Testa, A., Boncio, P., Baize, S., Mirabella, F., Pucci, S., Pace, B., Riesner, M., Pauselli, C., Ercoli, M., Benedetti, L., Di Chiara, A., and Civico, R.
The NE‐dipping Anghiari normal fault, bounding to the west the Sansepolcro basin in the Upper Tiber Valley (northern Apennines), is thought to be a synthetic splay of the Altotiberina low‐angle normal fault (LANF), an active ENE‐dipping extensional detachment whose seismogenic behavior is debated. In order to assess the Anghiari fault capability to break the surface during strong earthquakes and be the source of historical earthquakes, we acquired high resolution topographic data, performed field survey and geophysical investigations (Seismic reflection, Ground Penetrating Radar, Electrical Resistivity Tomography) and dug three paleoseismological trenches across different fault sections of the Anghiari fault. The acquired data reveal for the first time the Late Pleistocene to historical activity of the Anghiari fault, constraining the age of seven paleo‐earthquakes over the last 25 ka, the youngest of which is comparable with one of the poorly constrained historical earthquakes of the Sansepolcro basin. The yielded slip rate is >0.2 mm/yr averaged over the last 25 ka and the recurrence interval is about 2,500–3,200 years. An analysis of the anisotropy of the magnetic susceptibility performed in one of the paleoseismological trenches revealed an extensional stress field, continuously acting during the sedimentation of the entire trenched stratigraphy. Our results confirm the ability of the Anghiari fault to generate surface faulting earthquakes. In addition, if the Anghiari fault does sole at depth into the Altotiberina low‐angle normal fault, this LANF could also be seismogenic and generate M > 6. Plain Language Summary: We analyzed the capability of the Anghiari fault (Upper Tiber Valley, Northern Apennines of Italy) to reach and break the topographic surface during a strong earthquake. This fault may links to the Altotiberina low‐angle normal fault (ATF), a detachment fault accompanied by a system of synthetic splays whose seismogenic behavior is still debated because of a lack of seismological and paleoseismological data. Thanks to geological, morphological, and geophysical analyses, four sites have been selected for paleoseismological investigation. The paleoseismological data revealed the seismic history of the Anghiari fault, assessing its capability to recurrently break the surface during strong earthquakes. In addition, an analysis of the anisotropy of the magnetic susceptibility performed in one of the paleoseismological trenches indicates that the stress field has been extensional, acting continuously during the sedimentation of the entire trenched stratigraphy. Our results suggest that the AF is capable to break the surface during strong earthquakes. This has potential implications for the seismogenic behavior of the ATF low‐angle normal fault, as the strong earthquakes (M > 6) may nucleate on the low‐angle fault plane if the AF is soling at depth into that detachment. Key Points: Revealed the Late Pleistocene—Holocene slip history of the Anghiari fault, considered a synthetic splay of a low‐angle normal faultThe capability of the Anghiari fault to rupture the surface during M > 6 earthquakes has been ascertainedThe results have potential implications for the activity and seismogenic behavior of a continental low‐angle normal faults [ABSTRACT FROM AUTHOR] more...