Volcanic Earthquakes

Many processes in and around volcanoes can generate earthquakes. Most of the time, these processes are faulting and fracturing that does not lead to an eruption. However, volcanic earthquakes do occur as magma, and volcanic gases rise to the surface from depth, which involves significant stress changes in the crust as the material migrates upward.

Volcano seismologists study several types of seismic events to better understand how magma and gases move towards the surface.

Seismogram signal examples from volcanic earthquakes: Volcano Tectonic (VT) Low Frequency (LF), hybrid (mix of VT and LF), Very Low Frequency (VLF), and Tremor. Volcano name/date in lower left. Credit: USGS
Seismogram signal examples from volcanic earthquakes: Volcano Tectonic (VT) Low Frequency (LF), hybrid (mix of VT and LF), Very Low Frequency (VLF), and Tremor. Volcano name/date in the lower left. Credit: USGS

Volcano-Tectonic (VT) Earthquakes

Deep (a) and shallow (b) volcano-tectonic earthquakes recorded at the Sinabung Volcano, Indonesia. The left column is the uncorrected, unfiltered signal waveform while the right column is the normalized spectrograms with red representing high power values, and blue, low values. (Oktory Prambada)
Deep (a) and shallow (b) volcano-tectonic earthquakes recorded at the Sinabung Volcano, Indonesia. The left column is the uncorrected, unfiltered signal waveform while the right column is the normalized spectrograms with red representing high power values, and blue, low values. (Oktory Prambada)

These types of volcanic earthquakes represent the brittle failure of rock – similar to that of tectonic quakes that occur along tectonic faults such as the El Pilar Fault, Los Bajos Fault, or the Central Range Fault. However, volcano-tectonic earthquakes can occur at volcanoes due to “normal” tectonic forces, changing stresses caused by moving magma, and the movement of fluids through pre-existing cracks. Distinguishing between these various processes can be tricky and often requires data from other disciplines (geodesy, hydrology, gas geochemistry, and geology). (USGS)

Long Period (LP) or Low-Frequency (LF) Earthquakes

Typical low-frequency earthquake signal recorded at the Sinabung Volcano, Indonesia. The left column is the uncorrected, unfiltered signal waveform while the right column is the normalized spectrograms with red representing high power values, and blue, low values. (Oktory Prambada)
Typical low-frequency earthquake signal recorded at the Sinabung Volcano, Indonesia. The left column is the uncorrected, unfiltered signal waveform while the right column is the normalized spectrograms with red representing high power values, and blue, low values. (Oktory Prambada)

These volcanic earthquakes are caused by cracks resonating as magma and gases move toward the surface. They are often seen before volcanic eruptions. Their occurrence is also part of the normal background seismicity at some volcanoes, and their occurrence does not necessarily indicate that an eruption is imminent. It is normally not possible to locate LP earthquakes. LP events can also be produced by non-magmatic processes, most notably glacier movement. (USGS)

Hybrid Volcanic Earthquakes

Typical hybrid earthquake signal recorded at the Sinabung Volcano, Indonesia. The left column is the uncorrected, unfiltered signal waveform while the right column is the normalized spectrograms with red representing high power values, and blue, low values. (Oktory Prambada)
Typical hybrid earthquake signal recorded at the Sinabung Volcano, Indonesia. The left column is the uncorrected, unfiltered signal waveform while the right column is the normalized spectrograms with red representing high power values, and blue, low values. (Oktory Prambada)

As the name suggests, a hybrid earthquake is somewhat a mixture between a VT and an LP. They tend to have impulsive starts but also contain a significant amount of low-frequency signals. They are thought to represent magma making its way to the surface at shallow depths and are often associated with periods of rapid dome growth. They have also sometimes been precursors to major dome collapses or switches in the direction of lava extrusion. These signals can often merge into continuous tremor, which sometimes occurs in bands several hours apart.

Tremors

Typical emission (e) and tremor (f) seismic signals recorded at the Sinabung Volcano, Indonesia. The left column is the uncorrected, unfiltered signal waveform while the right column is the normalized spectrograms with red representing high power values, and blue, low values. (Oktory Prambada)
Typical emission (e) and tremor (f) seismic signals recorded at the Sinabung Volcano, Indonesia. The left column is the uncorrected, unfiltered signal waveform while the right column is the normalized spectrograms with red representing high power values, and blue, low values. (Oktory Prambada)

This type of volcanic earthquake is a continuous high-amplitude seismic signal that can be caused by multiple processes, including long-lived resonance due to the extended flow of magma movement through cracks, continuous occurrence of VT or LP/LF events that are so closely spaced in time that they can’t be visually separated, and explosions. (USGS)

