hazards, such as storm surge and tsunami, and study longer-term coastal processes, including El Niño effects and sea-level rise. These gauges have been tsunami-enabled to record and store sea-level measurements quickly at 1-minute intervals to better describe the various waves that make up a tsunami, which is often at wave periods of 8 to 20 minutes. NIWA, port companies, regional and district councils operate various sea-level gauges which complement the operational near real-time monitoring undertaken by GeoNet (New Zealand’s geological hazard monitoring system).

For sea-level data updated daily or on demand, see our sea-level monitoring network

For sea-level data updated at 5-minute intervals, see GeoNet's tsunami gauge data 

Acknowledgement: Sea-level gauge data is regularly supplied to NIWA by agencies listed in Table 2 and we acknowledge their ongoing contribution.

For more information, contact: Dr Rob Bell, Principal Scientist Coastal Modelling, NIWA, rob.bell@niwa.co.nz.

Table 2: Location of source of sea-level gauge datasets