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

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.