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<figure><img src="image_137.png" /></figure>

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EDITED BY

Aybige Akinci,
Istituto Nazionale di Geofisica e
Vulcanologia (INGV), Italy

REVIEWED BY

Alessandro Vuan,
Istituto Nazionale di Oceanografia e di
Geofisica Sperimentale, Italy
Basil Margaris,
Institute of Engineering Seismology and
Earthquake Engineering (ITSAK), Greece

*CORRESPONDENCE

J. Lin,
jiayue.lin@polimi.it

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RECEIVED 24 May 2022

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PUBLISHED 24 August 2022

CITATION

Lin J and Smerzini C (2022), Variability of physics-based simulated ground motions in Thessaloniki urban area and its implications for seismic risk assessment.
Front. Earth Sci. 10:951781.
doi: 10.3389/feart.2022.951781

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# Variability of physics-based simulated ground motions in Thessaloniki urban area and its implications for seismic risk assessment

J. Lin* and C. Smerzini

Department of Civil and Environmental Engineering, Politecnico di Milano, Milan, Italy

An accurate characterization of earthquake ground motion and its variability is crucial for seismic hazard and risk analysis of spatially distributed portfolios in urban areas. In this work, a 3D physics-based numerical approach, based on the high-performance spectral element code SPEED (http://speed.mox.polimi.it/), is adopted to generate ground shaking scenarios for strong earthquakes (moment magnitude M<sub>w</sub>=6.5–7) in the Thessaloniki area (Northern Greece). These simulations account for kinematic finite-fault rupture scenarios and a 3D seismic velocity including the two main geological structures present in the area (Thessaloniki and Mygdonia basins). The numerical model is successfully validated by comparing simulated motions, on the one hand, with the recordings of a real small-magnitude (M<sub>w</sub>4.4) earthquake and, on the other, with empirical Ground Motion Models for the historical M<sub>w</sub>6.5 1978 earthquake. The sensitivity of results to the velocity model, anelastic attenuation, and non-linear soil effects is evaluated. The variability of the ground motion intensity measures in Thessaloniki as a function of the finite-fault rupture realizations (causative fault, magnitude, hypocenter location) is explored to gain insight into its potential impact on seismic risk assessment in urban areas.

KEYWORDS

earthquake ground motion, 3D physics-based numerical simulation, finite-fault rupture scenarios, spatial correlation, seismic risk

# 1 Introduction

The characterization of earthquake ground motion and of its spatial variability is a key component of seismic risk modeling, especially for spatially distributed structures or infrastructure systems, such as bridge networks and building portfolios in large urban areas.

Empirical Ground Motion Models (GMMs) and ShakeMaps (Worden et al., 2020; Wald et al., 2021) represent the reference approach for earthquake ground motion prediction because of their consolidated utilization in the frame of probabilistic seismic hazard and risk analyses. Empirical GMMs are derived from the statistical processing of recordings of past earthquakes to provide the probability distribution of prescribed
ground motion Intensity Measures (IM) as a function of essential
explanatory variables, such as earthquake magnitude, source-to-
site distance, and soil conditions (Douglas and Edwards, 2016).
Driven by the increasing availability of recordings, state-of-the-
art GMMs are calibrated on comprehensive datasets by using
robust mixed-effect regressions techniques (Stafford 2014) and
by relaxing the assumption of ergodicity, i.e., the variability at a
single site from a specific source is assumed identical to that
derived from multiple sites over large regions (Villani and
Abrahamson 2015; Kotha et al., 2020; Sgobba et al., 2021a;
Caramenti et al., 2022). By providing region- and site-specific
adjustments of the model parameters, non-ergodic models
proved to offer significant improvements in terms of median
shaking accuracy and reduction of GMM variability
(i.e., standard deviation), especially in the far-field.

Despite these advancements, the main issue is that the paucity of ground motion recordings in the proximity of the earthquake source persists, implying that empirical GMMs are poorly constrained in the near-source region, and they are subject to high uncertainty. Recently, Paolucci et al. (2022) showed that the predictive performance of recent GMMs is still poor when assessed on the NEar-Source Strong-Motion recorded dataset NESS2.0 (Sgobba et al., 2021b), because of the still insufficient sampling of the GMM calibration datasets in the near-field. Furthermore, because of the limited within-event spatial sampling of the calibration datasets, in empirical GMMs the spatial correlation of ground motion is reproduced through simplified approaches based on the stochastic simulation of spatially correlated random fields under the hypotheses of ergodicity, isotropy, and stationarity (see review in Schiappapietra and Douglas 2020). However, such assumptions are hardly found in near-source conditions and may not be suitable to reproduce scenario- and region-specific features of ground motion spatial correlation and cross-correlation (see Chen and Baker 2019; Schiappapietra and Smerzini 2021). This may negatively impact on seismic risk assessments of regional-scale infrastructures or urban areas, where ground motion scenarios preserving a realistic spatial correlation structure are needed (Schiappapietra et al., 2022).

