SEATTLE -- Scientists who study volcanoes say the first sign that an eruption is imminent is often a swarm of tiny earthquakes -- so small that humans don't feel them and they register near zero on the Richter scale.

But to an observant scientist, these tiny movements are a sign that molten rock is beginning to move.

The problem, said University of Washington grad student Kate Allstadt, is that on Mt. Rainier in particular, the mountain isn t the only thing making noise. The glaciers on the mountain crack and pop, creating tiny quakes of their own. The worry is that when the glaciers become more active, all that popping and cracking can mask the true signs of an eruption.

The small earthquakes, and the behavior of small earthquakes, is what we really look at to monitor the volcanoes, said Allstadt.

Mt. Rainier is considered the most dangerous volcanic mountain in the continental United States, as an eruption could threaten huge population centers in Puget Sound. When volcanoes erupt, as Mt. St. Helen s did in 1980, glaciers melt, sending torrents of melt water and mud down through river valleys called lahars, sweeping away everything in their path. Mt. Rainier has the most glacial ice of any volcanic mountain in the lower 48 states.

Allstadt and U.W. volcanologist Steve Malone were on Mt. Rainier using special radar as recently as this week to monitor movement of the glaciers. Quakes caused by glaciers are more common in the winter, when big snows regularly fall on peaks throughout the Cascades.

Malone, now a professor emeritus, has studied the seismicity of Cascade volcanoes since the 1970s, and was heavily involved in studying the Mt. St. Helens eruption. Discriminating them has always been somewhat of a question, Malone said, referring to several cases in the 1990s when swarms of tiny quakes drew the attention of scientists that Mt. Rainier could be on the move.

Malone said scientists suspected the swarms in the 1980s were the result of glacial cracking, but couldn t be sure. Allstadt s research is aimed at giving scientists a better ability to distinguish quakes caused by glaciers from those caused by magma.

That intelligence could empower emergency planners to issue evacuation warnings to people living in nearby communities like Orting, Puyallup, Ashford and others.

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