PSI - Issue 78

Franco Braga et al. / Procedia Structural Integrity 78 (2026) 2176–2183

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response spectra are well highlighted (Memarpour et al., 2016) in Fig. 3 1, in which spectra with progressive directionality (thin lines) are compared with spectra taken from technical codes (thick lines). • Fling step that originates from residual displacements due to tectonic deformations, in a direction parallel to the fault characterized by speed pulses of great amplitude and consequent low frequency signals of strong intensity whose effects on the response spectra are well highlighted (Memarpour et al., 2016) in Fig. 3 2 in which spectra endowed with step failure (broken lines) are compared with some normal spectra (solid lines). a b

Fig. 2. Spectra (averages of 68 forward directivity accelerograms) compared with spectra of various standards.

Both the described phenomena cause the motion at the source to be of an impulsive type, with signals characterized by fairly homogeneous frequency contents over wide intervals: at high frequencies, due to the forwad directivity; at low frequencies, due to the step failure. As a consequence of the above, the prevailing signals that depart from the source are quasi-white noises with high amplitude. The limited path that brings the signal to the surface does not allow to modify its characteristics, nor by limiting its intensity, nor by modifying the intervals of the frequency content, as is the case for deep earthquakes which are those described by the seismic codes. Finally, another peculiar feature of "near fault" earthquakes is the fact that the vertical component of the seismic action is decidedly higher than the usual and almost impulsive, thus determining overcompression or decompression of the vertical bearing elements (walls, pillars). 3.2. Peculiar characteristics of the earthquake of August 24, 2016 Studies conducted on the seismic source that caused the event in question demonstrated that the distribution of the main events and the aftershocks on August 27 indicated a main plane with a variable direction between N140° 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. 3 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 c

Fig. 3: Pseudo acceleration spectra (AMT and AMTS) with a) E-W for the “Forward directivity”, b) N-S for the “Fling step”; c) vertical from which unusual intensities are appreciated The E-W component, with a propagation direction almost perpendicular to the fault, shows a signal clearly