Due to the ever-growing increase of computational
resources, physics-based numerical simulations (PBS) of
source-to-site seismic wave propagation have been
gradually playing a promising role in responding to the
existing knowledge gaps in earthquake ground motion
prediction (Bradley et al., 2017; McCallen et al., 2021a;
2021b; Paolucci et al., 2021; Touhami et al., 2022). Based
on the numerical solution of the elastodynamics equation,
PBS provides ground motion time histories reflecting the
physics of the seismic wave propagation problem, from the
fault rupture to the propagation path and complex site effects
in shallow geology. PBS motions can be used: 1) to
complement recordings, especially in those conditions
where data are still sparse, such as in the near-source

region of strong earthquakes, 2) to calibrate region-specific
spatial correlation models, 3) to constrain site amplification
studies in complex geological configurations (e.g. alluvial
basins), 4) to gain insight into the physics of the complex
interactions between the source process and the ground
shaking, up to the coupling with the structural response,
5) to provide site-specific waveforms for structural analysis,
also at spatially dense locations, as well as scenarios for
region-specific hazard and risk assessments.

With this background, the goal of this work is to construct
and validate a set of earthquake ground shaking scenarios
from 3D PBS for the area of Thessaloniki (Northern Greece),
given their future use in region-specific seismic risk
assessments. The case study of Thessaloniki is considered
because of the detailed knowledge on the geological and
seismotectonic context, and vulnerability and exposure
data (Crowley et al., 2020; Riga et al., 2021), which are
critical ingredients for seismic risk evaluations. Starting
from previous works (Smerzini et al., 2017; Smerzini and
Pitilakis 2018), an updated version of the 3D numerical
model of the broader Thessaloniki area is built in this
work by bringing improvements to the geological and
geophysical model.

PBS are carried out by the open-source computer code SPEED—Spectral Element in Elastodynamics with Discontinuous (Mazzieri et al., 2013, http://speed.mox.polimi.it/), developed at Politecnico di Milano. In the recent past, SPEED has been extensively used for the validation of PBS addressed to real earthquakes in Italy and worldwide (e.g. Paolucci et al., 2015; Evangelista et al., 2017; Infantino et al., 2020), for region-specific seismic hazard and risk evaluations (Smerzini and Pitilakis 2018; Stupazzini et al., 2021), and the construction of a dataset of broadband near-source simulated ground motions for earthquake engineering applications (Paolucci et al., 2021).

The paper is organized as follows. After providing in Section 2 an overview of the study area, the 3D numerical model is presented in Section 3, emphasizing the main changes introduced in this work with respect to the previously published version of the model for the same region. In Section 4 the numerical model is validated on the recordings of a real earthquake with moment magnitude M<sub>w</sub>4.4, which occurred on 12th September 2005 in the Mygdonia graben. A set of sensitivity tests is shown for the calibration of model parameters. After the small magnitude event's validation, the M<sub>w</sub>6.5 1978 earthquake simulation is addressed in Section 5 by comparing simulated ground motion intensity measures with empirical GMMs and by assessing the spatial correlation of spectral accelerations across different vibration periods. Finally, Section 6 illustrates the set of 60 physics-based earthquake scenarios rupturing two different active fault systems around Thessaloniki, namely the Gerakarou-Langadhas and the Anthemountas faults, with M<sub>w</sub> in the range between 6.5 and 7.0.
<figure><img src="image_138.png" /><figcaption>**FIGURE 1**

Overview of the study area with indication of the main seismogenic faults (according to the GreDaSS database) and the Thessaloniki urban area (superimposed brown contour). The extent of the SPEED model as well as the epicenters of the real M<sub>w</sub>4.4 and M<sub>w</sub>6.5 earthquakes considered in this work for validation purposes are shown.</figcaption></figure>

## 2 Study area

The city of Thessaloniki is the second largest and the most relevant financial center in the territory of Greece, with more than one million inhabitants. Located in Central Macedonia and the inner part of the Thermaikos Gulf (see Figure 1), the city has an extensive industrial zone encompassing strategic infrastructures and a major international port functioning as a major gateway for the Balkan hinterland (Raucoules et al., 2008). The port of Thessaloniki is one of the most important harbors in Southeast Europe. It serves the needs of 15 million inhabitants of its international mainland and handles approximate trading of 16,000,000 tons of cargo annually.

