Predicting volcanic eruptions 43 years after the Mount St. Helens blastFree

At the time of the 1980 eruption, there was only one seismometer to detect activity on Mount St. Helens, Thelen said. Today, there are at least 20 monitoring sites on the volcano.

The U.S. Geological Survey Cascades Volcano Observatory and the Pacific Northwest Seismic Network use a number of tools to track volcanoes. One of the biggest is earthquake patterns.

The equipment used now is sensitive enough to tell what caused an earthquake. For instance, if it was from a fault in the earth’s crust, or from liquid, such as magma, moving through a crack. That can give scientists a general idea of where magma is stored and where it comes out, Thelen said.

One indicator alone doesn’t necessarily mean an eruption is imminent. Take the last 30 days, when Mount St. Helens has seen 14 total earthquakes and Mount Rainier 40, according to the Pacific Northwest Seismic Network. But the seismic data, along with satellites that track changes in volcanoes’ shapes, the types of gasses released, and the geological history of the mountains, help give scientists a picture of what’s happening underneath all that rock, snow and ice. It can also help them model what could happen in the future.

Because Mount St. Helens has erupted more recently than any other volcano in the Cascade Range, scientists are able to track the signals it gives off before it erupts more easily than other volcanoes, such as Mount Rainier, Thelen explained.

“We don’t necessarily know the personality of the other volcanoes,” he said.

Still, scientists monitor the other volcanoes in the range and will often compare their activity to that of similar volcanos located elsewhere, Thelen said.

All of that monitoring still doesn’t mean scientists can predict the odds that a volcano will let loose on a given day.

Generally, the forecast window for an eruption in a system that is not very active, like the Cascades, would be one or two weeks. It often takes about seven days for an eruption to ramp up, Thelen said.

The Volcano Observatory issues weekly public alerts based on volcanic activity to help people understand the risks.

As of last week, the volcano alert level for Oregon and Washington was normal.

Because it is the most active volcano in the Cascades, Mount St. Helens has the most complete set of instruments used to predict activity, Thelen said. Other high threat volcanoes, such as Mount Rainier or Mount Hood, have sufficient monitoring, but he said that more is still needed.

This summer, Thelen said his agency is working to improve its network at Glacier Peak. In the next few years, they’ll do the same at Mount Baker. The observatory is also installing a lahar detection network near Mount Rainier to help determine where a volcanic mudflow would travel during an eruption.

In a severe eruption, lahar debris flows, often described as rivers of concrete that can be laden with rocks and downed trees, could pose serious hazards to communities in their paths.

At Mount St. Helens, an immediate concern for volcanologists is reworking monitoring equipment that was cut off by a landslide this month near the Johnston Ridge Observatory. The slide has indefinitely closed parts of State Route 504 and a number of recreation areas.

Long term, scientists would like to get to the point where they can predict eruptions weeks, months, or years out. But they’re not there yet, Thelen said. Although the monitoring technology is significantly better than it was in past decades, it still doesn’t allow for very direct measurements of what’s happening in the earth’s crust. That will likely take more time, along with further research and new technology.

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