Two fresh jolts shake Tongans

The Free Church of Tonga at Veitongo, on Tongatapu, lost its steeple in the quake. Picture / Matangi Tonga Online

Two aftershocks rattled Tonga early yesterday, 24 hours after a large quake exposed alarming cracks in a tsunami warning system.

The system is meant to prevent another disaster like the Indian Ocean tsunami in 2004.

Aftershocks of 6.0 and 5.4 hit the same region of Tonga shortly after midnight, the US Geological Survey reported. There was no sign of any damage in the capital and no tsunami warnings were issued.

On Thursday, a 7.8-magnitude earthquake struck in the early hours, prompting a warning centre in Hawaii to issue a tsunami alert for Fiji, New Zealand, Tonga, Niue, American Samoa, Samoa and Wallis-Futuna.

The Honolulu-based Pacific Tsunami Warning Centre lifted the alert within two hours, after a wave of less than 60cm was recorded.

But Tonga did not receive Thursday's alert because of a power failure and the alarm system only operated on Fiji's main island - one of about 110 inhabited islands in Fiji.

The failings raised troubling questions about how alerts issued in Hawaii reach remote communities scattered across the earthquake-prone Pacific. "If people don't get it [the warning], it's not worth anything, but we don't have people in every country who can help keep their sirens running and their power running. It's frustrating," said Barry Hirshorn, a geophysicist at the warning centre.

GNS Science's Dr Warwick Smith said the 7.8-magnitude earthquake was "marginal" for producing a tsunami, unlike the 9.3-magnitude quake in the Indian Ocean on December 26, 2004.

Dr Smith said the earthquake also occurred 60km below the ocean surface, and did not significantly affect the ocean floor. "To make a tsunami, you need to push a lot of water out of the way. In the Indian Ocean tsunami, for instance, a patch of seabed 1200km long was lifted 10m ... now that's a lot of water pushed out of the way."

The depth of the earthquake and its 170km distance from Tonga also meant the damage to the island nation was limited, he said.