Chain of quakes rattle northwestern US state

A series of small earthquakes around Mount St Helens in Washington state startled residents as far away as Portland, Oregon, and Seattle early on Wednesday.

The Pacific Northwest Seismic Network said a magnitude-3.9 quake hit around 12:38 am (local time). Dozens of people throughout the region reported feeling it, including those about 112km to the south in Portland.

Even though quakes have been located near Mount St Helens, they're not caused by the magma underneath the volcano, said Bill Steele, seismic network outreach director.

Mount St Helens is best known for a spectacular eruption in 1980 that coated the region in ash and blew off the mountain's peak. It killed 57 people and came within minutes of a magnitude-5.1 earthquake.

Earthquake swarms

The larger quake on Wednesday was followed minutes later by a magnitude-2.7 temblor. Dozens of smaller aftershocks have been hitting since then. 

Steele said it's fairly typical to see earthquake swarms in the area though somewhat unusual to have one with a magnitude-3.9 quake.

Seth Moran, the scientist-in-charge at the US Geological Survey Cascades Volcano Observatory said that despite the abundant seismic activity, it's believed to be "pretty normal tectonic activity that just happened to be close" to the mountain, The Seattle Times reported .

"We don't think it's related to Mount St Helens because the pattern of earthquakes, and the style of how they occur, is very different," Moran said.

The pattern of one large shock followed by a series of weaker aftershocks is "typical, predictable and not that impressive" for the region, Moran said.

Scientists continually record activity in and around Mount St Helens, including tiny temblors and gas releases.