GRANTS PASS (AP) — Scientists listening to underwater microphones have detected an unusual swarm of earthquakes off the central Oregon Coast.

Scientists don’t know what the earthquakes mean, but they could be the result of magma rumbling underneath the Juan de Fuca Plate — away from the recognized earthquake faults off Oregon, said geophysicist Robert Dziak of the National Oceanic and Atmospheric Administration and Oregon State University’s Hatfield Marine Science Center in Newport.

They hope to send out the OSU research ship, Wecoma, to take water samples, looking for evidence that sediment on the ocean bottom has been stirred up and chemicals in the water that would indicate magma is moving up through the crust, Dziak said.

There have been more than 600 quakes over the past 10 days in a basin 150 miles southwest of Newport. The biggest was magnitude 5.4 and two others were more than magnitude 5.0, OSU reported. They have not followed the typical pattern of a major shock followed by a series of diminishing aftershocks, and few have been strong enough to be felt on shore.

It looks like what happens before a volcanic eruption, except there are no volcanoes in the area, Dziak said.

The Earth’s crust is made up of plates that rest on molten rock, which are rubbing together side to side and up and down. When the molten rock, or magma, erupts through the crust it creates volcanoes. That can happen in the middle of a plate. When the plates lurch against each other, they create earthquakes along the edges of the plates.

In this case, the Juan de Fuca Plate is a small piece of crust being crushed between the Pacific Plate and North America, Dziak said.

On the hydrophones, the quakes sound like low rumbling thunder and are unlike anything scientists have heard in 17 years of listening, Dziak said. Some of the quakes have also been detected by earthquake instruments on land.

The hydrophones are leftover from a network the Navy used to listen for submarines during the Cold War. They routinely detect passing ships, earthquakes on the ocean bottom and whales calling to each other.