There have been more than 100 tiny earthquakes in the past two months near a Washington state mountain that blew its top off in 1980.

The last two earthquakes under Mount St. Helens today were magnitude 0.58 and 0.61 at depths of 1.45 kilometres and 1.65 kilometres, respectively.

"The earthquakes have low magnitudes of 0.5 or less; the largest a magnitude 1.3," the USGS revealed in a May 5 statement on its website. "Earthquake rates have been steadily increasing since March, reaching nearly 40 located earthquakes per week.  These earthquakes are too small to be felt at the surface."

Mount St. Helens is 150 kilometres south of Seattle. When it erupted on May 18, 1980, 57 people were killed and hot ash fell across a large swath of the Pacific Northwest.

According to the USGS, recent "volcano-tectonic earthquakes beneath Mount St. Helens are likely associated with the recharge of the volcano".

"After the 2004-2008 eruption, subtle inflation of the ground surface and seismicity indicate that the magma reservoir beneath Mount St. Helens is slowly re-pressurizing, as it did after the conclusion of the 1980-1986 eruption," it added. This is to be expected and it does not indicate that the volcano is likely to erupt anytime soon."

Mount St. Helens remains the most seismically active volcano in the Cascade Range.

The USGS stated that unlike the recent spate of earthquakes nearer the surface, the 1980 eruption was "marked by a vertical streak of earthquakes" extending more than 15 kilometres below the surface. "Scientists believe that these earthquakes occurred when the May 18 eruption drained magma from deeper parts of the magmatic system, leaving voids of unsupported rock that then failed and produced earthquakes."

In 1792, Captain George Vancouver named the mountain after a British diplomat, Alleyne Fitzherbert, the 1st Baron St. Helens.