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Feature

Watching for the Waves

Dr Willem de Lange

Computer models and historical data are being used to build up a picture of which earthquakes around the Pacific Basin are likely to produce major tsunami (known as teletsunami) that can cross the ocean to cause damage on the New Zealand coast.

The large Nazca Earthquake last November had the potential to generate a teletsunami affecting New Zealand. The initial reports indicated that the earthquake was shallow, had a Richter magnitude of 7.4 and was located off the coast of Peru. More than 95% of historic tsunamis have been generated by shallow submarine earthquakes with Richter magnitudes greater than 7.2. The Pacific Tsunami Warning Center assessed that no destructive tsunami was generated, but that sea level disturbances could occur in some areas.

Following this bulletin, a watch was maintained for any such disturbances at several locations around New Zealand -- none eventuated. Subsequently the location and magnitude of the earthquake was revised, giving a Richter magnitude of 7.3 and an on-land epicentre.

Although no sea level disturbance resulted, the Nazca Earthquake does provide useful information for defining the teletsunami hazard for New Zealand. It is possible to sub-divide the Pacific Ocean Basin and the surrounding seas into a number of tsunami-generating regions. For each of these regions, the characteristics of the minimum earthquake capable of generating a teletsunami affecting New Zealand can be defined. Further, it should be possible to define the likely coastal response for ranges of earthquake magnitudes exceeding the threshold.

The National Geophysical Data Center of the National Oceanic and Atmospheric Administration in Boulder, Colorado have defined a standard set of 10 tsunami-generating regions for the Pacific region, covering areas such as Hawaii, New Guinea and the Solomon Islands, Japan, the Aleutian Islands and Alaska, South America and so forth.

Based on historical data and numerical modeling undertaken by the US Department of Commerce, it is possible to make a rough assessment of the earthquake teletsunami hazard for each of these regions.

We can ignore some regions as potential generators of teletsunamis affecting New Zealand. The areas on negligible hazard are Hawaii, New Guinea and Solomon Islands, Indonesia and Philippines. These areas are excluded either because the potential tsunami-generating mechanisms are incapable of generating a teletsunami, as in Hawaii or, as for the other areas, the travel paths of any teletsunami reaching New Zealand are indirect with very high energy dissipation. None of these areas have been associated with a teletsunami recorded in New Zealand.

Next we have regions that represent a minimal hazard. These are New Zealand and South Pacific Islands, Japan and the Kuril Islands and Kamchatka. All these regions have generated teletsunamis recorded in New Zealand. However the travel paths of the northwest Pacific tsunamis involve considerable energy dissipation so the resulting tsunami are small, with waves less than 0.5 metres high. The southwest Pacific tsunamis are generated by tectonic structures that direct the tsunami waves away from New Zealand, so that the resulting waves are also small in New Zealand.

Based on historical data, very shallow earthquakes with Richter magnitudes greater than 8.25 located on the east coast of the northwest Pacific islands are needed to produce a significant teletsunami (0.25 m) in New Zealand. Shallow earthquakes larger than Richter magnitude 7.5 near the Kermadec Islands can generate waves of up to 0.75 m in northern New Zealand. For sources further north, much larger earthquakes are needed to create a detectable wave.

The Aleutian Islands and Alaska represent a moderate hazard. Only one historic teletsunami has been recorded from this region, on 28 March 1964. This event produced wave heights up to 1.5 m around the coast. It seems likely that only very shallow earthquakes with Richter magnitudes 8 and above will produce a significant tsunami in New Zealand.

The West Coast of South America represents the greatest hazard. Four teletsunamis have been recorded in New Zealand, and three of the most destructive events (in 1868, 1877 and 1960) originated from this region.

All the historical teletsunamis have been associated with shallow earthquakes with Richter magnitudes greater than 8.2, and produced wave heights up to 5 m. However numerical modelling suggests that magnitudes as low as 7.6 may produce a significant wave in New Zealand.

This leaves one region of uncertainty: the west coast of North and Central America. No historic teletsunamis have been recorded from this region. Numerical modeling suggests that the tectonic structures in Central America may direct the tsunamis towards the northwest Pacific so that New Zealand would not be affected. However, further north, tsunamis may be focussed towards the southwest Pacific. Research in Oregon suggests that tsunamis are generated in this region by very large long-return period earthquakes. It is likely that this area represents a moderate to high teletsunami hazard.

Over time with more earthquakes to add to our information it should be possible to refine this assessment and define the minimum earthquake event required to trigger a response around the New Zealand coast. It remains to define what that response will be.

Dr Willem de Lange Waikato University