MICROEARTHQUAKES IN THE TYRRHENIAN SEA AS REVEALED BY JOINT LAND AND SEA-BOTTOM SEISMOGRAPHS

Five independent ocean-bottom seismographs (OBS) were operated from 17 to 28 May, 1987 in the Southeastern Tyrrhenian Sea, at a depth range of 1000-2800 m. These were concomitantly run with nine Vesuvian Observatory and University of Calabria temporary digital seismic stations along the Calabrian coast together with fifteen permanent seismic stations in Southern Italy. The latter stations also belong to the University of Calabria, and to the Italian National Institute of Geophysics. About 200 local shocks, mainly microearthquakes (ML < 3), were recorded by the OBS network. At least three swarms were located approximately beneath the subsea volcano Alcione (at depths of 2-7 km), beneath Diamante seamount (7-16 km), and, on land, beneath Mt. Sacro (7-10 km). Some features of the observed seismicity are typical of active volcanic areas. Up to 30 tectonic earthquakes were also located, either by the OBS array alone or by sea-bottom and land-based stations together. More than half of them occurred beneath the Tyrrhenian Sea, with the others occuring on the Apennine Peninsula. The majority of the earthquakes originated in the upper crust (1-10 km). Some concentration of shocks along the subsea slope was observed. Three shocks were located in the Benioff zone of the Calabrian arc. Some very small shocks were also distinguished in the mantle between the Benioff zone and the Moho, which may support hypotheses of deep roots (up to the asthenosphere) for volcanism in island arcs. © 1990.