April 19 (Reuters) - An earthquake with a magnitude 7.5 struck off Papua New Guinea on Saturday and a tsunami warning was briefly issued for the Pacific Island nation and neighbouring Solomon Islands, but there were no immediate reports of damage.
The quake, at a depth of 10 km (6 miles), struck 68 km southwest of Panguna on the island of Bougainville, the U.S. Geological Survey said, revising down the magnitude from an initial 7.8.
The Pacific Tsunami Warning Center later cancelled a tsunami warning for Papua New Guinea and the Solomon Islands and there was no threat to neighbouring Australia or across the Pacific Ocean.
At least six strong tremors have hit near Bougainville in the past week or so, including a magnitude 7.3 on April 11, but there have been no reports of major damage.
"Certainly it has been very active, more active than usual," said Jonathan Bathgate, a seismologist at Geoscience Australia. "(The spate of earthquakes) is relieving some pressure on this faultline, but we can't rule out another large earthquake."
The quake would have been felt strongly on Bougainville and nearby islands, but given its position on the so-called "Pacific Ring of Fire" where earthquakes are frequent, extensive damage was unlikely, Bathgate said. However, a local tsunami may have been generated, he added. Readings showed a small wave had been generated, the Pacific Tsunami Warning Center said.
In 1998, a magnitude 7 earthquake triggered a tsunami that smashed into villages near Aitape on Papua New Guinea's north coast and killed more than 2,000 people.
Resource-rich Bougainville, which neighbours the Solomon Islands, fought a war for independence from Papua New Guinea in the 1990s, leading to the closure of the Panguna copper mine, majority-owned by Rio Tinto Ltd. Bougainville is now an autonomous region of Papua New Guinea.
(Reporting by Lincoln Feast in Sydney; Editing by Alison Williams and Janet Lawrence)
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