While Mount St. Helens shot off fireworks this week, scientists' hopes to capture an underwater volcano in action fizzled.

Water samples and photos of the seafloor 200 miles off the coast of Vancouver Island showed no evidence that fresh lava had pushed up to the surface.

"We combed the area pretty thoroughly," Seattle oceanographer Ed Baker said yesterday, shortly after the research ship docked at the University of Washington.

But the rapid-response expedition to the Juan de Fuca Ridge will still provide valuable information about the dynamics of a little-known landscape, said Baker, of the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory.

"We didn't find any smoking gun, but there may be some fingerprints yet at the crime scene."

The thousands of small earthquakes that rattled the area beginning Feb. 28 might not have signaled an eruption, but they might have rearranged the plumbing in nearby hydrothermal fields — underwater geysers populated by giant tube worms and other creatures that thrive in super-heated waters.

Chemical analysis of water samples collected over the vent fields already hint at some changes, and more detailed studies will provide a fuller picture, Baker said.

And a more thorough look at the type of ground-shaking during the earthquakes could make it possible in the future to distinguish between underwater earthquakes and actual eruptions.

While signals from seismographs and a network of acoustic sensors showed that magma was moving underground, scientists couldn't tell until they reached the area whether the molten rock had breached the surface, Baker said.

Ocean ridges are the places where new seafloor is formed as molten rock wells up from deep inside the earth. But very little is known about that process, said James Cowen, a University of Hawaii oceanographer who, along with Baker, led the expedition launched March 5.

"We may only see one of these events once every three years," Cowen said. "We've had so few opportunities to study what is the fundamental process of ocean crust formation."

With funding from the National Science Foundation, the team of about 20 scientists monitors underwater earthquakes and keeps instruments packed and ready to go so they can respond quickly when an underwater eruption starts.

This time, they were especially lucky. The University of Washington research vessel — normally booked years in advance — was available.

"We were out there within a week of the first earthquakes," Baker said. "That's one of our fastest responses ever."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com