What are tsunami? Tsunami are a series of waves caused by a displacement of waterTsunami:• Are long-period waves – up to 100 kilometres

• Travel quickly in the open ocean – up to 700 kilometres per hour

• Slow down and increase in height as they approach land

• Behave like a rapidly rising/falling tide.

The difference between wind and water waves is shown to the right:

What causes tsunami?Tsunami are mainly caused by earthquakes, landslides and volcanic activity under the ocean.

Landslides, earthquakes and wind and waves diagrams courtesy Willem de Lange, University of Waikato.

The difference between wind and water waves

Landslides Earthquakes Explosion Impact

Types of tsunamiThere are three main types of tsunami:• Local source: travel time between the source, such as an earthquake in the

Southern Tonga-Kermadec Trench, and impact is one hour or less.

• Regional source: travel time between the source, such as Samoa or Tonga, and impact is between one and three hours.

• Distant source: travel time between the source – such as South America or Japan - and impact is greater than three hours.

Where do most of Whitianga’s tsunami come from? Most tsunami that impact Whitianga are distant source or local source – from regions “0” and “2” in the diagram (top right).

What are we most concerned about?We are most concerned about local tsunami events generated from large earthquakes along the Tonga-Kermadec Trench subduction zone – the tectonic plate boundary between the Pacific and Australian plates.

Above images sourced from Natural Disasters Association (www.n-d-a.org)

Image courtesy GNS Science

How subduction zones along plate boundaries produce large tsunami.

Tsunami source areas in the Pacific

How do we know that Whitianga is vulnerable to tsunami? New Zealand is impacted by numerous tsunami every centuryThe most recent was the Japanese tsunami on 11 March 2011.• The wave height of the recent Japan tsunami was 1.6 metres at

Whitianga – the highest recorded level on the mainland.

• Large distant tsunami events in the past century include the SE Asian tsunami in 2004, and the Chilean tsunami in 1960.

• Large South American earthquakes have produced tsunami that have impacted Whitianga over the past 500 years.

• Ancient (paleo) tsunami deposits along the north-east coast of the North Island show evidence of a very large tsunami around the year 1450.

Ancient (paleo) tsunami evidenceThe diagram mid-right shows the estimated maximum height of ancient tsunami deposits of various ages along the north-east coast of New Zealand. These deposits tend to support other research indicating that the Tonga-Kermadec Trench is capable of producing a large tsunami from a maximum 9.4 magnitude earthquake event.

Ancient (paleo) tsunami evidence in the North IslandImage from Goff et al, 2010

Background image: Whitianga, fishing from shell sand beach (courtesy Waikato Regional Council)

Large South American earthquakes since 1562 and their impacts

Year SourceEarthquake magnitude

Maximum run-up in source region (m)

Maximum run-up in NZ (m)

1562 South-central Chile unknown 16 Unknown

1575 Central Chile “ 25+ “

1586 Off Lima, Peru “ 26 “

1604 Arica, Northern Chile “ 16 “

1730 Valparaiso, Chile “ 16 “

1746 Callao, Lima, Peru 8.6 24 “

1835 Conception, Southern Chile unknown 14 “

1868Arica, Northern Chile/ Southern Peru 9.1 18

10 (Chathams); 4 (Mainland)

1877 Inquique, Northern Chile 9.0 21 3.5

1960 Southern Chile 9.5 25 5

Distant source tsunami• Distant tsunami take three or more hours from the time of the event until

they reach New Zealand.

• Because of their distant location, there is usually enough time for official warnings and co-ordinated evacuations of coastal communities.

• The 1960 Chilean tsunami took 12½ hours to arrive at Whitianga. This event arrived as a series of waves that inundated Whitianga over a period of about six hours.

Maximum elevations of various distant tsunami eventsThere are two primary factors that determine the impact of tsunami generated by earthquakes – the size of the event and the direction of the fault.1. New Zealand is particularly vulnerable to earthquakes generated in Southern Peru, due to the orientation of the plate boundary.

2. The primary energy from the 1960 Chilean tsunami event was directed towards the north-western Pacific region. Hawaii and Japan suffered substantial loss of life and damage from this event, with tsunami run-ups being almost eleven metres and more than six metres respectively.

