EAST LONDON - East London tidal gauge monitors are calming down after the weekend's abnormal activity, according to vessel traffic controller Derek Potgieter at the harbour here.

"The tide range is only about a quarter of a metre now, as opposed to a range of over a half a metre on Sunday," he said.

The KwaZulu-Natal and Eastern Cape coasts experienced unusual tidal activity and sea currents in the wake of the earthquake that struck south-east Asia at the weekend, sending giant waves across large areas of the Indian Ocean.

In the Port Elizabeth area one person is missing, believed drowned, as a result of higher than usual swells, but by yesterday the muted effects of the 9.0 magnitude earthquake on South Africa's coastline had largely subsided.

The commander of Port Elizabeth's National Sea Rescue Institute (NSRI), Ian Gray, said there had been a "phenomenal tidal variance" in Algoa Bay on Sunday.

"The sea was rising and dropping two to three metres outside the normal tidal variance." The tidal cycle was also affected with larger-than-usual swells.

"What usually takes 12 hours (between tides) was happening in 20 minutes."

One person was washed out to sea by a high swell at Blue Horizon Bay on Sunday and is presumed drowned. Gray said that at one stage the longshore current off the bay appeared to have reversed.

From Durban, NSRI duty officer Alan Cutten said the KwaZulu-Natal coast had experienced some "unusual tidal activity", and in the channels around Durban harbour current speeds had increased.

A Richards Bay harbour official said water levels had risen about three metres during the weekend.

The SA Maritime Safety Association said it had issued no general warning to local shipping in the Indian Ocean.

A CSRI technologist who monitors swell activity around the country, Jan Kuipers, said there had been nothing unusual about swells or waves during the weekend, and the SA Navy, which monitors tide activity, would not comment.

Geoff Brundritt, professor of oceanography at the University of Cape Town, said the tsunami - a wave of energy travelling in a straight line through the ocean as a result of an earthquake - took about 12 hours to reach Africa.

"The eastern and southern coast was exposed, but Mozambique was sheltered by Madagascar," he said.

"Most of the activity here happened after lunch on Sunday."

He said the tsunami would have reached Argentina, halfway around the world from the quake's epicentre, by yesterday about noon. "It radiates out from the epicentre of the quake, gradually losing intensity."

There was no way to predict the intensity of energy of a tsunami until it hit a shore and the height of the wave could be measured. The last quake this serious happened in 1883, when the Sumatran island of Krakatoa "exploded". That tidal wave was recorded at Port Elizabeth.

South African municipalities formulated disaster management plans and, while quakes and subsequent tsunamis were unusual in the Indian Ocean, they should be included in such plans.

"You can't track a wave. You can only know it's coming, but you can't know how high it will be unless you're in touch with places closer to the epicentre.

"The main thing is to have a system to get people off the beach, because of course human life is more important than material damage." - DDR-Sapa