A small earthquake rumbled beneath Mount Hood today but it wasn't related to Mount St. Helens' venting of steam and ash, according to scientists.
"All these volcanoes have a common tectonic setting," said Willie Scott, geologist with the United States Geological Survey Cascades Volcano Observatory, in Vancouver, Wash. "But each system operates quite independently on their own plumbing system and own magma systems."
Preliminary data at the Pacific Northwest Seismograph Network at the University of Washington in Seattle showed that the earthquake was about 2.7 in magnitude, said lab technician John Patrick.
Scott said it was just a coincidence that a quake shook Mount Hood as Mount St. Helens was belching steam and ash.
When Mount St. Helens erupted in 1980, a swarm of earthquakes shook Mount Hood. But the two events likely had different causes, say scientists.
Volcanic activity usually doesn't cause earthquakes in other locations, said Carolyn Driedger, a geologist with the Cascade Observatory.
The quake on Mount Hood this morning was likely tectonic, she said, a result of rocks breaking up due to geologic plates sliding under North America.
Volcanic quakes, on the other hand, are caused by magma rising, or groundwater getting heated, Driedger said.
Small quakes shake Mount Hood each year, said John Ewert, geologist and vulcanologist also at the Cascades Volcano Observatory.
More active is a bulge south of the Three Sisters mountains in Central Oregon. A five-inch bulge has been monitored there since the 1990s.
Magma is occurring at a deep level in that system, said Scott, causing visible changes in topography over a large area.
He said scientists are keeping close tabs on the bulge but haven't had any indication there will be an eruption there any time soon.