Tsunami Hazards

Offshore causes of tsunamis

Earthquakes, submarine landslides, volcanic eruptions, meteor impacts

What do offshore causes of tsunamis lead to

Cause displacement of the seabed and the associated water column

What produces tsunami earthquakes

Subduction zones

How do tsunami earthquakes in subduction areas work

As the subduction plate moves down the friction pulls the front edge of the overriding plate back “scrunching it up” which creates tension. Eventually the tension is too much which causes the overriding plate to break free (earthquake) springing seaward which raises the sea floor and water above it. The bulge behind the leading-edge collapses thinning the plate and lowering coastal areas. Part of the tsunami races towards nearby land growing taller as it comes into shore. Another part heads across the ocean toward distant shores

When was the Tatapouri tsunami and what caused it

Occurred in 1947 and was caused by a local offshore seismic event

When was the Chile earthquake and how did it impact NZ

occurred in 1960 and lead to significant flooding for several hours. Important to note that since the source was so far away there was no noticeable wave but the water level rose over several hours

How do you find prehistoric tsunamis

Must find where tsunami reserves are stores (usually in estuaries), look for evidence of rapid subduction (over years the subduction rises above water and things grow on it, when it releases it goes below water again)

What is the only example of a New Zealand tsunami generated by a volcano

Krakatau Volcano tsunami 28 to 29 August 1883, 30m at source but generally <1.2m in NZ

New Zealand sources of tsunamis

Submarine landslides (cook straight), Fiordland earthquakes

When was the Fiordland earthquake tsunami and how tall was it

2003, it was 3-4m tall

How fast do tsunamis travel

About as fast as a jet airliner (800-900 Km/hr)

What happens when tsunamis reach the coast

They slow down and amplify (get higher)

What are two considerations when it comes to tsunami arrival

Wave height and run-up

Wave height definition

The vertical distance of the water surface above normal sea level at time of measurement (usually lower in open ocean because of depth)

Run-up definition

The maximum vertical height that the water reaches on land above normal sea level. significant predictor of damage potential as it estimates how far inland the water can travel

When was the Wairarapa earthquake

1855

What did the Wairarapa earthquake lead to

Caused a landslide that created a tsunami that has ~8m runup on the south Wairarapa coast. Miramar Isthmus was overtopped by 1m of water

Isthmus definition

Narrow strip of land with sea on either side forming a link between two larger areas of land

When was the Goose Bay tsunami and what caused it

Occurred in 2016 and was caused by a local earthquake with a magnitude of 7.8. 6-7 meters of runup.

Examples of prehistoric tsunamis in NZ

Boulder Bank, and Vernon Lagoons in Marlborough

When was the Hikurangi Subduction Zone Earthquake and what did it cause

occurred in 2011 and caused a severe tsunami in Tohoku Japan

Steps of probabilistic tsunami hazard analysis

Identify and map tsunami sources submarine volcanoes & landslides, subduction zones etc.), estimate recurrence, Determine attenuation (source to site distance), Determine hazard/risk at site