Evaluation of complex site effects through experimental methods and numerical modelling: The case history of Arquata del Tronto, central Italy
Introduction
The evaluation of local seismic response at sites characterized by complex geological and morphological features represents a crucial challenge. Difficulties mainly arise from both the reconstruction of a suitable and robust geological/geotechnical subsoil model and from limitations of geophysical and continuous numerical methods in such contexts (Pagliaroli et al., 2015).
Historical earthquakes in many cases showed the concentration of building damage in the villages located on the top of a relief (Paolucci, 2002), evidencing the importance of topographic amplification effects. Despite this observation, few instrumental strong motion stations are deployed on these sites and generally, when available, data show that recorded amplification is higher than numerically predicted (Geli et al., 1988; Pagliaroli et al., 2011). This discrepancy mainly arises from i) the difficulty to represent the complex geological and geotechnical site characteristics in the model adopted for the numerical analyses (generally homogeneous media) and ii) the above-mentioned limitations of the continuous numerical models, which are not able to return the discontinuous behavior of jointed rock mass.
On August 24th, 2016, an Mw 6.0 earthquake (Azzaro et al., 2016) with epicentral area located near the village of Accumoli (Lazio Region), hit central Italy (Fig. 1). The mainshock was followed by aftershocks located southeast and northwest of the epicenter, and by Mw 5.9 and Mw 6.5 seismic events occurring on 26th and 30th October, respectively, about 25 km to the NW of the previous mainshock (Fig. 1).
The impact of the 2016–2017 seismic sequence and in particular of the August 24th, 2016 event was highly destructive, causing ~300 casualties and extensive and irregularly distributed damage. After the earthquakes of 2016–2017, quantitative seismic microzonation (SM) studies were performed in order to obtain a clear background on site effects to perform a correct reconstruction of the municipalities (Pergalani et al., 2019). In particular, this paper refers the case study of some hamlets located along the Tronto River Valley where a severe and heterogeneous pattern of damage was observed, suggesting the potential role of site effects in amplifying/localizing the ground motion. Arquata del Tronto, Borgo, and San Francesco villages (all located in the Municipality of Arquata del Tronto) have been heterogeneously damaged by the August 24th, 2016 earthquake (Galli et al., 2016; Pagliaroli et al., 2019), although the distance among these three sites is less than 500 m. Fig. 2 reports a zonation of the damage distribution defined after the reconnaissance activity following the August 24th event (Lanzo et al., 2018) and some representative pictures. The damage categories proposed by Bray and Stewart (2000) were considered, ranging from D0 (no damage) to D5 (collapse of the structure). In particular, the level of damage increases moving from Borgo (damage D1–D2) to San Francesco (D2–D3) and Arquata del Tronto (D4–D5). Considering the quite similar vulnerability of buildings, the mutual proximity of villages, and their comparable distance from the August 24th, 2016 mainshock epicenter (~9 km), the observed damage pattern could be related to changes in seismic motion caused by the particular local geological, geotechnical, and morphological conditions of the study area.
Arquata del Tronto village is built on an elongated WNW-ESE-trending ridge in the central Apennines thrust-belt (central Italy), made of the alternation of different rocky lithofacies Late Miocene in age, partially fractured and with high angle dipping strata, characterized by strong weathering at the shallow depths, while Borgo and San Francesco rest in part on Quaternary sediments filling a valley floor. In this particular geological scenario, ground shaking may be affected by different factors: (i) stratigraphic amplification due to shallow continental deposits resting on the bedrock; (ii) effects ascribed to the topographic features such as focusing/defocusing phenomena and resonance of the relief (Faccioli et al., 1997); (iii) coupling between topographic and stratigraphic effects or “atypical topographic effects” (Marzorati et al., 2011; Massa et al., 2014; Pagliaroli et al., 2015).
Historical documents reported that also after the Mw = 6.9 Valnerina earthquake (January 9th, 1703) an increasing level of damage was observed moving from Borgo toward Arquata del Tronto ridge (VII–VIII MCS, IX MCS respectively; Rovida et al., 2016), thus confirming the possible occurrence of site effects.