The broader Thessaloniki area sits in Central Macedonia, a high-seismicity region characterized by an extensional tectonic regime associated with complex NW-SE, NE-SW, E-W, and NNE-SSW trending faults (Tranos et al., 2003; Paradisopoulou et al., 2006). The outcropping rocks forming the pre-alpine and alpine basement belong to the NNW-SSE-trending alpine Circum Rhodope Belt Thrust System (CRBTS), characterized by several NE-dipping asymmetric anticlinoria and synclinoria and repeated SW-directed thrust sheets (Tranos et al., 1999). Above this basement, NW-SE- and E-W-trending basins and grabens of tectonic origin, filled with Neogene and Quaternary sediments, are present. These basins were formed by an extensive extensional deformation associated with high-angle normal faults (Pavlides and Kilias, 1987;

Tranos et al., 1999). Among these basins, the E-W-trending Mygdonia graben is within the study area, located around 25 km northeast of Thessaloniki.

The seismicity of the city of Thessaloniki is mainly associated with the activity of the Mygdonia and the Anthemountas fault systems (see Figure 1, according to the Greek Database of Seismogenic Sources-GreDaSS: http://gredass.unife.it/; Caputo et al., 2012), which were responsible for severe earthquakes with magnitudes up to M<sub>w</sub>7.0 (Papazachos and Papazachou 1997). The Mygdonia source represents a large fault zone bordering the southern margin of the Mygdonia basin. It consists of three major fault segments: from west to east, 1) the NW-SE trending Langadhas Fault (GRIS102), 2) the EW trending Gerakarou Fault (GRIS101), which was reactivated during the Mw6.5 20 June 1978 earthquake, and 3) the Apollonia Fault (GRIS103), with a WNW-ESE strike and steeper dipping angles with respect to the previous fault segments. The Anthemountas source is a 40 km-long fault zone bounding the narrow E-W-striking Anthemountas basin to the south of Thessaloniki. It is separated into two segments: from west to east, 1) the Angelochori Fault (GRIS251) and 2) the Souroti Fault (GRIS252).

Destructive events, such as those occurring in the VIII century (677, M<sub>w</sub>=6.4 and 700, M<sub>w</sub>=6.5), the Assiros (5 July 1902, M<sub>w</sub>=6.5) and Thessaloniki (20 June 1978, M<sub>w</sub>=6.5) earthquakes, were generated by the seismic rupture of the Mygdonia seismogenic source. Instead, the Vasilika (1,677,
<figure><img src="image_139.png" /></figure>

<caption>FIGURE 2</caption>

(A) Overview of the 3D numerical model and details on (B) mesh discretization in shallow layers and on the seismic faults, namely (C) GER-LAN fault and (D) ANTH fault. (E) Computational features. (F) Geometry of the seismic faults included in the numerical model. (G) 3D wave propagation model adopted for the sedimentary basins and for crustal rock layers (Shear-wave velocity: Vs; compressional wave velocity: Vp; soil density: ρ; quality factors: Qs and Qp).

M<sub>w</sub>=6.2) and the Thessaloniki (22 June 1759, M<sub>w</sub>=6.5) earthquakes are associated with the Anthemountas fault system.

In Figure 1, the superimposed black box indicates the size of the SPEED model, described in detail in the following Section.

## 3 An updated 3D numerical model of the Thessaloniki area

A 3D physics-based numerical approach, through the spectral element code SPEED (Mazzieri et al., 2013, http://speed.mox.polimi.it/), is used to simulate the seismic wave propagation during both real and scenario earthquakes occurring around the city of Thessaloniki. Smerzini et al. (2017) constructed the first 3D spectral-element model of the area and validated it on the available instrumental (one accelerometric recording in the city center) and macroseismic observations. In this work, an update of this 3D model is proposed concerning: 1) the inclusion of the Mygdonia basin in the 3D numerical model, which was neglected in Smerzini et al. (2017), 2) modification to the seismic velocity profile of the outcropping bedrock layer of the crustal model to provide more

realistic velocities at shallow depths following Cotton et al. (2006); 3) new mesh of the model to provide adequate discretization of the low-velocity sediments of Mygdonia basin. Further details about model updates will be provided below.

The final 3D model extends over a volume of 82 × 64 × 31 km³ and it is discretized using an unstructured hexahedral mesh capable of propagating frequencies up to 1.5 Hz, with a third-order spectral degree, leading to a total of 100 million degrees of freedom, see computational information from Figure 2E. Numerical simulations were carried out on the Galileo100 and Marconi100 Cluster at CINECA, the largest high-performance computing center in Italy (www.cineca.it).

