The effects of conduit length and acoustic velocity on conduit resonance: Implications for low-frequency events

Low-frequency seismic events at volcanoes are modelled as the seismic wavefield from a magma-filled conduit embedded in a solid country rock using a finite difference method. The effects of varying the conduit length and the impedance contrast between the magma and the country rock are examined, generating a range of possible signals. Short-windowed spectrograms are used to look at the time–frequency relationships within the events in detail, and some of the possible variations are identified using a series of schematic spectrograms. The numerical results are compared to examples of observed seismic data from Soufrière Hills Volcano, Montserrat. While the spectra of the observed events are often different to the spectra of the numerical results, the spectrograms have similar features and show that the low-frequency events from Montserrat are composed of discrete subevents.