Three hundred miles off the Pacific Northwest coast, the seafloor has been rumbling.

Over the past five months, there were hundreds of small earthquakes on most days at Axial Seamount.

Then on April 24, there was a spike: nearly 8,000 earthquakes. The seafloor level dropped more than two meters. Temperatures rose.

An eruption is not a threat to coastal residents, researchers say, because the earthquakes are small, mostly magnitude 1 or 2, and the seafloor movements are relatively gradual, so they won't cause a tsunami.

The volcanic activity has no relationship to the Cascadia Subduction Zone, which scientists watch closely for signs of a much larger and more destructive earthquake.

To Bill Chadwick, an Oregon State University geologist, the eruption at Axial Seamount was not a surprise.

He had predicted it would happen this year. He predicted the previous eruption, in 2011, too.

Chadwick hopes the lessons he and his collaborator, Scott Nooner at the University of North Carolina Wilmington, learn from Axial Seamount can eventually be applied to volcanoes on land.

Land volcanoes have thicker crusts and are influenced by large earthquakes and other nearby volcanoes, among other things, so predictions are more difficult, Chadwick said.

"Axial Seamount is a pure example, if you will," he said. "It has relatively simple plumbing."

Chadwick and other scientists watch the signals at Axial Seamount in real-time via a cable laid out on the seafloor. The cable is part of the Ocean Observatories Initiative funded by the National Science Foundation.

The instruments that sent back the volcano's measurements were installed only last summer, said Chadwick, who works out of OSU's Hatfield Marine Science Center in Newport. He is also affiliated with NOAA's Pacific Marine Environmental Laboratory.

"Volcanoes like this have very fluid magma that is supplied from below, seemingly continuously," Chadwick said. "It's like a balloon filling with air. The seafloor actually rises -- that's what we're measuring."

Super sensitive pressure sensors can detect shifts as small as two millimeters, he said.

The earthquakes that University of Washington geologist Bill Wilcock detected are caused by magma forcing its way through the rock, Chadwick said.

Though the signs seem to point to an eruption, it will likely be months before Chadwick can travel out to Axial Seamount to do the measurements that will say for sure. He has a trip planned for August.

-- Carli Brosseau

cbrosseau@oregonian.com

503-294-5121; @carlibrosseau