Call it a Valentine’s Day tradition on a monumental scale.
A 4.3-magnitude earthquake occurred 6 miles north of Mount St. Helens on Monday morning — exactly 30 years to the day after an even bigger earthquake occurred at nearly the exact same spot. Monday’s quake was followed by more than a half-dozen much smaller aftershocks, all in the same general area.
Scientists said the seismic activity does not indicate magma rising at Mount St. Helens. They’ve detected none of the shallow quakes or gas typically associated with an imminent volcanic eruption.
But they said Monday’s quake — just like the one 30 years ago — is at least indirectly related to the volcano.
Felt as far away as south Puget Sound and Astoria, Ore., Monday’s first temblor was the biggest in the Pacific Northwest in two years. Scientists suspect the quake, at 10:35 a.m., may have occurred in a bit of post-eruption settling in the landscape surrounding Mount St. Helens, which last erupted between 2004 and 2008.
The quake generated a specific kind of shear wave felt over a wide area, said Bill Steele, seismology lab coordinator at the University of Washington in Seattle.
In a curious twist of timing and fate, this type of surface wave is named for the scientist who discovered it: early 20th-century British geophysicist Augustus Love.
“It was very rich in Love waves,” Steele said, with scant trace of irony. “Those Love waves coming out were probably what people were feeling.”
If so, it’s not the first Valentine’s Day that the volcano has been massaged by waves of Love.
Post-eruptive settling likely triggered a 5.5-magnitude quake in the vicinity of Johnston Ridge on Feb. 14, 1981, according to seismologists in Vancouver and Seattle. They said the massive and sudden expulsion of magma during the catastrophic eruption of May 18, 1980, probably increased stress in a well-documented seismic zone that runs north-to-south underneath Mount St. Helens.
In effect, the earth shifted nine months after the eruption.
“That was a fairly significant adjustment,” said Seth Moran, a seismologist with the U.S. Geological Survey in Vancouver. “You had a cubic kilometer of material removed from depth.”
The earthquake and aftershocks on Monday were not as large as the ones in 1981.
However, Moran said the location and depth — roughly 3 miles below the Johnston Ridge Observatory — bears striking similarities to the 1981 temblor. The volcano spurted a new lava dome between 2004 and 2008, which may have created new strain in the seismic zone below Johnston Ridge.
Seismometers detected a swarm of smaller earthquakes in the general area about three weeks ago.
“We’re about three years removed from the end of the last eruption, in January of 2008,” Moran said. “Perhaps these earthquakes are in response to the end of that eruption.”
In that case, it may be well to keep this in mind for some future Valentine’s Day excursion to Mount St. Helens: If this hill’s a rockin’, don’t come a-knockin’.
“One can’t rule these things out completely,” Steele said.