High precision relocation of microearthquakes at Mt. Etna (1991–1993 eruption onset): a tool for better understanding the volcano seismicity

The beginning of the 1991–1993 large eruption at Mt. Etna was marked by a seismic swarm and a sudden increase of volcanic tremor amplitude. Several earthquakes were recognized as clusters of shallow events with nearly identical waveforms (multiplets). We have analyzed these signals with a cross-spectral technique that allows us to locate the relative position of the events within a multiplet, obtaining a precision higher than with traditional routines. The relocated shocks occurred some kilometers away from the propagating eruptive fracture, east of both the summit craters and the modeled eruptive dykes. They clustered in a relatively small volume, elongated in the NNW–SSE direction, the depth of which ranged from about 1100 to 2000 m below sea level. These earthquakes are interpreted as the passive brittle response of the volcanic basement to the intrusion of the eruptive dyke, rather than being directly linked to the magma transport.