Volcanic tremor-source mechanisms and correlation with eruptive activity

Signal characteristics and intensity patterns of volcanic tremor are discussed with regard to eruption phenomena. Although a generally valid relationship cannot be given, volcanic tremor is strongly correlated to the fluid dynamics of the volcano. A major difficulty in the interpretation of volcanic tremor is the lack of an appropriate source model. It is assumed that flow transients of quite different kinds act as seismic sources. The significant correlation of volcanic tremor to external forces (barometric pressure, rain-fall loading, earth tides) is discussed and included in a relation with magma-flow instabilities. Two-phase (liquid-gas) flow instabilities play an essential role in the origin of volcanic tremor and, possibly, in the occurrence of volcanic eruptions. A model is discussed, in which one cause of tremor is seen in self-sustained pressure oscillations following transitions between subsonic and supersonic two-phase magma flow (modified Steinberg and Steinberg model).