Multiplet and particle motion analysis of very-long-period (VLP) events at Stromboli Volcano, Italy, during a period of low surface activity in 2018

Very long period (VLP) seismic signals observed in volcanic environments are thought to be produced by magma and gas flow through conduits. Stromboli Volcano, Italy, typically produces hundreds of VLPs per day. These have been generally attributed to the flow of gas slugs through the shallow plumbing system and thus linked to the mechanism thought to drive Strombolian explosions. During a 6-day-long seismo-acoustic campaign in May 2018 (a period characterized by relatively low activity) we recorded 1900+ seismic events, the majority of which have significant energy in the VLP (2-100 s) band. We used a coincident STA/LTA trigger to identify seismic events in continuous waveform data and then used the PeakMatch algorithm (Rodgers et al., 2015) to identify seismic multiplets, with a focus on VLPs. To identify explosions, we applied the same coincident trigger to infrasound data, and manually identified gas jetting events using spectrograms and high-pass-filtered (20 Hz) waveforms.

We identified ~250 explosions and ~550 jetting events. Seismic multiplet analysis identified two main families of repeating events. Family 1 (F1) has over 500 events and Family 2 (F2) has over 200 events based on a 0.7 correlation threshold. We find that F1 VLPs do not coincide in time with explosions or jetting events (we term these "silent VLPs"), or with lava effusion (Marchetti and Ripepe, 2005; Ripepe et al., 2015). F2 VLPs often coincide with explosions and have a higher dominant period (8-10 s) compared to F1 (3-4 s). The infrasonic explosion signals coincide with the high-frequency portion of the corresponding seismic signal, and follow the onset of the VLP signal by several seconds. VLP peak amplitudes are generally larger for F1 events. The differences in dominant frequency and peak amplitudes suggest that F1 VLPs are generated by a different process and are not simply a smaller version of F2 VLPs. One possibility is that "silent" (F1) VLPs are the signature of gas feeding the passively degassing central vent rather than driving explosions at the NE crater. However, the two VLP generating processes occur in approximately the same location based on particle motions. Thus additional analysis of F1 and F2 VLPs is necessary to understand their mechanisms during our study period and how these relate to previously documented "silent" VLPs at Stromboli.