Monitoring Volcano Seismicity Provides Insight to Volcanic Structure
Moving magma and volcanic fluids trigger 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:
Volcano-tectonic (VT)earthquakes represent brittle failure of rock, the same process that occurs along purely "tectonic" faults such as the San Andreas Fault. At volcanoes, VT events can occur due to "normal" tectonic forces, changing stresses caused by moving magma, and 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) to work out what's going on.
Long-period (LP) or low-frequency (LF) earthquakes are caused by cracks resonating as magma and gases move toward the surface. They are often seen prior to volcanic eruptions, but 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. LF events can also be produced by non-magmatic processes, most notably glacier movement.
Tremor is a continuous high-amplitude seismic signal that can be caused by multiple processes, including long-lived resonance due to 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.
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.