A volcano tectonic earthquake or volcano earthquake is caused by the movement of magma beneath the surface of the Earth. The movement results in pressure changes where the rock around the magma has a change in stress. At some point, this stress can cause the rock to break or move. This seismic activity is used by scientists to monitor volcanoes. The earthquakes may also be related to dike intrusion and/or occur as earthquake swarms. Usually they are characterised by high seismic frequency and lack the pattern of a main shock followed by a decaying aftershock distribution of fault related tectonic earthquakes. Magma can not move through the newly compressed crust as easily. This means it tends to pool in magma chambers beneath the surface and between the converging tectonic plates. Many of the famous and most well known volcanoes are of this type, including those of the Ring of Fire. As the plates move, magma underground may be forced in and out of these chambers and form intrusions into surrounding crust. This movement is capable of causing the unstable rocks around it to cave in or shift. The movement of this magma as described, causes measurable seismic activity.
Scientists monitoring volcanoes have noticed that magma movement may lead to earthquake swarms depending on the movement of magma and the interaction with rock beneath the ground. Additionally, the volatility of volcanoes and the accompanying earthquakes has been shown to be linked to dike induced stress and the interaction this causes between the magma, rock, and wall of the chamber. Such tectonic events can predict eruptions in long-dormant volcanoes and the size of the magma pocket. However, for individual volcanoes, the relationship between earthquakes and magma movement or possible eruptions has resulted in approximately 200 of the world's volcanoes being seismically monitored. The recording of several years of background seismic data has allowed classification of volcanic earthquakes. These earthquakes tend to occur in swarms as opposed to mainshock–aftershock sequences, have smaller maximum sizes than tectonic structure earthquakes, have similar waveform patterns, increase in number before eruptions, and occur near or beneath the site of the eruption. Volcano tectonic earthquake seismicity typically originates lateral to the site of the volcanic eruption to come, at tectonic fault structures a few kilometres away.