External modulation of volcanic activity

Time and again records of seismovolcanic activity that show a diurnal and s emi-diurnal modulation have been related to the Earth's tidal stress field as an external triggering mechanism. Several seismic data sets from differe nt volcanoes are examined, including Stromboli volcano, Italy, Mt Ruapehu, New Zealand, and Merapi and Batur, Indonesia, where in some cases weather d ata are also incorporated. A comparison with theoretical body tides in the spectral domain reveals strong evidence against a correlation between seism ovolcanic activity and tidal stress. However, other parameters such as temp erature and barometric pressure show the same diurnal and semi-diurnal modu lation 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.