When earthquakes struck beneath the South Pacific ocean on Thursday, islanders fled to high ground, fearing a repeat of last week's devastating tsunamis that killed some 150 people on Samoa, American Samoa and Tonga.

Three subsea quakes, measuring 7.8, 7 and 7.3 on the Richter scale, were registered beneath the seas between Vanuatu and the Solomon Islands, but the waves only trickled in.

The Pacific Tsunami Warning Centre had issued widespread alerts to the island resorts, Australia, New Zealand and Indonesia, and Hawaii and the Philippines were placed on tsunami watch. But the warnings were cancelled later on Thursday and relieved islanders returned to their homes.

Last week's quake off the coast of Samoa, at a magnitude of 8, was only slightly bigger than the largest of Thursday's quake.

So why did one cause a devastating tsunami and the other a tiny wave?

Experts say magnitude isn't the only thing that determines the likelihood of a tsunami.

DEPTH MATTERS

Subsea earthquakes occur when tectonic plates shift dramatically or collide beneath the sea bed. But not all quakes will break through the sea bed as some will have their epicentre very deep into the rock.

"If the quake breaks the sea bed, it is more likely to cause a tsunami," Peter Sammonds, professor of geophysics at University College London, told AlertNet.

"But it also depends at what angle it breaks the sea bed: if it's movement is more vertical, it's going to have more of an affect in terms of generating a tsunami than if it hits at a shallow angle," he added.

If the energy from the quake vertically jolts the sea bed, this causes the displacement of large volumes of water. Waves then move across the ocean at great speed away from the quake's epicentre. The tsunami slows as it reaches shallower water but the waves increase in height.

Experts agree that tsunamis are more likely to happen from shallower and larger earthquakes than ones that occur much deeper beneath the sea bed and are smaller.

The December 2004 Indian Ocean tsunami, the worst ever recorded with a death toll of almost 230,000 people, was triggered by an earthquake of a magnitude between 9.1 and 9.3 that occurred at a depth of 30 kilometres (19 miles) below sea level. The sea floor overlying the epicentre lifted up several meters, according to the US Geographical Survey.

With earthquakes of a magnitude above 7, there is the potential for a tsunami, but not always, as seen on Thursday in the South Pacific.

"Not all big earthquakes produce tsunamis. There are big ones, deep ones, that don't produce them," said Wang Dailan, oceanographer at the NOAA Pacific Tsunami Warning Centre in Hawaii.

However, in the Pacific region, anything above 7.5 produces a local tsunami warning, Dailan added, as the probability of a tsunami is high.

SHAPE ALSO IMPORTANT

Other factors that would impact a tsunami include the shape of the sea floor between the quake and the land and the distance from the shore of the quake's epicentre.

There are other tsunamis that are not caused by a quake but rather by a submarine landslide, Sammonds said.

Dailan said it was quite unusual to have so much seismic activity in the same region in such a short space of time.

"In 2008, there were three or four magnitude 8 earthquakes in the whole year. Now we have close to that in just a few days," he said.

Sammonds disagreed, saying what made these quakes so noticeable was the fact that they occurred so close to heavily populated areas.

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