Seismic site effects in the Red Zone of Amatrice hill detected via the mutual sustainment of experimental and computational approaches

Abstract

This paper presents the results of numerical analyses of ground motion in the Red Zone sector of Amatrice hill, violently struck by the 2016-2017 Central Italy seismic sequence. The methodologies used in processing the data to define the numerical model are firstly described. The results obtained from the computational analyses are then presented and discussed by comparing them with experimental data set of weak motion recordings. Computational analyses were performed via both a 2D-numerical FEM model and a pseudothree-dimensional hybrid model (SiSeRHMap) which develops multispectral maps taking into account topographic effects. Starting from available geological data and geophysical measurements, an original and specific subsoil GIS model was developed and utilised to perform the computational analyses. The preliminary map for fundamental periods computed from the subsoil model is in good agreement with the experimental data. A restricted set of weak ground motions acquired from an accelerometric station located in a reference site was used as input for the numerical analyses, while the signals of the corresponding events recorded at the top of the hill were used as targets in the reliability evaluation analysis of the outputs. In the area of Amatrice hill, which is characterized by a complex geological and topographical context, the reliability analysis shows a good performance of the hybrid model compared to the 2D-FEM model in the prediction of seismic response. Agreement generally was also good with regards to the experimental and computational results, both in relation to the amplitude and to the shape of the spectral amplification that change depending on the hill sector. Considering the predictive reliability of the models, a high amplification, due to topographic effects, was observed for the Red Zone by performing a back-simulation of the 24th August 2016 main shock. The analysis results highlight also that the maximum amplification factors, based on the definition of the Housner intensity, occur in the interval of periods 0-0.5 s covering the fundamental period range of the buildings in this area.

Near-Source Simulation of Strong Ground Motion in Amatrice Downtown Including Site Effects

“…Here, the amplification is larger in the northern sector and varies from the central part to the edge, where [25] observed a clear directionality of the resonant peak (on the contrary, [34] concluded that maximum directions are linked to the presence of noise sources). These observations suggest a lateral variability of the geological conditions, due to the spatial variation of the impedance contrast, and the presence of topographic effects, due to the morphology of the area [25,29,30]. quake a few hundred meters away from the historical center (AMT station).…”

“…This short review highlights that there is no single interpretation on the explanation of the ground motion experienced in Amatrice, neither on the causes of the variability of heavy damage [25,29,30,34]. Indeed, the ground motion complexity, due to a combination of nearsource and site effects, is difficult to reproduce with standard simulation techniques [21,29].…”

“…The closest seismic station to Amatrice downtown that recorded the 24 August, Mw 6.0, earthquake was AMT, belonging to the National Accelerometric Network [37] and able to record seismic events with a Ml ≥ 2.5 [17]. AMT was at an epicentral distance of about 8.5 km (Rjb = 1km; [38]) and recorded the maximum horizontal PGA for the event on the EW component (8.5 m/s 2 ; [2]); the difference with the NS component, that recorded half of the PGA value, was probably caused by topographic effects [29], seismic amplification [25], or directivity effects [14].…”

“…The Amatrice basin is filled by about 60-m-thick Quaternary fluvial deposits, laying above the Messinian siliciclastic deposits of the Laga Formation, which play the role of seismic bedrock for the area [27,28]. They are made by conglomerates and sands alternations, few tens of meters thick, which can generate a seismic impedance contrast that, together with the topographic effects, is able to generate a strong amplification effect [25,29,30].…”

Ground motion prediction maps using seismic microzonation data and machine learning

“…Ground motion prediction maps, referred to the Central Italy event occurred on August 24, 2016, (i.e., the first destructive event of the Central Italy seismic sequence for which a great amount of studies have been published) are shown in § 4 to enlighten the capability of the proposed ML approach to gauge the ground motion variability at the urban scale. Moreover, with reference to § 4.1, the ground motion profiles, based on the proposed ML approach, are compared with results obtained by means of two completely different methodologies: 2D numerical modelling of seismic site response (Gaudiosi et al, 2021;Giallini et al, 2020;Grelle et al, 2020) and with the mean values predicted by the Italian ShakeMaps (http://shakemap.rm.ingv.it/shake4/). Finally, in the § 4.2, referring to the seismic event occurred on October 30, 2016 (i.e., the strongest of the Central Italy seismic sequence), the spatial correlation structures of the maps are analyzed in terms of sill and range in order to provide the relation between local site effects and spatial resolution of ground motion maps.…”