Before it let loose this week, Mount St. Helens started to hum.

Now geologists are trying to figure out whether the subtle tremor that preceded Tuesday's explosion might help them predict future outbursts.

They've already factored the tremor into their automated alarm system. If the volcano starts rattling again the way it did Tuesday, the computer will page scientists at the U.S. Geological Survey (USGS), day or night.

After examining the seismic signals, they can decide whether another explosion might be imminent — and whether to issue warnings, said University of Washington seismologist Steve Malone.

But don't expect perfection.

Nobody knows yet whether the tremor will be a useful tool or a dead end in the quest to better forecast eruptions.

"We'll be wrong sometimes," Malone said. "Our understanding of how volcanoes work is still cursory in many respects."

Tuesday's earthquakes and explosion were modest by volcanic standards, and the mountain has since quieted down. But such outbursts are also capable of triggering more dangerous eruptions.

"We don't know if one of these relatively small events, once it gets going, could release enough pressure that it accelerates the magma coming up from below, so you get a runaway phenomenon," Malone said. "We don't think that's likely, but it's one of the possibilities."

A few hours before Tuesday's explosion, USGS seismologist Seth Moran spotted the unusual seismic signal. "It wasn't an increase in earthquake size or number," he said. "It was an increase in the background shaking."

A more powerful type of tremor, created by the movement of magma, often signals an eruption. But in this case, the shaking was so slight that seismologists weren't sure what to make of it.

They're kicking around several possible explanations, from the rattling of an underground magma plug to seepage of gas-rich, explosive magma.

The researchers are going back to examine the seismic records from another explosion in January, to see if the same type of pattern preceded it.

"What we'd really like to do is learn to anticipate when these events are about to occur," said USGS geologist Jon Major.

Even 15 minutes' warning would help route airplanes away from a volcanic plume, which can kill jet engines.

Tuesday's explosion, which sent a plume 36,000 feet into the air, was probably the biggest bang at Mount St. Helens in 20 years, Malone said.

The ongoing eruption, which started with earthquakes and a series of small explosions in late September and early October, is also setting records for the mountain.

Lava has been pushing up through the crater floor at a prodigious rate, building a new dome that now stands as tall as the Empire State Building and encompasses enough rock to fill Safeco Field 16 times, seismologists say. At one point, the volcano was spitting out a dump-truck load of lava every second.

The growth rate has slowed slightly, but the dome is still expanding by 20 or 30 feet every day — and didn't miss a beat after Tuesday's explosion.

The new dome is now about half the size of an older lava dome that took six years to build after the 1980 eruption that blew off the volcano's top and killed 57 people.

Volcanoes usually build domes through short eruptions that spew out a lot of material, or long-term trickles of lava. Mount St. Helens has been breaking those patterns.

"What makes this unique is how long it's been going at such a good clip," Moran said.

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