Distinguishing the wavefield of volcano-seismic events on Mt. Etna: Achieving wavefield separation combining a seismic array and a rotational sensor

Mt. Etna volcano is Europe’s most active volcano, showing pre- and co-eruptive seismic signals as tremor and long-period (LP) events. Understanding those signals contributes to hazard assessment and risk management during volcanic eruptions. In our study we examine the wavefield composition of LP events and volcanic tremor on Mt. Etna. Both are characteristic seismic signals generated by fluid-driven volcanic activity. By combining results from a seismic array and a rotational sensor co-located with a seismometer (6C station), we decipher their wavefield.

For seismic data from August - September 2019 we calculate and compare directional and phase velocity estimates. Back azimuths (BAz) of LP events and tremor from the seismometer array and the 6C station are compared to reference network BAzs which are obtained from locations estimated by the Istituto Nazionale di Geofisica e Vulcanologia-Osservatorio Etneo (INGV-OE) on Mt. Etna.

We observe varying seismic tremor and surface activity which we associate with different eruption phases. During these tremor phases, either the array or 6C BAz estimates agree well with the INGV-OE reference. LP event BAz directions from both methods show a southward shift in comparison with the INGV-OE reference. Local heterogeneities might cause the larger southward deviation of the 6C BAz results in comparison with the array.

Array slowness results indicate that tremor and LP events were primarily composed of surface waves. Rotational sensor recordings further indicate a wavefield dominated by SH-type waves. Together with the array results, this suggests a Love-wave dominated wavefield. The combination of rotational sensors with seismic arrays significantly enhances our ability to constrain the wavefield in complex volcanic settings.