Volcano seismicity is an important tool in monitoring and forecasting activity at volcanoes globally. Volcanic earthquakes show diverse spectral characteristics, of which shallow long-period (low-frequency) seismicity and long-duration tremor are generally interpreted as indicators of fluid migration, and as potential eruption precursors. Here we show that a common low-cohesion volcanic sediment from the Campi Flegrei caldera (Italy) produces low-frequency and long-duration seismicity as it undergoes deformation in dry conditions. We employed acoustic-emission rock-deformation experiments at a range of strain rates to produce events that, when normalized for scale, were spectrally indistinguishable from the long-period and tremor seismicity observed in natural volcanic settings. The generation of these signals was enhanced at lower laboratory strain rates. Correlated X-ray tomography of the samples before and after deformation constrained the source as distributed damage.