Seismic Efficiency during Volcanic Unrest: Insights from the 2010 Eyjafjallajökull Eruption

Identifying patterns in precursory signals may aid forecasting over month to year timescales. Here we examine the relationship between seismicity and ground deformation during unrest prior to the 2010 Eyjafjallajökull eruption. We find that the ratio between seismic moment and horizontal GPS ground displacement is constant within two distinct phases, but with a step increase from one to the other. This step-change is attributed to a change in source between two sills, and is expected given the change in deformation associated with different source geometries and deformation mechanisms. We use displacement data to estimate source volume change, assuming each source has a fixed geometry through time. We can then calculate seismic efficiency (the ratio between seismic moment and volume change, assuming a constant shear modulus). An increase in seismic efficiency is also present between sources, indicating different growth mechanisms. The shallower source has a lower proportion of aseismic deformation, consistent with previous observations showing clusters of seismic events, interpreted as a number of magmatic ‘lobes’ separated by seismogenic zones. This case study provides new insights into subsurface processes prior to eruptive activity, specifically controls on the seismic efficiency of intrusions.