Analysis of vibrations recorded inside the cemetery area of Incisa, central Italy

Seismic stations are usually used to record seismic event and, therefore, they are recommended to be installed far from railways and traffic roads in order to avoid the superposition of ambient noise signals to those provoked by an earthquake. In this paper, instead, seismic stations, placed intentionally in areas near railway and traffic roads, are used to characterize the subsoil spectral properties and to assess the effect of vibrations due to trains and vehicles. A cemetery in the green countryside near Florence is chosen as a reference case study to deal with this topic. Most of the buildings in the cemetery area are affected by an extensive crack pattern. In January 2020 five seismic stations were installed in order to evaluate if the trains running in the tunnels of the regional and high-speed railway lines located below and in the vicinity of the cemetery and the vehicles traveling on the nearby A1 highway and regional road can produce vibrations in the ground that justify the observed damage pattern. Collected data are analyzed using the Nakamura technique in order to estimate the dynamic properties of the ground and compared to the limits provided by the current regulations. Furthermore, the trend of the Root Mean Square average over the entire recording period is computed as well. From the obtained results, it is possible to highlight that the average daily oscillation level increases from early morning until 7 p.m. and then it decreases, and also that the highest amplitudes of transients are concentrated in the late evening and during the night, when the background noise is lower. Furthermore, the computed values of the maximum and average amplitudes are lower than those that can cause damage to buildings as defined by the guidelines, the eigenfrequency of the ground falls in a range far from that ascribable to the cemetery buildings, so that the resonance effects can be excluded. In order to confirm these results, the amplitude of ground shaking due to recorded transients is compared to that produced by two earthquakes (a 3.4 Mw local earthquake at more than 100 km and a Mw 6.6 teleseism from Turkey) which occurred during the monitoring period. One can conclude that it seems unlikely that the shaking produced by nearby vehicles and trains could be responsible for the observed damage.