America’s deadliest volcano rattles with unexplained seismic bursts

Washington’s Mount Rainier has been sending up a flurry of strange signals for days, briefly raising concern that something inside the volcano might be shifting.

This towering stratovolcano looms over more than 3.3 million people across the Seattle-Tacoma metro area, threatening to cripple entire communities with ashfall, flooding, and catastrophic mudflows if it erupts.

Since Saturday, instruments on Mount Rainier have picked up what looked like constant vibrations beneath the surface, thousands of tiny tremor-like bursts blending into one another.

The unusual seismic rumblings were detected by the Pacific Northwest Seismic Network (PNSN), where seismometers on Mount Rainier recorded three straight days of persistent, high-energy signals across the volcano’s west flank.

At first glance, the pattern resembled a volcanic tremor: a kind of nonstop hum or roar that forms when magma, hot water, or gas is moving around inside a volcano.

That doesn’t mean Mount Rainier is on the verge of erupting, but such signals can sometimes appear before volcanic activity ramps up toward a critical level.

Geologists typically watch for signs that these tremor-like patterns are escalating, their intensity increasing, small earthquakes beginning inside the volcano, or the ground around Mount Rainier starting to swell.

If this volcano eventually explodes, it won’t be scorching lava flows or choking clouds of ash that pose the greatest danger, but the lahars: violent, fast-moving mudflows capable of tearing across entire communities in minutes.

Large lahars can crush, bury, or carry away almost anything in their paths, according to the US Geological Survey (USGS).

The last eruption of Mount Rainier was a minor one in 1884. Its last major eruption occurred about 1,100 years ago, and the last significant magmatic eruption was around 1,000 years ago

The activity at Mount Rainier started with a sharp spike around 5am ET on November 15. After that, the line becomes increasingly fuzzy, showing vibrations that never seem to calm down.

Later analysis, however, suggested that ice buildup on one of the seismic stations may have distorted the readings, creating the appearance of relentless tremor-like noise. 

This highlighted how challenging it can be to monitor heavily glaciated mountains like Rainier, and how even false alarms serve as a reminder of the volcano’s very real hazards.

Data showing what appeared to be tremors can also be a result of wind buffering a tower, rockfall, snow sloughing and equipment malfunction. 

Mount Rainier has already experienced massive swarms of earthquakes earlier this year, sparking fears of an imminent volcanic explosion.

In July, over 1,000 earthquakes rocked the area over a span of more than three weeks, in the largest ever seismic swarm recorded at the mountain.

That swarm easily surpassed the last large string of earthquakes at Mount Rainier, which came in 2009, lasting only three days and producing around 120 minor earthquakes.

The swarm kicked off on the morning of July 8, unleashing up to 41 minor earthquakes per hour through the end of the month. 

The 1980 eruption of Mount Saint Helens, located just 50 miles from Mount Rainier, may have provided a concerning preview of what the Pacific Northwest can expect from Rainier’s expected future eruption.

That volcanic event also produced a dangerous lahar that destroyed more than 200 homes, over 185 miles of roads, and contributed to the total death toll of 57 people.

These tragic events have helped scientists better understand the threat that lahars pose to human communities around active volcanoes, and experts are currently preparing for the terrifying possibility of a lahar forming at Mount Rainier.