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Monitoring the seismicity of volcanoes helps to understand the structure of volcanoes
Moving magma and volcanic fluids trigger earthquakes
Many processes can generate earthquakes in volcanic lands. Most of the time, these processes are faults and fractures that do not necessarily lead to an eruption. Volcanic earthquakes occur when magma and volcanic gases rise to the surface from deep underground, involving large stress changes in the crust as the material migrates upwards.
Volcano seismologists study several types of seismic events to better understand how magma and gases move towards the surface:
Seismic monitoring networks for volcanoes are very heterogeneous across countries and volcanoes. Volcanic earthquakes occur when magma and volcanic gases rise to the surface from deep underground, resulting in large stress changes in the crust, as volcano-tectonic (VT) earthquakes represent brittle fracture of the rock, the same process that occurs along purely “tectonic” faults.
1-On volcanoes, VT earthquakes can occur due to “normal” tectonic forces, changing stresses caused by moving magma, and fluid movement through pre-existing cracks. Distinguishing between these different processes can be tricky and often requires data from other disciplines (geodesy, hydrology, gas geochemistry, and geology)
2- Long-period (LP) or low-frequency (LF) earthquakes are caused by cracks that resonate as magma and gases move to the surface. They are often observed before volcanic eruptions, but their occurrence is also part of the normal background seismicity of some volcanoes and their occurrence does not necessarily indicate that an eruption is imminent. LF events can also be produced by non-magmatic processes, including glacier movement.
3-The tremor is a continuous high-amplitude seismic signal that can be caused by multiple processes, including long-lived resonance due to prolonged movement of magma through fissures, the continuous occurrence of VT or LP/LF events that are so close together in time that they cannot be separated visually, and explosions.
4- Surface seismic arrays can be parasitic or useful for monitoring. The sources can be anthropogenic, rockfall, icequakes, landslides, pyroclastic flows, and lahars, explosions.
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.
Seismogram signal examples from volcanic earthquakes: volcano tectonic (VT) Low Frequency (LF)/Deep Long-Period (DLP), hybrid (mix of VT and LF), very low frequency (VLF), and Tremor.
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