More than 400 earthquakes rattled Washington's Mount Rainier over just 12 hours on Tuesday, sparking fears that the catastrophic volcano could soon erupt.

The US Geological Survey (USGS) issued an update at 1:00pm PT, reminding the public that Rainier is far from extinct, but 'is still active.'

This huge, stratovolcano looms over nearly 90,000 people living in cities such as Seattle, Tacoma, and Yakima in Washington as well as Portland, Oregon.

Even though Rainier has not produced a significant eruption in more than 1,000 years, this weeks swarm of earthquakes caught the attention of the USGS as such events are often associated with volcanic unrest.

A USGS geophysicist clarified that the current tremors at Rainier are not being driven by rising magma, but are likely the result of hot fluids.

Alex Iezzi explained that hot water is probably circulating through pre-existing fractures in the rock beneath the surface, which can generate small, frequent earthquakes as the pressurized fluids cause the surrounding rock to shift and crack. 

However, Rainier is still closely monitored, as changes in these fluid-driven systems can sometimes precede more serious volcanic activity.

A network of webcams, seismometers, GPS stations and infrasound sensors at the volcano are watching it around the clock, looking for any changes that signal an eruption.

Volcanologists have said that it is only a matter of time until Rainier, arguably the most dangerous volcano in the US, unleashes on the Pacific Northwest. 

'Mount Rainier keeps me up at night because it poses such a great threat to the surrounding communities,' Jess Phoenix, a volcanologist and ambassador for the Union of Concerned Scientists told CNN.

But for now, the USGS has found 'no indication that the level of earthquake activity is cause for concern, and the alert level and color code for Mount Rainier remain at GREEN / NORMAL.'

Tuesday's earthquakes were very small, measuring up to a 1.6 magnitude. 

'Earthquakes are too small to be felt at the surface and will likely continue for several days. There would be no damage caused by such small events,' the USGS said.

The Pacific Northwest Seismic Network, which monitors seismic activity in the area, detected 25 earthquakes as of 11:20am PT, with a magnitude 2.3 as the strongest.

However, this is the largest swarm of earthquakes since 2009 that saw over 1,000 tremors.

The volcano typically experiences a rate of about nine earthquakes per month and swarms every one to two years.

'Earthquakes are one of several parameters we monitor to indicate what a volcano is doing,' said the USGS.

'Right now, this swarm is still within what we consider normal background levels of activity at Mount Rainier.'

The USGS Cascades Volcano Observatory (CVO) added: 'Instruments do not show any detectable ground deformation at the volcano, and no anomalous signals have been seen on the infrasound monitoring stations.' 

When this volcano eventually blows, it won't be lava flows or choking clouds of ash that threaten surrounding cities, but the lahars: violent, fast-moving mudflows that can tear across entire communities in a matter of minutes.

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

'Tacoma and South Seattle are built on 100-foot-thick ancient mudflows from eruptions of Mount Rainier,' Phoenix said.

This is a strong indication that these populous cities lie within the path of potential lahars generated by a future eruption.

Volcanic eruptions usually cause lahars by rapidly melting snow and ice that covers the volcano's slopes, which then destabilizes loose dirt, rock and volcanic debris and causes it to flow rapidly downward.

But it doesn't always take an eruption to trigger a lahar, according to the Seismological Society of America.

Rarely, these powerful mudslides can form as the result of gradual weakening of the volcano's slopes due to past eruptions, or heavy rainfall after an eruption.

The deadliest lahar in recent history resulted from a 1985 eruption of the Nevado del Ruiz stratovolcano in Tolima, Columbia.

Within hours of the eruption, a torrent of mud, melted snow and rock inundated the town of Armero and killed an estimated 25,000 people.

This event, now known as the Armero tragedy, was the costliest volcanic disaster in history, according to The International Disaster Database. The total economic impact was estimated at $1 billion.

The 1980 eruption of Mount Saint Helens, located in Washington just 50 miles from Mount Rainier, 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.