HOW A QUAKE CAN MAKE AN ISLAND
Asmall island that appeared in the Arabian Sea off Pakistan after a recent earthquake most likely formed when the shaking released methane gas and water trapped in undersea sediments.
The gas and water forced part of the seabed to the surface, experts say.
“It looked as if a section of shallow sea floor had simply been pushed up,” says Game McGinsey, a volcanologist with the U.S. Geological Survey. Photographs of the island, which measures roughly 30 metres by 75 metres and rises about 18 metres above the water, showed a roughtextured surface suggesting the sea floor had risen and cracked, he says.
Mud volcanoes
McGinsey says the way the island was created is similar in some ways to that of a so-called mud volcano, in which gas and water force mud up through vents to the surface. In those cases, the flow of mud normally continues for some time, similar to the way lava flows from a conventional volcano. There are some long-lived mud volcanoes in the region, McGinsey says, but this one appeared to be a one-time event, with no sign of continuous flow. “It’s not a mud volcano in the classic sense.”
The 7.7-magnitude quake struck on Sept. 24, killing more than 500 people and flattening homes in the southwestern province of Baluchistan.
It was followed by a 6.8-magnitude aftershock that killed at least 15 people.
The initial quake was centred about 65 kilometres north of the city of Awaran and about 400 kilometres from the port town of Gwadar, where the new island appeared in shallow waters about a half-hour later. Townspeople and scientists who visited the island told news agencies that it was muddy and rocky and was emitting flammable gas. Methane, the main component of natural gas, is highly flammable.
Release of methane
The quake occurred in the Makran subduction zone, a vast and complex tectonic feature stretching from Pakistan to Iran, where three plates — the Indian, Arabian and Eurasian — meet.
As the Arabian plate slides under the Eurasian, sediments containing water and methane are compressed, says Michael Steckler, a geophysicist with the Lamont-Doherty Earth Observatory, part of Columbia University.
“In subduction zones you get a lot of overpressure,” Steckler says.
Even a relatively far-off earthquake can produce enough shaking to fracture the sediments and release the gas and water, he says.
The methane is created through the action of bacteria on organic matter, and would have been trapped in the sediments as free molecules of gas.
The Arabian Sea is also home to large quantities of methane hydrates, icy cagelike structures of water molecules with methane molecules inside. Immediately after the island formed there was speculation that hydrates, not free methane, had been released.
But hydrates form only under high pressure and low temperatures.
Carolyn Ruppel, director of the survey’s hydrate research program, says water in the area was too shallow, and temperatures in sediments far too high, for hydrates to exist.
Washed away
Similar islands formed in the Arabian Sea after an 8.1-magnitude earthquake in 1945. (A recent study by scientists in Germany showed that quake set off the release of free methane from sediments, releases that continue today.) Islands also formed after quakes in 1999 and 2010.
Such islands eventually disappear, eroded by the action of tides and waves. Steckler says that the one that formed in 1999, for instance, was gone in a few months, a victim of monsoon surges.