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In this paper we present a new study on the High Tiber Valley earthquake occurred on April 26, 1917. Using the digitized data from mechanical seismograph records, we computed the source parameters like focal mechanism and moment magnitude from moment tensor (MT). The study of historical earthquakes from an instrumental perspective is crucial because of the complexity of problems associated with the study of seismograms of moderate to large earthquakes occurred from the late 19th century until the early 1960s. Since historical earthquake records show significant uncertainties in phase arrival times and have been recorded by seismograph generally with short natural period, we developed a code to compute the MT based on a forward modeling technique, which uses the amplitude spectra of the full waveform length and the first P-arrival polarities to constrain the P- and T-axes. The best solution is determined by the best fit between the observed and synthetic amplitude spectra and from the coherency between the observed and the theoretical first P-arrival polarities. The 1917 High Tiber Valley earthquake is one of the most important 20th century earthquake occurred in the Italian Peninsula for which the focal mechanism and moment magnitude from seismic records are not available. Additionally, we apply a multidisciplinary approach to characterize the source of this earthquake, combining instrumental, macroseismic, geological and tectonic data and investigations. The computed MT results in a north-south normal fault mechanism (strike: 147°, dip: 29°, slip: −94°), which is consistent with the strike estimated from the macroseismic data (157°). The moment magnitude calculated from the MT and that derived from the macroseismic data are Mw=5.5±0.2 and Mw=5.9±0.1, respectively.