Seismology

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 volcano. It is a continuous, rhythmic ground vibration caused by the movement of fluids (magma, pressurized gas, or hydrothermal water) inside a volcano’s plumbing system. Unlike a typical tectonic earthquake, which appears on a seismogram as a sharp “crack” or “snap” that begins and ends clearly, a tremor looks like a continuous squiggle of sustained ground motion—resembling the sound wave of a steady wind instrument or a roaring engine—that can persist for minutes, hours, or even days.

The Physics: Why It Vibrates

The most intuitive analogy is a musical instrument. Think of a volcano’s plumbing system as a giant pipe organ.

  • Resonance: When magma or gas is forced through a narrow conduit, crack, or chamber at high velocity, it creates turbulence and sets up oscillations in the surrounding rock. Just as air forced through the reed of an oboe makes it vibrate at a specific pitch, fluid moving through volcanic conduits causes the surrounding rock walls to resonate at specific frequencies—typically 1–10 Hz (cycles per second) for volcanic tremor.
  • Harmonic Tremor: In some cases, this vibration becomes remarkably steady and rhythmic, producing a series of equally spaced frequency peaks (harmonics) on a seismogram’s spectrogram. Seeing a harmonic tremor is one of the strongest indicators that magma is actively moving toward the surface and that an eruption may be imminent.
  • Non-harmonic tremor: Broadband, irregular tremor is often associated with turbulent multiphase flow (mixtures of gas bubbles and liquid magma), or with vigorous hydrothermal fluid circulation.

Differentiating Seismic Signals

Volcanologists distinguish between three main types of seismic events near volcanoes:

  1. Volcano-Tectonic (VT) Earthquakes: High-frequency (5–20 Hz), sharp shocks caused by rock fracturing as magma forces it apart or as the crust adjusts to changing pressure. These have waveforms similar to tectonic earthquakes and can be located with conventional seismic analysis. Meaning: “The rock is cracking open.”
  2. Long-Period (LP) Events: Lower frequency (1–5 Hz) signals caused by sudden pressure changes in fluid-filled cracks. They are thought to represent “resonance” events—a crack briefly resonates like a struck bell when a fluid pressure pulse passes through it. Meaning: “Gas pressure is spiking in a crack.”
  3. Volcanic Tremor: Continuous, sustained, low-to-intermediate frequency vibration (0.5–10 Hz). Unlike VT and LP events, tremor is not a brief event but a prolonged signal. Meaning: “Fluid is flowing continuously through the system.”

The transition from isolated LP events to continuous tremor is often one of the most significant signs that a volcanic system is escalating from passive unrest to active magma ascent, and is used as a key criterion for raising volcanic alert levels.

Tremor and Eruption Forecasting

The detection and characterization of volcanic tremor is a primary tool for eruption forecasting. Several practical applications illustrate its importance:

Escalating Activity and Alert Levels

At most volcano observatories, the onset or intensification of tremor automatically triggers escalation in monitoring response. At Kīlauea Volcano Observatory in Hawaii, the sustained tremor accompanying the 2018 lower East Rift Zone eruption was tracked in real time, allowing authorities to issue timely evacuation orders as the eruption migrated downrift. The tremor signal provided continuous tracking of the dike’s propagation.

Depth Estimation

The dominant frequency of tremor can provide information about the depth of the source. Lower-frequency tremor tends to originate from greater depths (larger, slower oscillation systems), while higher-frequency tremor typically comes from shallower conduits. By deploying arrays of seismometers, volcanologists can use tremor amplitude patterns and arrival times to triangulate the approximate location and depth of tremor sources.

Tremor Amplitude and Eruption Rate

Research at several volcanoes has shown that the amplitude of volcanic tremor correlates with the flux of magma or lava moving through the system. At Kīlauea, tremor amplitude tracked the effusion rate (volume of lava per unit time) of erupting lava flows in near-real-time—providing a passive, continuous eruption rate monitor without requiring direct observation.

Case Studies

Mount St. Helens (1980)

In the weeks leading up to the catastrophic May 18 eruption, seismometers recorded increasingly intense harmonic tremors interspersed with hundreds of VT earthquakes per day. Scientists at the USGS identified the tremor as evidence of continuous magma movement within the edifice. While the scale and timing of the lateral blast were not precisely predicted, the weeks of escalating seismicity (including tremor) provided enough warning for an exclusion zone to be established—saving thousands of lives.

Holuhraun (Iceland, 2014)

A massive dike intrusion from the Bárðarbunga volcano traveled approximately 45 km laterally beneath the Dyngjujökull glacier before erupting at Holuhraun on the Icelandic plateau. The dike’s propagation was tracked in near-real-time by Iceland’s seismic network through the migration of tremor and VT earthquake swarms—moving at an average rate of ~5 km per day for 5 days before the eruption began. This provided emergency managers with both warning and a running estimate of where the magma was headed.

Kīlauea (ongoing monitoring)

Because Kīlauea is one of the world’s most continuously monitored volcanoes, its tremor record extends back decades and shows clear correlations with eruption rate, lava lake level changes, and summit deflation events. The 2018 drainage of the summit lava lake was accompanied by dramatic changes in tremor character, providing a real-time window into the dynamics of the draining system.

Infrasound: The Acoustic Dimension

Volcanic tremors are not just ground vibrations—they also radiate energy into the atmosphere as infrasound—pressure waves at frequencies below 20 Hz (below the threshold of human hearing).

  • Detection Range: Infrasound travels with relatively little attenuation over very long distances. Eruption-related infrasound from major eruptions has been detected at distances exceeding 10,000 km by specialized microphone arrays. The 2022 Hunga Tonga eruption generated infrasound waves that circled Earth multiple times.
  • Remote Monitoring: Because infrasound can be detected through clouds, at night, or from satellites, it provides a monitoring capability that complements direct seismic observation. The infrasound signature of a volcanic explosion is distinct from other natural and man-made sources, allowing automated detection algorithms to flag eruption events.
  • Physical Explanation: The generation of infrasound by eruptions involves both the direct acoustic radiation from the exploding gas-rock mixture at the vent and the coupling of ground tremor into the overlying atmosphere.

Distinguishing Volcanic Tremor from Other Sources

Not all sustained seismic signals near volcanoes are volcanic in origin. Volcanic monitoring networks must filter out:

  • Cultural noise: Traffic, industrial machinery, and wind turbines can produce sustained vibrations.
  • Ocean and river noise: Coastal or near-river seismometers pick up the continuous microseismic “hum” of ocean waves and turbulent river flow.
  • Glacial tremor: Ice movement at glacier-clad volcanoes (like those in Iceland or Alaska) generates sustained seismic signals that can mimic volcanic tremor.

Distinguishing volcanic tremor requires analysis of frequency content, polarization, spatial coherence across multiple seismometer stations, and correlation with other monitoring parameters (gas emissions, ground deformation, visual observations).

Volcano-tectonic earthquake is the rock-fracturing seismic event that often accompanies magma ascent. Long-period event is a shorter-duration, resonance-type seismic signal associated with fluid movement. Harmonic tremor is the highly regular, musically structured variety of volcanic tremor considered a strong eruption precursor. Infrasound refers to the sub-audible acoustic waves generated by eruptions and volcanic tremor coupling with the atmosphere.