Tremor is a continuous vibration of the ground around volcanoes. The signal lasts for a period from minutes to days.
The dominant frequency is 1-5 Hz. Harmonic tremor is a low frequency sine wave with smoothly varying amplitude. Spasmodic tremor is a high frequency, pulsating, irregular signal.
Volcanic tremor is a sustained seismic signal which is observed during either quiescient or active stages. The seismic signal results from the interaction between
hydrothermal-magmatic fluids and the surrounding rocks.
Palmieri invented the first seismometer at Mt Vesuvius volcano in 1855.
Eruptions at Piton de la Fournaise volcano are always accompanied by volcanic tremor.