Volcanic Tremor
"A continuous seismic signal indicating the movement of magma or gas beneath a volcano."
A volcanic tremor is a reliable distress signal from a waking giant. It is a continuous, rhythmic ground vibration caused by the movement of fluids (magma, pressurized gas, or hydrothermal water) inside a volcano.
Unlike a typical tectonic earthquake, which sounds like a sharp “crack” or “snap” on a seismogram, a tremor looks like a continuous squiggle—resembling the sound wave of a steady wind or a roaring engine.
The Physics: Why It Vibrates
Think of a volcano’s plumbing system as a giant musical instrument, like an organ pipe.
- Resonance: When magma pushes through a narrow conduit or crack, it creates friction and turbulence. This oscillation makes the surrounding rock resonate at a specific frequency.
- Harmonic Tremor: Sometimes, this vibration becomes incredibly steady and rhythmic (monochromatic). Seeing a harmonic tremor on a seismograph is one of the strongest indicators that magma is actively moving toward the surface and an eruption may be imminent.
Differentiating Signals
Volcanologists distinguish between three main types of seismic events:
- Volcano-Tectonic (VT) Earthquakes: High-frequency, sharp shocks caused by rock breaking as magma forces it apart. Meaning: “The rock is cracking.”
- Long-Period (LP) Events: Lower frequency signals caused by sudden pressure changes in fluids. Meaning: “Gas is bubbling.”
- Tremor: Continuous, low-frequency vibration. Meaning: “Fluid is flowing continuously.”
Case Studies
- Mount St. Helens (1980): In the weeks leading up to the cataclysmic eruption, harmonic tremors became so constant that they formed a “continuous release of energy,” warning scientists that the magma chamber was destabilizing.
- Holuhraun (Iceland, 2014): Massive dyke intrusion created days of intense tremor as magma traveled horizontally underground for 40 km before erupting.
Infrasound
Tremors are not just ground vibrations; they often couple with the atmosphere to create infrasound—sound waves too low for humans to hear (< 20 Hz).
- Detection: Specialized microphones can detect these silent roars thousands of kilometers away, providing early warning data even when the volcano is obscured by clouds.
FAQ
Q: Can you hear a tremor with your ears? A: Usually, no. The frequency is often too low. However, if you are very close to the vent, the ground vibration can sometimes be felt as a physical buzzing in your feet, and the accompanying gas release might sound like a jet engine or a freight train.
Q: Does every tremor mean an eruption? A: Not always. Sometimes magma moves, causes tremors, and then stalls underground (an intrusion) without ever breaking the surface.