Relocation of microearthquake swarms in the Peloritani mountains – implications on the interpretation of seismotectonic patterns in NE Sicily, Italy

The Peloritani Mountains in northeastern Sicily make part of the Appennine-Maghrebian Chain, which forms the highly deformed southern margin of the European Continent. In this zone the NW–SE-striking ‘Aeolian-Tindari-Giardini’ System (ATG) separates two areas of seismicity. To the west of the ATG fault system, seismic activity below a depth of 40 km is essentially absent. To the east of this fault system,we note a significant presence of intermediate depth and deep events, which mark the subduction zone in the Calabrian Arc. Between 1994 and 2003 300 microearthquakes could be located with fair accuracy near the ATG fault system. Their depths range from less than 5 to 40 km, with greater depths occurring to the east of the ATG.We examined the resolution capability of the standard location by applying a grid search location for typical events. The distribution of the residuals shows trends recognizable in the standard locations that are in part an artefact of a non-ideal conditions of the standard locations, such as the station configuration, the use of an unsuitable velocity model and inconsistencies of arrival time pickings. By applying relative location techniques (the double-difference method and a master-event technique) we were able to reduce the scatter of hypocentres significantly.
We focused in particular on earthquake families with similar waveforms and estimated the geometrical extent of hypocentre clusters. Compared to the standard location the dispersion of hypocentres decreased by an amount of over 90 per cent and the volume occupied by the foci contracted to 1 per cent. The significance of these geometries was tested with Monte Carlo experiments and by interchanging the master events. The cluster geometries are consistent with the dislocation patterns as inferred from fault-plane solutions but do not show a simple relation to the ATG. The role of fluid flow of plutonic origin may be invoked as a possible trigger mechanism. This hypothesis is supported by the presence of geothermal anomalies in the vicinity as well as by an upward migration trend in foci.

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