Continuous GPS constraints on active extension in the Umbria-Marche Apennines, northern Italy

A recently established network of 20 continuously operating GPS stations in the Umbria-Marche region of the northern Apennines, Italy, records strain accumulation associated with a system of active normal faults. Crustal velocities reveal roughly 3 mm/yr of relative motion accommodated between the Tyrrhenian and Adriatic coasts, but extensional deformation is mostly concentrated in a fairly narrow ~30 km-wide belt which lies just west of the Apennines topographic high. An apparent NW directed component of velocity, possibly related to broader scale geodynamic processes associated with Nubia-Eurasia convergence, or other causes, is superimposed upon the extensional deformation signal. The locus of the extensional component of deformation coincides with the low angle (~20°) NE dipping Altotiberina fault, which is clearly imaged in seismic reflection profiles (CROP03) and highlighted by microseismicity (0.6 ≤ M ≤ 3.0). However, larger earthquakes in the region (~M5-6) appear to occur on antithetic normal faults with high angle (~60°) SW dip (e.g., 1979 Norcia, 1984 Gubbio, and the eight 1997-8 Colifiorito earthquakes). Low angle normal faults such as the Altotinerina fault are widely documented, but whether or not displacement along low angle normal faults is mechanically possible remains an important unresolved issue. Collettini and Holdsworth (Journal of the Geological Society of London, 161, 2004) proposed a model for low angle Altotiberina fault slip by aseismic frictional-viscous creep at fairly shallow crustal levels. Based on the statistical characteristics of our preliminary data analyses, we show that the new continuous GPS network in the Umbria-Marche Apennines will be well suited to test this hypothesis as more data are collected and horizontal and vertical rates become more precise.