Deformation-controlled long-period seismicity in low-cohesion volcanic sediments

Volcano seismicity is an important tool in remotely monitoring and forecasting activity at volcanoes around the world. Volcanic earthquakes show diverse spectral characteristics, with shallow Long Period (Low Frequency) seismicity and long duration tremor generally interpreted as indicators of fluid migration, and as potential precursors to eruption. Here we show that a common low-cohesion volcanic sediment from Campi Flegrei caldera (Italy) produces Low Frequency and long duration seismicity whilst undergoing deformation in dry conditions. We employ acoustic-emission rock deformation experiments at a range of strain rates to produce events which are spectrally indistinguishable when normalised for scale from Long Period and tremor seismicity observed in natural volcanic settings. Generation of these signals is enhanced at lower strain rates. Correlated X-Ray tomography of samples before and after deformation constrain the source as sub-grain scale diffuse damage. Given the ubiquitous nature of slow edifice deformation, and the frequent occurrence of such low cohesion materials in the upper edifice of volcanoes, we suggest low frequency seismicity and tremor in volcanic settings do not require fluid movement. Instead, these characteristic signals can be an indicator that deformation within the edifice is being accommodated by weak volcaniclastic materials.