1960 Chile tsunami maximum amplitude plot

Map images courtesy NOAA / Pacific Marine Environmental Laboratory / Center for Tsunami Research

Travel times from the 1960 Chilean tsunami

Tsunami from Southern Peru earthquake, 15 August 2007, magnitude 8.0

Tsunami from Japan earthquake, 11 March 2011, magnitude 9.0

Volcanoes

Earthquake from Tonga-Kermadec subduction zone

Landslides

Local earthquakefaults

Local tsunami sources

Local source tsunamiThere are numerous local sources for tsunami to the north-east of Whitianga.• Earthquake: local faults in the Bay of Plenty, and large earthquakes from the plate

boundary in the Tonga-Kermadec Trench

• Volcanic: Mayor Island, and eruptions from undersea volcanoes on the Kermadec Ridge to the north-east of New Zealand

• Landslide: failure of large undersea canyons off East Cape, and landslide hazards from White Island.

Local source tsunamiThe single biggest risk is from a large earthquake along the plate boundary in the Tonga-Kermadec Trench. The Tonga-Kermadec Trench may produce:• an earthquake of magnitude 9.4 about once every 600-800 years, which could

produce tsunami run-ups of 15-20 metres along the north-east coast of the North Island

• an earthquake of magnitude 9.0 about once every 400 years, which could lead to run-ups of 6.5 metres in Whitianga

• Whitianga is particularly at risk from fault segments “2” and “3”, as waves from these segments are directed straight into Mercury Bay

• recent scientific work suggests that the plates may be locked in places along the Tonga-Kermadec Trench.

Image courtesy GNS ScienceTonga-Kermadec events and segments

Scientific modelling and tsunamiScientific modelling facts• The model used is 3DD Suite Computational Marine and Freshwater Laboratory

developed by Dr. Kerry Black.

• The model deals well with some of the major challenges of tsunami modelling such as non-linear flow, wetting and drying on land and fast moving flows on dry surfaces.

• The model has demonstrated the ability to accurately replicate tsunami impacts on a real-world scale such as the SE Asian tsunami event of 2004, and the impacts of the 1960 Chilean tsunami on Whitianga (in line with eyewitness reports).

• The model relies heavily on the use of detailed data on seafloor elevations (bathymetry) and detailed data on land elevations in the town (LIDAR).

• The modelling assumes a magnitude 9.0 earthquake along segments 2 and 3 of the Tonga-Kermadec Trench with a 450 kilometre rupture length, 100 kilometre horizontal displacement and 15 metre vertical displacement

• The modelling was done by Dr. Gegar Presetya at the University of Waikato as part of a PhD thesis.

Whitianga inundation maps produced by the modelling

1960 Chilean inundation

Tonga-Kermadec Trench inundation

Tsunami zonesTwo types of zones have been identified,each with a slightly different purpose:!, Hazard Zones: based on the modelling results from the Chilean and Tonga-Kermadec Trench, the hazard zones show the areas of maximum probable impacts to Whitianga. The zones are determined by the combination of inundation depths and flow speeds:• High Hazard Zone (red): inundation depth of between 1.5 metres and three metres,

and flow speeds of between nine and 29 kilometres per hour

• Medium Hazard Zone (yellow): inundation depth of between 0.5 metres and 1.5 metres, and flow speeds of between 1.8 and nine kilometres per hour

• Low Hazard Zone (blue): inundation depth of less than 0.5 metres, and flow speeds of less than 1.8 kilometres per hour.

The hazard zones are primarily used to assist with land-use planning due to the high degree of accuracy, although they are also used to inform evacuation planning.

", Evacuation zone: this zone identifies all land above 20 metres in elevation, and has been developed by Councils and emergency services to provide an indication of safe areas in the event of a worst-case scenario (magnitude 9.4 event on the Tonga-Kermadec Trench). It is anticipated that the inundation in a worst case scenario would be significantly higher than the area covered by the hazard zones.

Whitianga tsunami evacuation zone

Whitianga tsunami hazard zones

The Japanese tsunami, !! March "#!!Facts and figures• The earthquake was a magnitude 9.0 “megathrust” event, with the epicentre

located 72 kilometres east of the Oshika Peninsula of Tohoku at a depth of 32 kilometres.

• The earthquake is the most powerful to have hit Japan, and one of the five largest earthquakes recorded in the world since records began in 1900.

• The tsunami waves produced by the event ran up to 38 metres above sea level in some areas, and travelled more than 10 kilometres inland in others.

• At May 2011, 15,000 deaths and 5,300 injuries had been confirmed, with almost 11,000 people still missing.

• The tsunami waves damaged the Fukushima nuclear power plant, left 4.4 million homes without electricity and 1.5 million homes without water, causing massive displacement of people. Insured losses alone are estimated at $15 - $35 billion (US).

• The earthquake shifted the island of Honshu 2.4 metres to the east.

Maps (above) courtesy New Scientist

Images below courtesy National Geographic NewsSlip map images sourced from http://supersites.earthobservations.org/sendai.php