External modulation of volcanic activity

Time and again records of seismovolcanic activity that show a diurnal and semi-diurnal modulation have been related to the Earth's tidal stress field as an external triggering mechanism. Several seismic data sets from different volcanoes are examined, including Stromboli volcano, Italy, Mt Ruapehu, New Zealand, and Merapi and Batur, Indonesia, where in some cases weather data are also incorporated. A comparison with theoretical body tides in the spectral domain reveals strong evidence against a correlation between seismovolcanic activity and tidal stress. However, other parameters such as temperature and barometric pressure show the same diurnal and semi-diurnal modulation and are, therefore, candidates for a possible external modulation of volcanic activity. Several mechanisms are discussed. For Stromboli a clear correlation between volcanic activity and barometric pressure is shown.