Quaternary faults and seismicity in the Umbro-Marchean Apennines (Central Italy): evidence from the 1997 Colfiorito earthquake

Analyses of structural and geomorphological data combined with remote sensi ng interpretation confirm previous knowledge on the existence of an extensi onal Quaternary tectonic regime in the Colfiorito area (Umbro-Marchean Cent ral Apennines), This is characterized by a maximum principal axis of Finite strain oriented approx. NE-SW, which is the result of a-progressive deform ation process due to pure and radial extension. Surface geological data, th e crustal tectonic setting (reconstructed using a CROP 03 seismic reflectio n profile), and seismological data relative to the autumn 1997 Colfiorito e arthquake sequence constrain the following seismotectonic model. We interpr et the seismogenic SW-dipping low-angle normal fault pictured by seismic da ta as an inverted thrust ramp located in the basement at depth between 5 an d 10 km, The surface projection of this seismogenic structure defines a cru stal box within which high-angle normal faults are responsible for the defo rmation of the uppermost crust, The regional patterns of pre-existing basem ent thrusts therefore control the seismotectonic zoning of the area that ca nnot be directly related to the high-angle normal fault systems which cut t hrough different crustal boxes; the latter system records, in fact, re-shea r along pre-existing normal faults. Moreover, Quaternary slip-rates relativ e to high-angle normal faults in the General Apennines are closely related to seismic hazard within each crustal box. (C) 1999 Elsevier Science Ltd, A ll rights reserved.