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

Forecasting of volcanic eruptions is still inadequate, despite technological advances in volcano monitoring. Improved forecasting requires a deeper understanding of when unrest will lead to an actual eruption. Shallow, long-period seismic events often precede volcanic eruptions and are used in forecasting. They are thought to be generated by resonance in fluid-filled cracks or conduits, indicating the presence of near-surface magmatic fluids. Here we analyse very-high-resolution seismic data from three active volcanoes-Mount Etna in Italy, Turrialba Volcano in Costa Rica and Ubinas Volcano in Peru-measured between 2004 and 2009. We find that seismic resonance is dependent on the wave propagation path and that the sources for the long-period seismic waves are composed of short pulses. We use a numerical model to show that slow-rupture failure in unconsolidated volcanic materials can reproduce all key aspects of these observations. Therefore, contrary to current interpretations, we suggest that short-duration long-period events are not direct indicators of fluid presence and migration, but rather are markers of deformation in the upper volcanic edifice. We suggest that long-period volcano seismicity forms part of the spectrum between slow-slip earthquakes and fast dynamic rupture, as has been observed in non-volcanic environments.