PSI - Issue 78

Franco Braga et al. / Procedia Structural Integrity 78 (2026) 2184–2191

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and N170° and a tilt between 45° and 70° SW. The seismogenic structure was oriented in a NNW-SSE direction (Apennine direction), extending approximately 25-30 km between the municipalities of Norcia and Amatrice, with a width of approximately 10-12 km and a depth of 10-12 km. The rupture surface extended beneath the town of Amatrice and at shallow depths. Fig. 4 shows a detailed analysis of the pseudo-acceleration spectra obtained by processing the recordings of the AMT and AMTS stations, with the event's distinctive characteristics highlighted (blue boxes). a b

Fig. 4: Pseudo acceleration spectra (AMT and AMTS) with a) E- W for the “Forward directivity”, b) N - S for the “Fling step

The E-W component, with a propagation direction almost perpendicular to the fault, shows a signal clearly indicating forward directivity at high frequencies (low periods), while the N-S component (with a propagation direction almost parallel to the fault) shows a signal clearly indicating fling step, with more intense pulses at low frequencies. Finally, in Fig. 5, the high intensity and concentration of the vertical acceleration component within a narrow frequency range are evident.

Fig. 5. Pseudo-acceleration vertical spectra (AMT and AMTS stations) showing unusual intensities

The analysis reported above clearly indicates that, for the town of Amatrice, the event of August 24, 2016, was a near-fault (or near-source) earthquake. 3.3. Near-Fault Earthquakes in Technical Codes The most advanced international seismic codes contain within them, for several decades, specific indications for "near fault" earthquakes and in particular: • Uniform Building Code 1997 (ICBO 1997): considers two "near-source" factors (Na and Nv) depending on the type of seismic source and its distance from the area in question; • ATC-40 (ATC,1996): the same factors prescribed in UBC 1997 are used; • ASCE/SEI 7-10 (ASCE, 2010): the need to consider the effects related to "near-source" earthquakes is prescribed, also giving specific prescriptions regarding the direction to be considered for each pair of horizontal components for sites falling in areas less than 5 km away from active faults;