Late Saturday night, Feb. 15, someone turned on the washing machine in one of the old wooden buildings at Tamarack Lodge—or so thought Bob Thompson, an attorney and mountaineer visiting from Los Angeles.
Then he thought it might have been an avalanche rumbling outside after the fresh snowfall.
“There was some serious creaking,” he said.
What Bob felt was a magnitude 2.4 earthquake that occurred at 8:30 p.m., one of many small earthquakes in a swarm centered beneath Mammoth Mountain that started on Feb. 3.
“The swarm has declined over the past couple of days,” said Dr. Margaret Mangan, Scientist-in-Charge for the U.S. Geological Survey’s California Volcano Observatory (USGS CalVO) in Menlo Park, in an interview Tuesday, Feb. 18.
There were as many as 130 to 140 earthquakes at the peak on Feb. 5, she said.
The levels are now around 10 to 20 quakes per day.
The normal background seismicity for this area runs between three and 10 quakes per day, and generally those are tiny, she said—with a magnitude of 1.0 or less.
The strongest quake was on Feb. 5, with a magnitude of 3.1. Most in the swarm have been magnitude 1.5 (M1.5) or less, Mangan said, which a human wouldn’t feel.
Typically, people will feel quakes at or above M3.0, she said.
Mammoth Mountain and the Long Valley Caldera region is one of eight volcanic areas in California that the USGS monitors for potential eruptions.
But this swarm doesn’t have them too concerned, Mangan said.
Given the area’s restless nature, however, they are vigilantly monitoring the swarms. Though these quakes have been relatively small, it is the strongest swarm in about a decade, she said.
For anecdotal perspective, the swarms in the 1980s, which occurred east of Mammoth Lakes, were much stronger—in the M6.0 range.
For mathematical perspective, the Richter scale, used to give quakes their magnitude number, is logarithmic—not linear. This means for every change of one unit, the magnitude is actually 10 times greater. So an M6.0 earthquake would be 1,000 times stronger than an M3.0 earthquake.
Instruments are in place here because scientists suspect there to be zones of partially molten rock deep under Mammoth Mountain and the Long Valley Caldera, at a depth of roughly eight to 10 km (five to six miles), Mangan said.
They monitor activity to see if any magma is rising toward the surface, she said.
This current swarm is centered under the southeast flank of Mammoth Mountain, and is likely caused by the migration of hydrothermal fluids. The partially molten rock supplies the heat, and density and pressure drive the migration, Mangan said.
“The migrating fluid pressurizes and weakens pre-existing faults causing rock to crack, producing earthquakes,” Mangan wrote in an update on the USGS CalVO website.
The forces driving the current swarm are the same that supply the stinking, steaming fumarole on Mammoth Mountain, as well as the carbon dioxide in the soil that suffocated the trees at Horseshoe Lake.
Trees take up carbon dioxide in their leaves, turning it to oxygen through photosynthesis—but their roots need to absorb oxygen directly, Mangan said.
The high concentration of carbon dioxide here denies oxygen and interferes with nutrient uptake.
“In the areas of tree kill, carbon dioxide makes up about 20 to 95 percent of the gas content of the soil,” Mangan said.
“Soil gas normally contains one percent or less carbon dioxide.”
Though the migrating fluids and gases are rich in carbon dioxide, “preliminary measurements made last week suggest carbon dioxide emissions at Mammoth Mountain fumarole have not changed notably,” Mangan wrote on the website.
Swarms like this one typically peak and decline, Mangan said, and she expects this event to follow suit.
The morning after Thompson felt the little earthquake, he said, nobody seemed to notice.
“In L.A., earthquakes are front-page news,” he said.
“I guess the new snow was too overwhelmingly exciting.”
For more information and updates, go to volcanoes.usgs.gov/observatories/calvo.