Strain pattern of the Southern Tyrrhenian slab from moment tensors of deep earthquakes: implications on the down-dip velocity

Abstract

Seismic strain is analysed along the slab face of the Southern Tyrrhenian subduction zone using focal mechanisms of deep earthquakes which occurred in the period 1960-1998. Results show that the slab is mostly affected by down-dip shortening which strongly increases below 250 km depth. Extensional strain is mainly confined to the direction perpendicular to the slab face. The dominance of down-dip shortening and the minor along strike inplane extension implies thickening of the slab below 250 km depth. Assuming constant seismic efficiency along the slab, this strain pattern also implies a decrease of the down-dip velocity below 250 km depth. We also locate lower magnitude intermediate-depth and deep earthquakes using arrival times since 1985 available from the Italian seismic national network. These data show that the slab reaches the deeper part of the upper mantle, as suggested by the occurrence of a few ??600 km depth earthquakes, and that a large portion of the Tyrrhenian slab, between 100 and 250 km depth beneath the offshore of the Calabrian arc, is aseismic. Only a short part of the Tyrrhenian slab is seismically continuous from the top to the bottom. The lack of seismicity may indicate either that aseismic subduction is occurring or that the slab is detached from its upper part. Although the data are still inconclusive, they suggest that an aseismic subduction is the most appropriate interpretation, considering recent tomographic images of the slab and the results of this study, which agree well with the presence of a neutral down-dip stress zone, as also observed worldwide in deep slabs.