Seismicity and 3-D substructure at Somma–Vesuvius volcano: evidence for magma quenching

We present new results on the velocity structure of the Somma–Vesuvius volcano, obtained by joint inversion of P- and S-wave arrival times from both local earthquakes and shot data collected during the TOMOVES 1994 and 1996 experiments. The use of a large set of earthquakes, recorded over a period of ten years by both temporary and permanent seismic stations, allowed us to enhance the resolution of the structure beneath the Somma–Vesuvius down to 5 km depth. The results obtained show the presence of a high Vp and Vp/Vs anomaly located around the crater axis, between 0 and 5 km depth, involving the volcano edifice and the carbonate basement westward deepening from the adjacent Apenninic belt. The whole available seismic catalogue between 1987 to 2000 (1003 events) has been relocated in the obtained 3-D velocity model. Seismicity appears to be clustered around the anomalous high rigidity body. Laboratory experiments at high temperatures and pressures on 1944 eruption lava samples, taken representative in composition of the magma masses erupted through the cycle 1631–1944, support the interpretation of this anomaly in terms of magma quenching along the main conduit, because of the exsolution of magmatic volatiles. The effect of volatiles from the melt at the eruption onset and through its explosive phases is to increase the solidus temperature well above its eruptive temperature, causing the immediate quenching of the system. This paper shows a good example of how seismic tomography and experimental petrology constrain magmatic models. Results have important implications for the hazard assessment at Somma–Vesuvius, and at other volcanoes worldwide where similar seismological evidence has been recently observed.