GFZ German Research Centre for Geosciences Potsdam

On 26 April 1917, an earthquake Mw 6.0 (known as the Monterchi earthquake) struck the upper Tiber basin, central Italy. This Quaternary basin is situated on the hanging wall of a segmented low‐angle normal fault (LANF) named the Alto Tiberina fault (ATF), whose capability to act with stick‐slip behavior is debated. We reanalyzed instrumental historical data and performed a relocation of the hypocenter along with a reassessment of the scalar moment and magnitudes (⁠Mw and Ms⁠); we calculate the fault radius and stress drop and propose a focal solution based on first‐arrival polarities. The methodologies that we applied strongly take into account the intrinsic uncertainties present in the historical data to obtain solutions that provide well‐defined error estimates. The hypocentral solution is consistent with the known macroseismic scenario and is plausible in terms of root mean square (rms) and error ellipse. The calculated focal depth at ≈8  km⁠, even considering the depth uncertainty of ±4  km⁠, locates the earthquake source inside the ATF footwall. The Mw distribution (median Mw 5.8) suggests an overestimate of the macroseismic Mw probably due to seismic amplification caused by site effects. A preferred focal solution (strike 60/dip 84/ rake −162) defines an antiapenninic‐oriented fault, with predominantly right‐lateral slip. Our results strongly suggest that the 1917 Monterchi earthquake did not nucleate on one of the splays of the low‐angle ATF but rather was generated by a deeper seismic source, which could be referred to as a structural transfer zone responsible for the Quaternary LANF segmentation.