December 14, 2011
by Alicia Hotovec-Ellis
Harmonic tremor is a kind of continuous seismic signal commonly observed on erupting volcanoes that resembles sounds made by many musical instruments, although at lower than audible frequencies of between 1 and 5 Hz. During the March 2009 eruption of Redoubt Volcano in Alaska, upward gliding harmonic tremor was observed immediately prior to six nearly consecutive explosive events (see the spectrogram below).
The spectrogram shows the dominant frequencies evolving over time, here the 9 minutes that lead up to an eruption. (Listen to the scream sped up 60x) The hum starts very low, around a few Hz, rising to disappear above 20 Hz about 30 seconds before the explosion. For reference, the lower limit of human hearing is around 15 to 20 Hz.
These signals were endearingly dubbed 'the screams' by seismologists at the Alaska Volcano Observatory because of their unusually high frequency content, up to a previously unheard of 30 Hz. Another surprise is that the spectral evolution of the signals is nearly identical from explosion to explosion, indicating a stable and repeatable process is at work in the moments before an eruption, but what?
In the ten hours before the first explosion with harmonic tremor, there was a swarm of over 1,600 repeating 'drumbeat' earthquakes. Over time, the earthquakes became increasingly frequent, eventually blending smoothly into tremor. (Listen to these drumbeats sped up 60x)
Alicia Hotovec (graduate student and PNSN duty seismologist) and collaborators have developed a theory that the harmonic tremor can be explained as many more of these small, repeating earthquakes that are close together in both time and space. Further evidence seems to suggest that these earthquakes may be caused by repeated stick-slip at or near the conduit wall as viscous magma works its way upward. A familiar analog to this case of harmonic tremor might be screeching tires or a squeaky door.
Although this is a rare and extreme example, if you ever happen to be near an active volcano and feel the ground humming, you may want to find cover...
From Alicia Hotovec, originally posted to the Earth and Space Sciences Department newsletter.
- Why earthquakes disappear
- PNW Earthquake Early Warning prototype goes live
- Packers versus Seahawks game analysis -- too exciting
- Panther versus Seahawk Game Analysis
- Seismology will again watch/help the Seahawk's playoff run
- Canadian ETS morphing to Washington one?
- Great ShakeOut, Great success!
- The Great ShakeOut 2014 is Tomorrow!
- Three Cascadia ETS events in past month??
- Is Mount St. Helens seismicity increasing?
- 2015 (5)
- November (1)
- October (2)
- September (1)
- July (1)
- May (1)
- April (1)
- March (2)
- February (3)
- January (4)
- October (1)
- September (1)
- August (1)
- June (1)
- April (3)
- March (4)
- February (1)
- January (2)
- December (1)
- November (2)
- October (3)
- September (1)
- August (3)
- July (2)
- June (4)
- May (4)
- April (2)
- 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)