Long-period seismicity in the shallow volcanic edifice formed from slow-rupture earthquakes

Abstract

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.AD 17/02/201