February 2, 2012
by John Vidale
Sometimes I wonder how we can spot volcano rumblings, given the variety of noises that come from our cities and mountains. We've written about several on this blog - grinding glaciers, sonic booms from fighter planes, roaring football crowds, tunneling machines, and helicopters, as well as earthquakes and tremor.
At 20 minutes past midnight last night, this signal appeared: a humming at 6 Hz, which dropped in a few seconds to 4Hz, then rose again as it disappeared, shown here to be coincident on five stations. It appeared on many stations - it is loud. It was also there for a few minutes at 10:03pm, 11:06pm, and 12:06am.
I was thinking it might be a tremendously loud helicopter, but Seth Moran of CVO (the Cascade Volcano Observatory) reminded me that we had seen similar and even more striking signals a few years back, and likely figured it out.
This spectrogram for the two stations B204 and B203 from February 2009 is quite different - the frequency band runs from 5 Hz at the bottom to 2 Hz at the top, so the signal starts lower period, rises in frequency for a half hour, then sinks back before fading. Much more detail is revealed because these are very quiet borehole stations, and I custom tailored the spectrogram to bring out the signal, unlike the plot from today, which from our routine monitoring. The time scale is counting 10^^5, or one hundred thousand seconds, so we are looking at about a 2-hour record.
The 2009 screams we finally traced to opening and closing of dams and operation of hydropower generators near Lake Merwin, Yale Lake, and Swift Reservoir.
Given the likely heavy stream flow, it is the right time of year to see something similar now. As is common, now that we've spotted this unusual signals, it is clear that there have been several similar but weaker bursts a day for at least the last 3 days.
In 2009, we were seriously uncertain what was happening for a week, but this time we're learned from our history.
- Three Cascadia ETS events in past month??
- Is Mount St. Helens seismicity increasing?
- Warm weather triggers snow avalanches at St. Helens
- Seismic recordings of a gas explosion in North Bend, WA
- Seismic signals generated by the March 22nd Oso Landslide
- Legacy web site content returns
- Ice avalanches on Cascade volcanoes
- A New View On What's Shaking on the Cascade Volcanoes
- Seismic Spectrograms - A new way to look at wiggles
- The final football game analysis
- September (1)
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- January (4)
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- The wech-o-meter takes over all of Cascadia
- Keystone Cops: Italy prosecutes seismologists for failure to predict deadly quake
- UFOs in eastern Washington? No, rather UTEs (Unidentified Terrestrial Events)
- New Sodo Seattle Liquefaction Array Installed
- Why we should constantly watch the deformation of the seafloor
- Mystery chirp near Newberry Volcano
- Planting seismographs causes earthquakes? or maybe ice-quakes?
- Tunneling rumbles south under Capitol Hill
- 15 years of mostly silent magma inflation near Three Sisters, Oregon
- Mount Hood earthquake swarm of Feb 23, 2012
- Web glitches: duplicate (and even triplicate!) earthquakes
- How earthquake magnitude scales work
- Mine blast masquerades as volcanic tremor
- The Spokane Swarm about 10 years ago
- Another hum around Mount St. Helens
- Slow slip: A new kind of earthquake under our feet
- PNSN and social media
- 3am M3.4 earthquake in St. Helens Seismic Zone
- The wrong kind of volcano noise
- Fast chatter on Rainier an hour ago
- Can slush-mageddon trigger earthquakes?
- Rainier Repeating Earthquakes Update and Comparison with Weather Patterns
- 22-minutes drumbeat icequakes(?)
- Mount Rainier popping away
- Repeating Earthquakes on Mount Rainier - are glaciers the culprit?
- Debunking another SEC football myth by the PAC-12
- One year ago, Seattle Seahawks 12th Man Earthquake
- The odds this year of a megaquake on the Pacific Northwest coast
- Is the plague of great earthquakes this decade a sign of increased danger?
- Nile Valley landslide talks to PNSN seismologists
- Good vs evil in central US earthquake hazard analysis
- Why does a volcano scream?
- Predicting big quakes from patterns of little ones
- 1-hour warning for Japanese M9 earthquake?
- Sound Transit train under Interlaken keeps a rollin'
- Invisible changes under the hood at the PNSN
- Sound Transit Tunneling Noise
- "Visionary" toads
- Earthquake early warning in the PNW
- November (1)
- December (13)