Monday 1 August 2011

Magnitude 6.8 Earthquake hits Bam Island, Papua New Guinea. 1 August 2011.

Just before 9.40 am local time on the 1st of August 2011 an earthquake with a magnitude of 6.8 hit Bam Island of the northeast coast of Papua New Guinea. The earthquake was very shallow, at about 17 km, which means it probably caused severe shaking at the surface. It was preceded by a smaller, magnitude 5 earthquake at about 4.10 am local time. There are no reports of any casualties, however given the remote location of the island this means little. There is little official contact between the Papuan Government in Port Moresby and the island. What little information is available is provided by missionary societies (the island officially converted to Christianity in 2008), who estimate the population to be between 2000 and 3000. No tsunami warning has been issued.

The location of Bam Island.

Bam Island is the easternmost of the Schouten Islands, a group of small volcanic islands off the northeast coast of Papua New Guinea (not to be confused with the Shouten Islands of Indonesian West Papua). Bam has a 685 m high, double-coned stratovolcano (cone shaped volcano formed by successive layers of ash and lava). This volcano was more-or-less constantly active from its discovery in 1872 until 1960, when eruptions abruptly stopped. It is possible that the earthquakes may herald the start of a new volcanic cycle (though it is hard to judge how probable this is).

Bam Island

The Schouten Islands contains two other volcanoes.

Kadovar is 18 km to the east of Bam. It is another stratovolcano, with a summit 365 m above sea level. The volcano may have erupted in 1700, though this is uncertain. There has been some more recent activity. In 1977 a fumarole (gas emission) briefly caused the island to be evacuated, and in 1981 the sea to the northeast of the island was stained an orange colour for several weeks.

Blup Blup is 23 km northeast of Bam and 11 km north of Kadovar. There have been no recorded eruptions on Blup Blup, though there is an area of hydrothermal activity off the west coast (i.e. submerged hot springs). There are stories of eruptions in 1830 and 1616 but Smithsonian Global Volcanism Program dismisses both of these.

Satellite Image of Blup Blup Island.

Papua New Guinea forms part of the Pacific Rim of Fire. It has a complicated geological structure, essentially lying on a transform plate margin, where the Pacific and Australian Plates are rubbing past one-another. However the situation is more complex, as there are a number of small plates caught between the two, possibly the result of fragmentation on the plate margin. The Schouten Islands lie on the margin between the South Bismark Plate and the Australian Plate.

The geological setting of the Shouten Islands.

The southern margin of the South Bismark Plate is being subducted beneath the Australian plate. As the plate is drawn into the earth's interior, the rocks are melted by the heat of the Earth's interior, and the lighter portion rises up through the overlying Australian Plate forming the volcanoes of the Shouten Islands and New Britain.