Now showing 1 - 2 of 2
  • Publication
    Origin of spurious single forces in the course mechanism of volcanic seismicity
    Single forces are often observed in the source mechanism of volcanic seismicity. However, their underlying causative processes are still doubtful. The reliability of single force observations must be assessed, prior to analyzing them in terms of physical mechanisms. Using numerical examples, we show that source mislocation and velocity mismodeling lead to strong spurious single forces. Layering in the velocity model produces converted S-waves and source mislocations modify the wavefield at the free surface (mainly through converted S- and surface waves). However, these waves can also be accurately reproduced in a homogeneous model by adding a vertical single force in the source mechanism, which mainly generates S-waves for large take-off angles. Hence approximate velocity models can lead to the appearance of strong single forces in source inversions. We conclude that, in moment tensor inversion, while single forces can be used in some cases to accommodate mismodeling errors, they cannot be reliably used to infer physical processes.
    Scopus© Citations 16  228
  • Publication
    Long-period seismicity in the shallow volcanic edifice formed from slow-rupture earthquakes
    (Nature Publishing Group, 2013-12-15) ; ; ;
    Despite recent technological advances in volcano monitoring, eruption forecasting is still inadequate. Improved forecasting requires a deeper understanding of when unrest will lead to an actual eruption. Shallow Long Period (low spectral frequency) seismic events are routinely employed as a primary tool in forecasting strategies as they often precede eruptions. They are universally explained as resonating fluid-filled cracks or conduits, indicating the presence of mechanically active near-surface fluids. We undertake very high resolution seismic field experiments at Mt Etna, Italy; Turrialba, Costa Rica and Ubinas, Peru, in which we find that seismogram resonance is propagation path related whilst the seismic sources comprise short pulses. Data analysis and numerical modelling show that slow-rupture failure in unconsolidated volcanic materials reproduces all key aspects of these new observations. Contrary to current interpretations, here we show that our observed Long Period events are not direct indicators of fluid presence/migration, but rather are markers for upper edifice deformation. This finding encapsulates this seismicity within growing observations of a spectrum of deformation rates in other non-volcanic environments, from slow-slip earthquakes through fast dynamic rupture. It calls for a reassessment of how lowfrequency seismic signals are interpreted in their key role in eruption forecasting.
      1065Scopus© Citations 125