Theory 012/Earth & Space /Volcanology
Tremor Pitch Tracks the Gas Fraction
Tremor is read as a resonance of the plumbing. The pitch may be measuring bubbles.
Volcanic tremor frequency is set by the gas volume fraction in the conduit rather than by conduit length, so a shift in pitch records a change in exsolution rather than a change in geometry.
01The evidence
Tremor frequency changes before eruptions in ways that are not accompanied by measurable geometry change, which points to a fluid property.
02The tension
Tremor is modelled as a resonant organ pipe, which makes frequency a function of length and wave speed. Geometry changes are inferred to explain the shifts, and those inferences are rarely confirmed.
03The mechanism
A bubbly liquid has a sound speed that falls sharply with gas fraction, reaching a minimum near a few per cent gas. The resonant frequency of a fixed conduit therefore drops as gas increases.
That makes pitch a gas-fraction gauge, and it predicts that the largest frequency shifts should occur at low gas fractions where sound speed is most sensitive.
04What this is not
Conduit models make pitch a geometry measure. This makes it a gas measure, which predicts shifts without geometry change.
05Predictions
- P1Frequency should track gas fraction at fixed geometry.
- P2The largest shifts should occur at low gas fractions.
- P3Pitch should recover when gas resolves out.
06Refutation
Frequency tracking conduit length at controlled gas would restore the resonance model.
07What it changes
It would turn tremor into a real-time gas monitor.