No one at Crater Lake National Park felt it, but the sleeping geological giant recently stirred in its long slumber.
For the first time since seismic equipment was installed at the park in 2011, three tiny earthquakes have been recorded, according to the Cascades Volcano Observatory, a U.S. Geological Survey facility in Vancouver, Wash.
The quakes measured on Oct. 25 were miniscule, registering only a magnitude .08, the observatory reported.
"The bottom line is the Crater Lake area is not necessarily dead," said Seth Moran, a seismologist at the observatory.
"But we've waited 7,000 years since the last eruption," he added of the dormant volcano. "It could be quite a while before another one. But it is worth paying attention to on a human scale."
Crater Lake is in a caldera formed when Mount Mazama blew its top some 7,000 years ago. The national park is visited by roughly half a million people each year.
Before last week's earthquakes, the most recent to have been recorded within the park were three quakes with magnitudes of 2.3, 2.6 and 2.4 occurring near Rim Village within 20 minutes of each other on the afternoon of Dec. 29, 1994, observatory scientists reported. The two largest of those were felt by people in the park, they noted. A 1997 USGS Crater Lake hazards report indicated they occurred along a system of regional faults.
"Those were literally the last earthquakes anyone knew about until now," Moran said. "Until the 25th, we hadn't seen anything like this."
However, because the Oct. 25 quakes were too small to be recorded outside the park by other instruments, their precise locations could not be determined. Based on how the quakes were recorded, the scientists believe they may have been centered near the northwestern corner of the crater rim at a depth of 5 to 10 kilometers.
The USGS' three seismic stations in the park each have highly sensitive GPS receivers.
"They are sending us data continuously," he said. "They can detect ground motion that is as little as a couple of millimeters."
Scientists are closely monitoring the data to keep tabs on any movement, he said.
"Whenever we put a new seismic network on a volcano, we always learn things we haven't learned before," he said. "These were quite a bit smaller than the ones in 1994. We don't know what the cause was.
"One of the complications is the fact there are some regional faults that run north and south of the lake," he added. "Geologists say these are active."
It may be the Oct. 25 quakes are related to the faults, he said.
"That's something we want to nail down," he said. "The equipment we have down there is not perfect, but it is good enough to detect earthquakes by looking at the data.
"These earthquakes were so small they didn't trigger anything," he added. "We are confident we can detect anything we need to. If we were to get an earthquake swarm, we would notice it."
Earthquake swarms immediately precede most volcanic eruptions, scientists observe. The observatory monitors volcano activity in Oregon, Washington and Idaho.
"Crater Lake is a fascinating area, but there is very little we know about it geophysically," he said. "It is a lot of terra incognito. We know a lot about Mount St. Helens. There has been more activity there. We also have more instruments on Mount St. Helens."
Mount St. Helens, which erupted in 1980, has two earthquakes each day on average, he said.
"We've now had three in two years at Crater Lake, so it has a much lower activity rate," he said.
In a 1997 USGS study of Crater Lake earthquakes, scientists estimated that while the recurrence of large earthquakes is unknown, it is likely between 3,000 and 10,000 years.
In recorded history, the largest was one in 1920 estimated to have been a magnitude 4.0 plus, the report said, noting it was before there were many seismometers in Oregon.
"Sleepers wakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks stop, start, change rate," it said of published reports.
Meanwhile, don't look for the caldera that was Mount Mazama to erupt anytime soon, Moran said.
"With Cascades volcanoes, you have to wait a couple of thousand of years between eruptions," he said.
Reach Mail Tribune reporter Paul Fattig at 541-776-4496 or email him at email@example.com.