SEATTLE - The Seismology Lab at the University of Washington keeps close tabs on anything that shakes around Mt. St. Helens.
Lately, the area just north of the crater, left over from the explosive 1980 eruption, has been shaking again. It always gets the attention of scientists.
"That is why we keep actually looking at the seismicity so carefully, so we can pick up any sign of unrest," said Silvio De Angelis, a volcano seismologist. He has studied quakes at other volcanoes around the world, as well as in the Cascades.
The current swarm now totals between 30 and 35 quakes over the last few days. Some are so tiny they barely register on seismographs. The biggest came on Sunday the 30th, with a magnitude of 2.6, strong enough to be felt by somebody close by. The quakes are about two miles deep.
The main swarm is happening five miles north of the crater, putting it under Johnston Ridge, site of an observatory where most tourists and visitors can view into the open side of the crater.
Some other small quakes, including a handful today, are happening under the crater itself, but don't concern scientists.
They've seen swarms in this area before.
In September 2004, quakes under the mountain grew from a few to thousands in a matter of days as magma made its way up to the surface. Scientists both at the Cascades Volcano Observatory and at the Pacific Northwest Seismic Network at UW don't think that is what's happening now.
These quakes are "tectonic," meaning they show signs of being typical of earthquakes involving shifting fault lines, not a sign of the ground being distorted by the underground forces of volcanoes.
But a swarm of quakes is always worth paying close attention to.
"This is why we are always on alert, and when there's a swarm of earthquakes like these, we always have our pagers with us," said De Angelis.