Earthquake waves surf to Sydney

01 September 2003

Unusual ocean-riding seismic waves explain why the recent New Zealand earthquake was felt across the Tasman but not in parts of New Zealand's South Island.

The earthquake that rocked the South Island of New Zealand on 21 August was felt 2000km away in Sydney, yet residents in Wellington, a mere 600km away, felt nothing.

Geoscience Australia seismologist Dr Mark Leonard explains that the energy from the magnitude 7.2 earthquake travelled to Australia through the ocean.

"Sometimes seismic waves from offshore earthquakes travel through the ocean. The waves move through the water and can travel long distances without losing much energy, much further than on land.

"The waves keep going until they run out of energy, or until they hit solid ground and the energy will pass from the ocean to the land.

"In the case of the New Zealand earthquake, the energy crossed the Tasman Sea and hit solid ground when it reached the NSW coast", said Dr Leonard.

This unusual mechanism is the reason why the energy was able to travel so far and why the earthquake was felt in Sydney but not in Wellington.

"This is a very unusual occurrence. The energy that hit Sydney was equivalent to a magnitude 3.0 earthquake located 50 kilometres off the coast from Sydney".

The data from this earthquake will provide a unique opportunity for Geoscience Australia seismologists to compare predictions of where the local geology amplifies earthquake energy with locations where people did and didn't feel the 7.2 earthquake.

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