This paper investigates the role of local conditions in Arquata del Tronto and in the surrounding hamlets of Borgo and San Francesco, through a multidisciplinary approach based on experimental and numerical methods, aiming at identifying the physical phenomena driving the site response and at justifying the observed irregular damage pattern.
We firstly give a short summary of the state of knowledge on complex topography site effects and a brief description of the regional geologic and seismotectonic settings. Then, the results of 1D and 2D numerical simulations obtained along four sections and at some representative points located in the three neighboring hamlets are presented. The main goal of this work is to quantitatively evaluate the irregular amplification pattern experienced during the August 24th, 2016 earthquake, to investigate the causes, and to offer a valid methodology for the evaluation of site effects in such complex configurations.
Section snippets
State of art of evaluation of topographic site effects
In the last decades, significant research efforts were devoted to the understanding of the physical phenomena causing the variability of seismic waves on isolated reliefs and across slopes. Focusing of seismic waves near the crest, interaction between incident and diffracted waves and 2D relief resonance are generally invoked to explain such wave field modifications (Geli et al., 1988; Bard, 1999; Faccioli et al., 2002). The first two phenomena generally lead to amplification at crest and an
Geological framework
The Arquata del Tronto territory is located within the Apennine Chain at the boundary between the northern and central Apennines Fold and Thrust Belt (Fig. 1a) (e.g., Boccaletti et al., 1990; Doglioni, 1991).
The study area was affected by multiphased contractional and extensional deformation, which involved reactivation/inversion of previous structures (e.g., Koopman, 1983; Di Domenica et al., 2014, and references therein). Quaternary NW-SE trending normal fault systems, 15 to 35 km long, are
Methods
This work integrates geological, geophysical, and geomechanical approaches aimed at defining the subsoil model of the study area and to understand the reason behind the spatial variation in the damage pattern observed in the aftermath of the August 24th, 2016 central Italy event. Considering the limitations of both the numerical and experimental methods for such complex sites, as discussed in section two, our estimation of ground motion amplification was made by integrating both methods.
The
Results
In this section we present the results of our investigations, within paragraphs dedicated to the different phases of the activities carried out in this work.
Discussion
Arquata del Tronto, San Francesco and Borgo villages experienced a different degree of damage (D4–5; D2–3; D1–2, respectively in Fig. 1) during the August 24th,2016 seismic event, despite their proximity (less than 500 m) and their similar distance to the earthquake epicenter.
The villages consist mainly of unreinforced masonry structures 2–3 stories in height. Very few structures were retrofitted with through-going iron bars. Isolate relatively modern reinforced concrete structures can be found
Conclusions
Local seismic response analyses were performed by using 1D and 2D numerical methods and experimental recordings (HVSR and GIT techniques) in the Arquata del Tronto area, following the heterogeneous damage pattern observed in the aftermath of the August 24th, 2016 central Italy event. In fact, despite their proximity (less than 500 m) Arquata del Tronto, San Francesco and Borgo villages experienced a different degree of damage (D4–5; D2–3; D1–2, respectively).
The numerical results can explain
Authors statement for publication
To the best of my knowledge everybody who participated substantially in the study is not omitted from the article.
To the best of my knowledge, all persons listed as authors qualify for authorship.
All persons who have made substantial contributions to the work but do not meet the criteria for authorship are listed in Acknowledgments section (technical help, writing assistance, general support, financial and material support).
Funding
The present work was performed, in part, in the frame of the Art. 14 of the OCDPC394 of the 19th September 2016, funded by the Italian Department of Civil Protection (DPC) and, in part, in the frame of the project “Microzonazione Sismica nei territori colpiti dagli eventi sismici del 2016-2017” ordinanza n.24 of the 12th-Maggio-2017 of the Presidency of Council of Ministers. All the activities were carried out under the umbrella of the CentroMS.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Part of data used in this work reflects the scientific research work resulted by a collaboration among: CNR-IGAG, ISPRA, POLITECNICO DI TORINO-DISEG, UNIVERSITY OF ROME “LA SAPIENZA”-DISG, UNIVERSITY OF CHIETI-PESCARA-INGEO and CNR-IAMC. We greatly thank all the members of this Working Group.
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