Abstract
The northern coast of Sicily and its offshore area represent a hinge zone between a sector of the Tyrrhenian Basin, characterized by the strongest crustal thinning, and the sector of the Sicilian belt which has emerged. This hinge zone is part of a wider W-E trending right-lateral shear zone, which has been affecting the Maghrebian Chain units since the Pliocene. Seismological and structural data have been used to evaluate the seismotectonic behavior of the area investigated here. Seismological analysis was performed on a data set of about 2100 seismic events which occurred between January 1988 and October 2002 in the Southern Tyrrhenian Sea. This paper focuses in particular on a set of data relating to the period from 6th September 2002, including both the main shock and about 540 aftershocks of the Palermo seismic sequence. The distribution of the hypocenters revealed the presence of two main seismogenic zones. The events of the easternmost zone may be related to the Ionian lithospheric slab diving beneath the Calabrian Arc. The seismicity associated with the westernmost zone is closely clustered around a sub-horizontal regression plane contained within the thinned Southern Tyrrhenian crust, hence suggesting that this seismogenic zone is strictly connected to the deformation field active within the hinge zone. On the basis of both structural and seismological data, the brittle deformation pattern is characterized by high-angle faults, mainly represented by transcurrent synthetic right-lateral and antithetic left-lateral systems, producing both restraining/uplifting and releasing/subsiding zones which accommodate strains developing in response to the current stress field (characterized by a maximum axis trending NW-SE) which has been active in the area since the Pliocene. The cluster of the seismic sequence which started with the 6th September 2002's main shock is located within the hinge zone. The distribution of the hypocenters relative to this sequence emphasizes the presence of a high-angle NE-SW-oriented deformation belt within which several shear surfaces are considered to be found sub-parallel to that established for the main shock. The kinematics of all these structures is consistent with a compressive right-lateral focal mechanism.
Highlights
Over the last few years, a research group, including structural geologists and geophysicists from the Universities of Palermo, Camerino, Padova and INGV, has been studying both the neotectonic and the seismicity of the area between the Southern Tyrrhenian and Northern Sicily, in an attempt to outline a seismotectonic model for this area, by: 1) reconstructing the sequential order of the neotectonic deformations on land; 2) identifying the offshore fault pattern, processing in different ways the morphobathymetric maps; 3) performing a fractal analysis of the fault grid and morphostructural lineaments; 4) relocating and analysing clusters of the seismic events; 5) studying the focal mechanisms of the most energetic shocks; and 6) characterizing the geometry of both the seismogenetic volumes and their fracturing styles
Second-order structures related to the UsticaEolie and Kumeta-Alcantara lines are represented by certain NW-SE trending faults, extending for less than 100 km in Northern Sicily and its offshore zone
The crustal seismicity of this sector of the Southern Tyrrhenian Sea is mostly located in the hinge zone between the portion of the basin characterized by the strongest crustal thinning, and the sector of the Sicily belt which had already emerged
Summary
The recent seismicity (1988-2002) around the border between the Southern Tyrrhenian Sea and Northern Sicily, including the 2002 Palermo seismic sequence, highlights some geological aspects from a seismotectonic perspective. Over the last few years, a research group, including structural geologists and geophysicists from the Universities of Palermo, Camerino, Padova and INGV, has been studying both the neotectonic and the seismicity of the area between the Southern Tyrrhenian and Northern Sicily, in an attempt to outline a seismotectonic model for this area, by: 1) reconstructing the sequential order of the neotectonic deformations on land; 2) identifying the offshore fault pattern, processing in different ways the morphobathymetric maps; 3) performing a fractal analysis of the fault grid and morphostructural lineaments; 4) relocating and analysing clusters of the seismic events; 5) studying the focal mechanisms of the most energetic shocks; and 6) characterizing the geometry of both the seismogenetic volumes and their fracturing styles. The present paper represents a preliminary attempt to improve our understanding of the Southern Tyrrhenian seismotectonic processes
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