Volcanic Earthquakes

Volcanic earthquakes are the result of rock fracturing within or beneath a volcano due to localized stress changes. These stresses may be due to the motion of magma (underground molten rock), hydrothermal fluids (hot water), or even gravitational stresses from the weight of the geologically recent addition of the the volcano on the surface of the earth. Changes in the rate, size or even type of volcanic earthquakes can be used to infer changes in the state of the volcanic plumbing system that may lead to volcanic unrest and even an eruption.

Volcanic earthquakes have the potential to cause cracks, ground deformation, and minor damage to local manmade structures. They typically are much shallower than the more common tectonic earthquakes caused by non-volcanic sources. They rarely exceed magnitude 5 though a few of the earthquakes preceding the 1980 eruption of Mount St Helens approached magnitude 5.

Seismic waveforms on volcanoes can be summarized as having four general characteristics:

Volcano-tectonic (VT) earthquakes are, simply put, caused by slip on a fault on or near a volcano. Volcanoes are often found in areas of crustal weakness and the mass of the volcano itself adds to the regional strain. VT earthquakes are usually associated with the interaction of a volcano's magmatic system with the regional stress field. VTs take place within cooler, brittle rock away from magma but caused by the stresses from the more distant injection or removal of magma from the volcanic system. They also can be caused by faults being weakened by hydrothermal solutions, which are driven by heat from a magma source at some distance below the volcano. Minor changes in a system of cracks with hydrothermal solutions can cause multiple small VT earthquakes without indicating a period of volcanic unrest.

On the other hand, a sequence of low-frequency (LF) — or long-period (LP) — volcanic earthquakes would be more indicative of magma or significant amounts of other fluids moving that may lead to volcanic unrest and an eruption. LF earthquakes are usually much shallower than VTs and their character may be due to the less brittle nature of either the very shallow volcanic rocks or due to their proximity to hot magma. In 2004, Mount St. Helens began dome building eruptions as magma was thrust upwards, accompanied by a transition from VT earthquakes to LF ones. Such LF earthquakes are usually interpreted as evidence for magmatic activity that may lead to an eruption. When these earthquakes occur continuously, back to back over long periods of time the result is observed as volcanic or harmonic tremor. Mount St. Helens exhibited both these low-frequency earthquakes and volcanic tremor months in advance of the 1980 eruption.

A challenge for the volcano seismologist is differentiating the signals caused by LF earthquakes from those caused by some other surface events. A short, sudden rock-fall or glacier jerk or serac collapse (the failure of a block of glacial ice) can generate a seismic signal that looks similar to a true volcanic, low-frequency earthquake. Interpreting subtleties of signal character, event location, size and quantity can be as much an art as a science when trying to predict volcanic unrest.

Volcano scientists at the USGS Cascade Volcano Observatory combine information obtained from seismic activity in combination with many other data sources from the volcano — including near and far field deformation, gas emissions, geochemistry, heat transfer and more — to determine the likelihood of an eruption and to estimate how dangerous the eruption is likely to be. This approach has been used successfully to predict the eruptions at many volcanoes around the world including Mount St. Helens in the 1980's and 2004. The irony is that while earthquakes can not themselves be predicted (Earthquake Prediction), the observation of volcanic earthquakes can be used to help predict volcanic eruptions.

To view current seismicity near the Cascade Volcanoes, visit our volcano seismicity pages. The Cascade Volcano Observatory also provides updates on volcanic activity.