Abstract

In this paper, by integrating regional tectonic data with detailed structural data and with earthquake informations, we try to verify and to constrain the similarity between the present and the geological stress fields and to reconstruct a coherent history of deformation for the Umbria-Marche regions, from Middle-Late Miocene to present. Then, we use the inferred structural processes and deformation mechanisms related to the Apennine mountain building to better understand the seismicity of the study area and to discriminate seismotectonic domains. To conclude, we try to recognize and assess the corresponding seismogenetic structures. The basic conclusions can be summarized as follow: The geometry and orientation of the deformation and stress field in the Umbria-Marche region have remained unchanged since the Middle-Late Miocene and have been always characterized by the superposition of normal faulting upon thrust-belt structure and by the coexistence, in contiguous areas in a given time, of contraction to the east and extension to the west. By comparing macroseismic epicentres, focus depths and mechanisms with crustal structure and independently known active faults, it appears evident that the pattern, frequency and distribution of the crustal seismicity is tectonically-controlled and that a contraction-extension pair is active also at the present time. In fact, along the Adriatic coast-line, the contractional deformation with involvement of Pleistocene layers prevails, whereas beneath the Apennine Range the extensional deformation prevails. Both T-axes in the inner domain and P-axes in the outer domain are oriented SW-NE or WSW-ENE and this coaxiality reflects the situation depicted by the geological stress field. On this basis, we have produced a seismotectonic zoning with three domains being clearly identified: Intramountain Seismic Zone, Foothills Seismic Zone and Coastal Seismic Zone. A plausible explanation for the genesis of the "active" and seismic deformations is given, speculating that the locally intense extensional upper crustal seismicity of the Intramountain Seismic Zone is confined to the hanging-wall of a major westward-dipping active extensional discontinuity, whereas the less frequent contractional seismic activity of the Coastal Seismic Zone is within the hanging-wall of an outer major active westward-dipping contractional discontinuity. An area, of overlapping between these two zones is represented by the Foothills Seismic Zone, where superposed epicentral patterns from different planes of weakness at different depths are the predictable result of the asymmetry of the deformation in the investigated area.

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