intrusion and/or occur as earthquake swarms.[3] 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.[2]
One possible scenario resulting in a possible volcano tectonic earthquake occurs in
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.[4] Where plates diverge such as at mid-ocean ridges the magma also can move into storage chambers and form intrusions again causing shifts in the rocks around them that can be detected as earthquakes. This is separate from earthquakes directly related to faults. However it is known than tectonic earthquake triggering has occurred with some volcanic eruptions, and there may be other associations of these in place and time with some eruptions, which could cause confusion because of the similar terminology.[5]
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.[3]
Importance
Nevado del Ruiz during the 1985 eruption. This eruption was one where seismic activity was monitored in order to determine that an eruption was imminent.
Volcano tectonic
magmatic intrusions
in near real time.
Use in monitoring volcanoes
Nearly every recorded volcanic eruption has been preceded by some form of earthquake activity beneath or near the volcano. This does not mean that always this activity will give sufficient warning of an eruption.[7] 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.[8] 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.[2] Such earthquakes have a large non-double couple component to their focal mechanism.[2]
Other types of seismic activity monitored in relation to
volcanoes and their eruptions are long period seismic waves (caused by sudden sporadic movement of magma that had previously not been moving due to a blockage), and harmonic tremor
