Massive Russian earthquake struck on 'megathrust fault'

By David Stanway

SINGAPORE (Reuters) -The 8.8 magnitude quake off Russia that triggered tsunami warnings across the Pacific occurred on what is known as a "megathrust fault", where the denser Pacific Plate is sliding underneath the lighter North American Plate, scientists said.

The Pacific Plate has been on the move, making the Kamchatka Peninsula area off Russia's Far East coast where it struck especially vulnerable to such tremors - and bigger aftershocks cannot be ruled out, they said.

With its epicentre near the city of Petropavlovsk-Kamchatsky, it was the biggest earthquake since the devastating Tohuku event in 2011, which caused a tsunami that sent Japan's Fukushima Daiichi nuclear plant into meltdown.

"The Kamchatka seismic zone is one of the most active subduction zones around the Pacific Ring of Fire, and the Pacific Plate is moving westwards at around 80 mm (3 inches) per year," said Roger Musson, honorary research fellow at the British Geological Survey.

"Subduction" events, in which one plate pushes under another, are capable of generating far stronger earthquakes than "strike slips", such as the one that hit Myanmar in March, where plates brush horizontally against one another at different speeds.

The Kamchatka area is particularly vulnerable and experienced a magnitude 9 event in November 1952, wiping out the town of Severo-Kurilsk and causing extensive damage as far away as Hawaii, Musson told Reuters.

Shallow "megathrust" events are more likely to cause tsunamis because they burst through the sea floor and displace huge volumes of water.

TSUNAMI RISKS

With a relatively shallow depth of 20.7 km (13 miles), Wednesday's earthquake was always going to create such tsunami risks, experts said.

"It is an offshore earthquake and when you have offshore earthquakes there is the potential for tsunamis," said Adam Pascal, chief scientist at Australia's Seismology Research Centre.

"If you have a relatively shallow earthquake it is more likely to rupture the surface of the ocean floor," he told Reuters.

"We've seen in some cases you can have large earthquakes like this and not cause a tsunami because they are too deep and the shearing doesn't express itself at the surface."

Tsunami waves of around 1.7 metres (5.5 feet) reached as far as Hawaii, less high than originally expected, but scientists warned that such waves do not have to be especially big to do damage to the relatively low-lying coastlines of Pacific island nations.

Parts of French Polynesia were told to brace for waves as high as 4 metres (13 ft).

The impact of a tsunami depends on its "run-up" as it approaches coastlines, Pascal said.

"If you have a very long, shallow run-up to the coast, a lot of the energy can be dissipated over that run-up, but if it is a very steep shelf before you get to the coast, the wave height can be higher," he said.

FORESHOCKS AND AFTERSHOCKS

Wednesday's quake has already triggered at least 10 aftershocks above magnitude 5, and they could continue for months, said Caroline Orchiston, director of the Centre for Sustainability at the University of Otago in New Zealand.

"This demonstrates that large-magnitude earthquakes generate aftershock sequences that start immediately, and some of these can be damaging in their own right," she said.

The 8.8 magnitude event on Wednesday came less than two weeks after a magnitude 7.4 earthquake in the same area, which has now been identified as a "foreshock".

"Earthquakes by their nature are unpredictable," said Pascal. "There are no precursors that are scientifically consistent in earthquake sequences. Before this morning, those other ones were the main shocks."

Bigger aftershocks cannot entirely be ruled out, he added, but their magnitude and frequency normally tend to decrease over time.

"You can expect large aftershocks to continue for some time, but the frequency of large, damaging events will reduce as time goes on," he said.

"There is always a chance of a larger event, but that larger event will usually occur relatively soon after, within days or weeks."

(Reporting by David Stanway;Editing by Alison Williams)