Activity of Mount Etna preceding the February 1999 fissure eruption: inferred mechanism from seismological and geochemical data

The February 1999 eruption of Mt. Etna took place through a fissure on the SSE flank of the cone of the summit SE Crater. This event was preceded by continuous activity since 1995, sometimes accompanied by violent outbursts from one or more of the three other summit craters (NE Crater, Voragine or Chasm, and Bocca Nuova), and finally by a series of 20 short-lived eruptions from the SE Crater between September 1998 and January 1999. These phenomena could be accounted for by invoking gradual invasion of a shallow small reservoir by more primitive, basic and gas-rich magma coming from depth. The shallow “chamber” is more likely to be a plexus of dikes, which had developed during the previous years (1995–1997), following variations of the local stress field owing to enhanced magma generation and accumulation at the top of the mantle. Magma injection and mixing is evidenced through geochemistry, whereas the state of stress of the volcanic pile and underlying crust is determined using earthquake distributions and focal mechanisms. The behaviour of the seismic tremor amplitude appears to be a good indicator of the state of unrest of the volcano, although not always directly linked to the relative energy of degassing phenomena.