Seismic heterogeneity of Mt Etna: structure and activity

SUMMARY A dense seismic array maintained temporarily for 7 weeks provides an approach to deducing the 3‐D heterogeneity of the structure of the upper crust beneath Mt Etna volcano, Sicily, Italy, from the inversion of P‐ and S‐wave arrivals of local earthquakes. Independent data from a different source‐array geometry using waves generated by distant shots at sea confirm a main lateral variation. A high‐velocity body underlies part of the volcano. Its contrast with the surrounding thick sedimentary pile and its location extending from the present summit to the southeast suggests that it may have been built by the development and migration of past eruptive centres followed by caldera collapse. Local low‐velocity anomalies near its sharp western edge may be associated with presently active features. These are: an extension of the region of the central craters from sea level to greater depth and a zone at 6 km depth, of possible proximity of magmatic material to sediments connected both to the upper central system and to greater depths. The structure under the western flank appears as highly heterogeneous. During the recording period, a sustained lava eruption of the Southeast Crater stopped and was followed by explosive activity at the central craters and a series of destructive earthquakes on the lower eastern flank in October 1984. No continuous migration occurred between these phases but they might be related through their locations with respect to structural heterogeneity. Seismic activity changed from a swarm of events just beneath the summit at sea level to a few isolated, much stronger events a few kilometres deeper and towards the edge of the central high‐velocity zone and eventually to destructive single earthquakes further away under the lower slopes.