Deep long-period volcanic earthquakes

Some earthquakes beneath volcanoes concentrate their energy in unusually low frequencies, even tens of kilometers deep. What change in magma or surrounding rock produces these deep long-period signals? Their recognizable compressional and shear-wave arrivals offer clues to a source far below the visible volcano.[1] [3]

Some earthquakes tens of kilometers beneath volcanoes vibrate at low frequencies while producing recognizable compressional and shear waves. Which physical changes in magma or rock generate that combination across different settings? Gas can exsolve when magma loses pressure or when crystallization concentrates volatiles. Source geometry matters: ideal spherical expansion produces compressional radiation, while pressure changes in a nonspherical sill can stress its walls and produce shear waves. A depth match alone does not test that radiation pattern.[1] [2]

Gas release, changing magma pressure and deformation around magma bodies are possible causes. Researchers have not established which processes produce the signals in each setting. Detecting an event also does not reliably mean an eruption will follow.[1] [3]

Gas-exsolution models remain candidates rather than a universal explanation. S-to-P tests depend on propagation and site corrections and compare limited source families. Different volcanic settings may produce similar low-frequency signals through different processes, so one successful local model need not explain the whole class.[1] [2] [3]

Deep magma cannot usually be watched directly. A physical explanation of these signals would let researchers use their waveforms to test what is changing inside the magmatic system, while respecting the weak connection between many events and eruptions.[1] [3]

An explanation would link a surface recording to otherwise inaccessible movement of magma, gas and stress deep beneath a volcano. For example, gas released from rising magma and gas released as stalled magma crystallizes describe different stages of its internal plumbing. Radiation, frequency and timing must distinguish such candidates; a shared depth or a burst of events alone cannot show that magma is ascending or that an eruption is imminent.[1] [2] [3]