Active aseismic creep on the Alto Tiberina low-angle normal fault, Italy

The existence of active low-angle normal faults has been questioned because the standard theory of fault mechanics precludes normal faults from slipping at low (<30°) dip angles, and there is an apparent deficiency of large normal fault earthquakes globally with a shallow dipping fault plane. The Alto Tiberina fault, Italy, is a low-angle normal fault (with dip 15°-20°) that has been clearly imaged with seismic reflection profiles and is highlighted by microseismicity. The fault is thought to be the main structure accommodating extension in the region. However, few large earthquakes have been documented on or near the fault for more than 2 ka. Analysis of data from a recently established network of continuous global positioning system (GPS) stations in the Umbria-March region, Italy, shows that the Alto Tiberina low-angle normal fault in the Northern Apennines is actively slipping at a shallow depth within the brittle crust. Crustal deformation models that best fit the GPS are consistent with both a shallow dip and active, mostly aseismic, creep of ∼2.4 ± 0.3 mm/a below 4 ± 2 km locking depth along the Alto Tiberina fault.