Tsunami

Tsunamis are waves created when the ocean floor is displaced by earthquakes, landslides or volcanoes.

The size of the wave, extent of inundation or erosion caused by a tsunami vary hugely and depend on the source, tide and meteorological conditions, and topography of the shore. Most tsunamis are so small that they are barely noticed. Small tsunamis can disrupt currents in the sea and within harbours. Occasionally there is a tsunami that is large enough to cause flooding of land.


What we know

Christchurch is most likely to get a distant source tsunami

Tsunami scientists have shown that the coastline of Pegasus Bay is different from the rest of the New Zealand coast in that a tsunami large enough to cause flooding of land is most likely to come from a distance source (i.e. an earthquake off the coast of South America) than from local or regional sources . This is because a distant source tsunami travels directly towards our coast, as opposed to the oblique angle created by a regional source wave. Tsunami scientists use the physics of earthquakes, wave generation and wave travel to model the likely effects of tsunamis.


Extent and consequence of tsunami could be severe

The latest tsunami modelling undertaken on behalf of Environment Canterbury maps areas susceptible to inundation from a distant source tsunami originating from a magnitude 9.485 earthquake at the Peru subduction zone. This indicates in a large tsunami, if the wave arrived at high tide, all the Regeneration Strategy project area could be inundated with a wave height of up to 12m at the coast. Most flooding would come from the Estuary/Ihutai side with about 14 hours of warning time . Note that the effects are significantly less if the tsunami arrives at low tide, which has been the case in past tsunami events in 1960 and 2010 .

Here's a map that shoes a distant source tsunami from a M9.485 earthquake off Peru.


Sea level rise increases the potential effect

Changes in long term sea level will have no impact on the occurrence of tsunamis, so will not affect the likelihood of a tsunami occurring. However, increases in the underlying water level that the tsunami is superimposed on could mean the impacts of tsunami are greater in the future.


Why this is important

Source and potential effects dictate management response

As the primary tsunami risk to the regeneration strategy project area is a distant source tsunami, there would be sufficient warning time to allow organised evacuation but with the potential effects much more damaging and severe. For this reason, tsunami risk is primarily managed in Christchurch through civil defence response and evacuation.


What we don’t know

As sea level rises, coastal scientists consider it is possible that the end of the spit could erode back. This would result in a wider Estuary/Ihutai mouth so that more water could flow in and out of the Estuary/Ihutai not only with tides, but also with storm surge and waves, and rarer events such as tsunamis. Because the Estuary/Ihutai coast is lower than the open coast, it is more vulnerable to flooding with such events, and this is expected to worsen with erosion of the spit, widening of the Estuary/Ihutai mouth and more water being able to enter the Estuary/Ihutai. How much this will affect future flooding in such events depends on many unpredictable factors and therefore cannot be fully defined.

Tsunamis are waves created when the ocean floor is displaced by earthquakes, landslides or volcanoes.

The size of the wave, extent of inundation or erosion caused by a tsunami vary hugely and depend on the source, tide and meteorological conditions, and topography of the shore. Most tsunamis are so small that they are barely noticed. Small tsunamis can disrupt currents in the sea and within harbours. Occasionally there is a tsunami that is large enough to cause flooding of land.


What we know

Christchurch is most likely to get a distant source tsunami

Tsunami scientists have shown that the coastline of Pegasus Bay is different from the rest of the New Zealand coast in that a tsunami large enough to cause flooding of land is most likely to come from a distance source (i.e. an earthquake off the coast of South America) than from local or regional sources . This is because a distant source tsunami travels directly towards our coast, as opposed to the oblique angle created by a regional source wave. Tsunami scientists use the physics of earthquakes, wave generation and wave travel to model the likely effects of tsunamis.


Extent and consequence of tsunami could be severe

The latest tsunami modelling undertaken on behalf of Environment Canterbury maps areas susceptible to inundation from a distant source tsunami originating from a magnitude 9.485 earthquake at the Peru subduction zone. This indicates in a large tsunami, if the wave arrived at high tide, all the Regeneration Strategy project area could be inundated with a wave height of up to 12m at the coast. Most flooding would come from the Estuary/Ihutai side with about 14 hours of warning time . Note that the effects are significantly less if the tsunami arrives at low tide, which has been the case in past tsunami events in 1960 and 2010 .

Here's a map that shoes a distant source tsunami from a M9.485 earthquake off Peru.


Sea level rise increases the potential effect

Changes in long term sea level will have no impact on the occurrence of tsunamis, so will not affect the likelihood of a tsunami occurring. However, increases in the underlying water level that the tsunami is superimposed on could mean the impacts of tsunami are greater in the future.


Why this is important

Source and potential effects dictate management response

As the primary tsunami risk to the regeneration strategy project area is a distant source tsunami, there would be sufficient warning time to allow organised evacuation but with the potential effects much more damaging and severe. For this reason, tsunami risk is primarily managed in Christchurch through civil defence response and evacuation.


What we don’t know

As sea level rises, coastal scientists consider it is possible that the end of the spit could erode back. This would result in a wider Estuary/Ihutai mouth so that more water could flow in and out of the Estuary/Ihutai not only with tides, but also with storm surge and waves, and rarer events such as tsunamis. Because the Estuary/Ihutai coast is lower than the open coast, it is more vulnerable to flooding with such events, and this is expected to worsen with erosion of the spit, widening of the Estuary/Ihutai mouth and more water being able to enter the Estuary/Ihutai. How much this will affect future flooding in such events depends on many unpredictable factors and therefore cannot be fully defined.