CRATER LAKE, Ore. — Last week the depths below Crater Lake shook with a "swarm" of small earthquakes — a reminder that the picturesque attraction remains an active volcano, National Park Service officials said. There is no current danger in the area, they underlined.
Information on the quakes came from the U.S. Geological Survey's (USGS) Cascade Volcano Observatory, who sent it along to the National Park Service.
"On June 9, at approximately 1800 UTC (11:00 Pacific) a M1.3 earthquake occurred under Crater Lake at a depth approximately 2 km below sea level. The event was followed by around 2 dozen events over the subsequent 12 hours identified through spectrogram observations," USGS said.
Only the first, main quake showed up at more than one monitoring station, USGS said. However, using the quake as a template to narrow down the search for others — what the agency called a "matched filter" approach — allowed scientists to detect as many as 50 other "events."
"The most useful interpretation of that result is that all of the observed earthquakes are occurring within a confined space, likely along the same fault patch," USGS said. "In a strict sense, this sequence is most like a main shock-aftershock sequence, which would argue for the source to be fault slip on a regional fault."
When USGS looked at their logs, the last recorded swarm was in May of 2015, but with only a handful of quakes recorded. In April 2014, there was a swarm of around 20 earthquakes at about the same depth and location. Before that, the agency recorded three quakes that were "not located," but were otherwise consistent with the most recent swarm.
"All this to say that the source region that we see in this earthquake may be the same one seen during the two swarms in 2014," USGS said.
Prior to about 7,700 years ago, Crater Lake was the site of Mount Mazama — a peak standing roughly 12,000 feet high when it was pulverized by volcanic eruption. The peak collapsed into the deep caldera, now host to the waters of Crater Lake. It is the deepest lake in the U.S. and the ninth deepest in the world.