Hawke's Bay faces out to the Pacific Ocean, which exposes it to tsunami triggered locally, regionally or from afar. Tsunami (pronounced tsu-na-mee) is a Japanese word meaning 'harbour wave'. It describes a series of fast travelling waves caused by large disturbances on the ocean floor, such as earthquakes, landslides or volcanic eruptions. In the deep ocean, tsunami pass almost unnoticed but they change dramatically as they approach shallower coastal waters - a 1-2 metre swell at sea can grow into waves over 30 metres high.
Previous impacts on Hawke's Bay
The few historical records of tsunami striking the Hawke's Bay coastline confirm that the area could be impacted. Damage in the past has been fairly minimal, partly because large stretches of coastline have been sparsely settled until comparatively recent times. However, the development of coastal communities now makes tsunami a greater threat.
The scarce details of past tsunami in Hawke's Bay are based solely on written European records of events. Localised damage has occurred - the result of earthquakes or undersea landslides. The subduction zone between the Pacific and Australian plates lies off the coast of Hawke's Bay and at least one huge ancient landslide has been mapped near the underwater fault-line.
In early February 1931, there were a number of reports of events related to the Hawke's Bay Earthquake:
- A large wave reported in Waikare Estuary appeared to have been caused by an earthquake-generated landslip on the opposite side of the estuary. The wave destroyed a woolshed and tossed fish onto grass about 15 m above the high-tide level.
- At Waikokopu Beach, west of Mahia Peninsula, three 'tidal waves' were reported, bringing fish and shellfish up onto the beach. A large rock was also thrown at a train, but the driver miraculously escaped without injury.
- A 3m surge or wave was also reported racing up the Wairoa River shortly after the earthquake.
Distantly generated tsunami have also been reported in Hawke's Bay. Large earthquakes off the coast of Chile triggered three that had a great impact in Hawke's Bay in 1868, 1877 and 1960. Records of the 1868 and 1877 tsunami are not extensive as many of the reports were destroyed by fire during the 1931 earthquake.
In 1868, there was a variation of 1.8 m in tide level in Napier and the water receded far enough to expose the wreck of the Montmorency, north of Westshore beach. In 1877, the tide ebbed and flowed every alternate hour, and Watt's Wharf in Wanganui was completely flooded due to the high water level.
More serious wave damage was reported in Napier in May 1960, with the gas main broken and 40 metres of a footbridge in Ahuriri Estuary destroyed. A number of pleasure boats were damaged or swept out to sea. Buildings were flooded and moved, endangering the lives of the Napier Sailing Club's caretaker and his family, and 1700 m³ of sand was scoured from the boat harbour.
The exact heights of the waves were not recorded, because the mechanical gauge malfunctioned, but they are thought to have reached up to three metres.
What Can You Do?
If you are at the coast and experience any of the following, move immediately to higher ground or go inland:
- You feel a strong earthquake (it's hard to stand up or walk steadily; there is significant household contents and building damage)
- You feel a weak earthquake that lasts a minute or more
- You see strange sea behaviour, such as the sea level suddenly rising or falling
- You hear the sea making loud and unusual noises such as roaring like a jet engine
Official warnings can be expected to come for sources more than three hours of tsunami travel time away from Hawke's Bay.
- An official warning from Civil Defence Emergency Management may be broadcast on radio or television.
- Warnings may also be by siren, telephone, loud hailer or other local arrangements.You may receive warnings from one or several sources. Respond to the first warning - don't wait for further messages before deciding to act.
- Listen to your radio and follow any official instructions
- Evacuate from the areas or zones identified in the warning.
- Take your 'Getaway Kit' with you.
- Stay away until the official 'all-clear' is given.
Warnings from friends, other members of the public, international media, Internet, etc. may be correct. Informal communication may be your only warning, especially for tsunami that are less than three hours tsunami travel time away.
- If you feel the threat is imminent, quickly get to high ground or, if the surrounding area is flat, go as far inland as possible.
- Verify the warning only if you can do so quickly (via NZ TV/radio broadcasts, Internet, Civil Defence Emergency Management, Police or Fire Services).
- The first or largest wave may not arrive for 6 hours after the forecast arrival time.