A small island that appeared in the Arabian Sea off Pakistan after an earthquake last week 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 said.

“It looked as if a section of shallow seafloor had simply been pushed up,” said Game McGinsey, a volcanologist with the United States Geological Survey. Photographs of the island, which measures roughly 100 feet by 250 feet and rises about 60 feet above the water, showed a rough-textured surface suggesting that the seafloor had risen and cracked, he said.

Dr. McGinsey said the way the island was created was 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, Dr. McGinsey said, 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,” he said.