Monitoring magma migration at Mt. Etna using the Seismic Amplitude Ratio Analysis method

The Seismic Amplitude Ratio Analysis method (SARA) was applied to data recorded during six days
before the May 13, 2008 eruption of Mt. Etna to test its potential as a forecasting attribute. By using
this method, the magma migration path, as well as the seismic migration, can be determined with
the amplitude of continuous data recorded at least at one pair of stations from a seismic network
near the eruption site. Due to the sudden changes in the seismic amplitude ratio calculated for
each pair of stations, the seismic migration trend, as well as the magma path at depths, were
clearly detected before the main eruption. The start and end times of the seismic swarms were
also determined. The standard practice to achieve similar results is to use volcanic tremors, which
must be pre-selected thus reducing efficiency and increasing the time needed. By using the whole
seismic signal, the method provides a simpler semi-automated alternative, especially for places
where it is not possible to record tremors continuously. This simple method is useful to reduce
uncertainties relative to hazardous magma propagation during volcanic unrest, as it helps to
improve the accuracy of locating seismic swarms and it allows determining the direction of magma
movement at depth before the eruption. We also analyzed the amplitude ratio trend using Mann-
Kendall and Sen’s estimator test. The results of these tests confirmed a positive and increasing
trend from the day before the eruption in most pairs of stations.