Seismic signals (velocity, vertical component) recorded during the 1998 – 2011 unrest at andesitic Volcán de Colima showing a typical tremor seismic signal. The vertical axis is the velocity (meters/second), while the horizontal axis is time (seconds). (Vyacheslav Zobin, Colima Volcano Observatory)
Seismic signals (velocity, vertical component) recorded during the 1998 – 2011 unrest at andesitic Volcán de Colima showing a typical tremor seismic signal. The vertical axis is the velocity (meters/second), while the horizontal axis is time (seconds). (Vyacheslav Zobin, Colima Volcano Observatory)

Rockfalls or Pyroclastic Flows

Seismic signals (velocity, vertical component) recorded during the 1998 – 2011 unrest at andesitic Volcán de Colima showing a typical pyroclastic flow seismic signal. The vertical axis is the velocity (meters/second), while the horizontal axis is time (minutes). (Vyacheslav Zobin, Colima Volcano Observatory)
Seismic signals (velocity, vertical component) recorded during the 1998 – 2011 unrest at andesitic Volcán de Colima showing a typical pyroclastic flow seismic signal. The vertical axis is the velocity (meters/second), while the horizontal axis is time (minutes). (Vyacheslav Zobin, Colima Volcano Observatory)

Rockfall or pyroclastic flow signals have often been the dominant type of seismic signal, particularly during periods of lava extrusion. They have an emergent start and a wide frequency range and are interpreted as material falling off the dome and traveling down its flanks. Signals from pyroclastic flows are similar to rockfalls but are generally of longer duration and higher amplitude.

The vertical components of the seismic signals (left side), recorded at a distance of 4 km from the crater at short-period station EZV5 and associated with the pyroclastic flows and rockfalls that were formed during the gravitational collapses of the lava body at Volcán de Colima. Corresponding video images of these events are shown on the right side. The arrows indicate the duration of the signal. (Vyacheslav Zobin, Colima Volcano Observatory)
The vertical components of the seismic signals (left side), recorded at a distance of 4 km from the crater at short-period station EZV5 and associated with the pyroclastic flows and rockfalls formed during the gravitational collapses of the lava body at Volcán de Colima. Corresponding video images of these events are shown on the right side. The arrows indicate the duration of the signal. (Vyacheslav Zobin, Colima Volcano Observatory)

Lahars or Mudflows

Seismic signals (velocity, vertical component) recorded during the 1998 – 2011 unrest at andesitic Volcán de Colima showing a typical lahar seismic signal. The vertical axis is the velocity (meters/second), while the horizontal axis is time (minutes). Note the longer time period for a lahar signal versus other types of volcanic seismic events. (Vyacheslav Zobin, Colima Volcano Observatory)
Seismic signals (velocity, vertical component) recorded during the 1998 – 2011 unrest at andesitic Volcán de Colima showing a typical lahar seismic signal. The vertical axis is the velocity (meters/second), while the horizontal axis is time (minutes). Note the longer time period for a lahar signal versus other types of volcanic seismic events. (Vyacheslav Zobin, Colima Volcano Observatory)

Lahars or mudflows can also create seismic signals that may be confused with pyroclastic flow signals, although they are usually of lower amplitude and longer duration. These signals usually build-up gradually and are stronger at stations close to ghauts (valleys).

Volcanic Earthquake Swarms

Ten-hour seismograms from a seismic station on Atka Island on August 6 & 7, 2008. The sudden appearance of numerous earthquakes was an early sign of an eruption that began later in the day on August 7. Not all swarms of earthquakes indicate eruptions, but nearly all eruptions are preceded by vigorous seismic activity. (University of Alaska Fairbanks Alaska Earthquake Center)
Ten-hour seismograms from a seismic station on Atka Island on August 6 & 7, 2008. The sudden appearance of numerous earthquakes was an early sign of an eruption that began later in the day on August 7. Not all swarms of earthquakes indicate eruptions, but nearly all eruptions are preceded by vigorous seismic activity. (University of Alaska Fairbanks Alaska Earthquake Center)

Most volcano-related earthquakes are too small to feel, generally quite shallow (usually within 10 km (7 mi) of the surface), and can occur in swarms consisting of dozens to hundreds of events. Most swarms usually don’t lead to eruptions, but most eruptions are preceded by swarms. Therefore, during any heightened periods of seismic activity at a volcano, seismologists work around the clock to detect subtle variations in the type, location, and intensity of seismic activity to determine whether or not an eruption may occur.

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