Tsunami waves possible along some coasts of not only Papua New Guinea but also Indonesia, the Solomon Islands and Vanuatu
* Quake downgraded to magnitude 7.9 from 8
* PTWC says tsunami threat has passed, cancels earlier alert
* PNG hospital evacuated - nurse (Updates with local residents, tsunami threat passing)
By Jonathan Barrett and Harry Pearl
SYDNEY, Dec 17 (Reuters) - People living along Papua New Guinea's coastline fled to higher ground on Saturday after a 7.9 magnitude earthquake struck east of the Pacific nation, triggering a tsunami warning for parts of its archipelago.
A couple of hours after the quake, the Pacific Tsunami Warning Center (PTWC) said the threat had largely passed, although it said government agencies should continue to monitor coastal tides.
"Based on all available data ... the tsunami threat from this earthquake has now passed," the PTWC said in an updated and final alert after the quake, which struck 157 km (98 miles) east of Rabaul.
The major tremor initially triggered a more widespread tsunami warning for the Pacific region, including Indonesia and the Solomon Islands, but that was pulled back after the U.S. Geological Survey (USGS) downgraded the magnitude slightly from 8. It also revised the depth of the tremor to 103.2 km from its original estimate of 73 kms.
Many residents in the northern parts of the autonomous region of Bougainville sought higher ground amid warnings that tsunami waves were possible.
"The town residents have vacated the whole place; those in the villages live higher up, so they're ok, it's just those near the coast," local resident Christabel Biasu told Reuters by phone.
Quakes are common in Papua New Guinea, which sits on the Pacific's "Ring of Fire", a hotspot for seismic activity due to friction between tectonic plates. There were no immediate reports of damage or casualties after Saturday's major tremor.
A nurse at Buka General Hospital in Bougainville said the quake was so strong it felt like the building she was sleeping in would topple. She said patients were being moved a few kilometres to higher ground.
"We have about 16 beds," she told Reuters by phone, declining to give her name. "Most of them are already a suburb up."
The small Pacific nation of Nauru, where one of Australia's offshore immigration detention centres is located, had a tsunami warning in place, according to the official government Twitter account.
Residents in other parts of Papua New Guinea, including the capital of Port Moresby and Kavieng in the northern tip of New Ireland island, said they had not felt the quake, and no evacuation plans were in place.
New Zealand also cancelled an earlier tsunami warning. (Writing by Jane Wardell; Editing by Ian Geoghegan)
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