Seismo-volcanic source mechanisms in hydrothermal systems

The observation of a unique very long period (VLP) seismic signal on 23 March 2012 at Soufriere Hills volcano, Montserrat triggered this research. This event, observed on a sparse seismic network, showed the limitations of routine processing steps to estimate correct source location and source mechanism. Firstly, it shows the absolute necessity to perform a proper restitution process where one has to consider spectral components of the lowest possible signal frequencies, as the interpretation of the ground displacement, and therefore, the deformation at the source changes drastically, in this case, from oscillatory behaviour to a static displacement. Further, it emphasises the often neglected influence of a seismic network configuration on our ability to correctly resolve the source mechanism when performing moment tensor inversion. Using numerical tests, it is shown that the moment tensor inversions using a low number of stations with poor azimuthal coverage can still produce an acceptable waveform fit at the surface, however, the time-histories of source components are not resolved. Volcano observatories can use this approach to improve their monitoring capabilities. In contrast to the Montserrat event, the VLP signals observed in a hydrothermal environment at Whakaari, New Zealand show an oscillatory waveform entirely produced by the source. Two source mechanisms are investigated: the effect of poroelastic wave propagation and a coupled fluid-solid system. By testing these two hypotheses we gain an interesting insight in how different characteristics of poroelastic media affect seismic waves in the frequency band typical in volcano seismology.