Non-double-couple mechanisms in the seismicity preceding the 1991-1993 Etna volcano eruption

The temporal evolution of the complete source moment tensor is investigated for 28 earthquakes that occurred at Mt Etna in the period August 1990-Dece mber 1991 preceding the biggest eruption of the last three centuries. We perform several tests to check the robustness of the results of inversio n considering different frequency ranges and different groups of stations. As well as the selection of good-quality data, the error analysis, statisti cally significant at the 95 per cent confidence level, is employed to valid ate the findings of the inversion and to distinguish between physical solut ions and artefacts of modelling. For events between 0.3 and 10 km depth: strike-slip mechanisms prevail on n ormal, inverse and dip-slip mechanisms; this is possibly due to the dyke-in duced stress dominating the overall stress field at the surface, producing a continuous switch of the tensile and compressive axes, The regional E-W t ension prevails at depth, as indicated by the prevalence of normal mechanis ms. An increment of the non-double-couple components is observed immediatel y before the eruption and can be related to movements of fluids? evert thou gh, for some events, the complex interaction between tectonic stress and vo lcanic activity cannot be excluded. The source time functions retrieved are in general simple and short but some show complexities, as one would expec t in volcanic seismicity. From the seismic scalar moment found, we extrapol ate an empirical moment-magnitude relation that we compare with other relat ions proposed for the same area and computed for the duration magnitude and the equivalent Wood-Anderson magnitude.