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 c ontinuous activity since 1995, sometimes accompanied by violent outbursts f rom one or more of the three other summit craters (NE Crater, Voragine or C hasm, and Bocca Nuova), and finally by a series of 20 short-lived eruptions from the SE Crater between September 1998 and January 1999. These phenomen a could be accounted for by invoking gradual invasion of a shallow small re servoir 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 develop ed during the previous years (1995-1997), following variations of the local stress field owing to enhanced magma generation and accumulation at the to p of the mantle. Magma injection and mixing is evidenced through geochemist ry, whereas the state of stress of the volcanic pile and underlying crust i s determined using earthquake distributions and focal mechanisms. The behav iour 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. (C) 2001 Published by Elsevier Scie nce B.V.