As shown in Figure 2, the model includes:

* 3D geological model for both Thessaloniki and Mygdonia;

* crustal model for rock materials, as in Smerzini et al. (2017), apart from the modification of the velocity of the first layer, as explained below;

* the Gerakarou-Langhadas (GRIS101-GRIS102) Faults, referred to as GER-LAN hereafter, and the Anthemountas (GRIS251-GRIS252) Faults, referred to as

<footnote>aNote: Qs=20; z<=200, Qs=50; z<=500, Qs=100; z<=1000, Qs=150 (z=depth from ground surface)</footnote>

<footnote>bNote: The values of ρ are calculated according to the approach from Cotton et al. (2006).</footnote>
<figure><img src="image_140.png" /><figcaption>**FIGURE 3**
(A) Digital Elevation Model of the study area. (B) Depth of geologic bedrock in Thessaloniki and Mygdonia basins. (C) Velocity cross-section (A-A') across the Mygdonia basin as indicated in Figure 3A. (D) V<sub>S</sub>, V<sub>P</sub> and ρ profiles within the Thessaloniki basin, Mygdonia basin and outcropping bedrock (V0 refers to the previous version in Smerzini et al., 2017; V1 refers to the model adopted in this work). Depth is measured with respect to the topographic surface.</figcaption></figure>

ANTH. The location and geometry of these faults were retrieved and adapted from the GreDaSS database (http://gredass.unife.it/) and are summarized in Figure 2F;

* ground topography.

Efforts were devoted to constructing a large-scale 3D geological model including, in the same computational domain, both the Thessaloniki basin and the Mygdonia basin. While the Thessaloniki basin model (3D shape and geophysical model) was taken from Smerzini et al. (2017), the 3D shape of the Mygdonia basin is taken from Maufroy et al. (2016). The velocity model of the soft soil deposits inside the Mygdonia basin was calibrated based on previous studies (Maufroy et al., 2016) and available recorded profiles. Specifically, a parabolic profile was

defined for both S and P wave velocity as a function of the depth from the topographic surface (z), while for soil density a linear profile was assumed, as follows:

For S wave velocity, $V_S = 200+15 \cdot z^{0.63}$

For P wave velocity, $V_P = 1500+32.8 \cdot z^{0.63}$

For soil density, $\rho = 2,075+0.55 \cdot z$

Besides, a frequency-proportional quality factor $Q_S = Q_0/f/f_0$ is assumed, with $f_0 = 1$ Hz (see sensitivity tests in Section 4) and $Q_0=V_S/10$, which is a rule-of-thumb for estimating Q based on the V<sub>S</sub> profile often used in the literature (see Laurendeau et al., 2018).

The 3D shape of both the Thessaloniki basin and the Mygdonia basin is shown in Figure 3B: the maximum sediment thickness reaches about 800 m and 500 m in the Thessaloniki and Mygdonia
<table><caption>TABLE 1 Numerical simulations performed in this study for validation and sensitivity purposes.</caption><thead><tr><th rowspan="2">Model label</th><th rowspan="2">Source type</th><th colspan="2">Thessaloniki basin</th><th colspan="2">Mygdonia basin</th><th>Outcropping bedrock</th><th rowspan="2">f<sub>0</sub> (Hz)</th></tr><tr><th>Point source (Mw4.4)</th><th>Finite-fault (Mw6.5 1978)</th><th>LE<sup>a</sup></th><th>NLE<sup>b</sup></th><th>LE<sup>a</sup></th><th>NLE<sup>b</sup></th><th>Hard rock</th><th>Soft rock</th></tr></thead><tbody><tr><td>6.5-HR-T-E</td><td></td><td>√</td><td></td><td>√</td><td></td><td></td><td>√</td><td></td><td>0.67</td></tr><tr><td>6.5-SR-T-E</td><td></td><td>√</td><td></td><td>√</td><td></td><td></td><td></td><td>√</td><td>0.67</td></tr><tr><td>6.5-SR-TM-E</td><td></td><td>√</td><td></td><td>√</td><td></td><td>√</td><td></td><td>√</td><td>1</td></tr><tr><td>6.5-SR-TM-N</td><td></td><td>√</td><td></td><td></td><td>√</td><td></td><td>√</td><td>√</td><td>1</td></tr><tr><td>4.4-SR-TM-E0.67</td><td>√</td><td></td><td></td><td>√</td><td></td><td>√</td><td></td><td>√</td><td>0.67</td></tr><tr><td>4.4-SR-TM-E1</td><td>√</td><td></td><td></td><td>√</td><td></td><td>√</td><td></td><td>√</td><td>1</td></tr></tbody></table>

basin, respectively. Figure 3C shows a representative cross-section of the V<sub>S</sub> model across the Mygdonia (A-A' in yellow line in Figure 3A).

Concerning the crustal model, the velocity of the uppermost layer was modified to provide a more realistic velocity profile of the outcropping bedrock in the area under study. Compared to Smerzini et al. (2017), where the outcropping bedrock layer consists of very hard rock with constant shear wave velocity V<sub>S</sub> = 2,000 m/s, the updated crustal model features the first layer (see Crust 1 in Figure 2G) with a gradient of V<sub>S</sub> from a minimum value of 1,150 m/s up to a value of 3,440 m/s at 1,000 m depth from the topographical surface, see continuous black line in Figure 3D. The gradient of this velocity profile was calibrated based on the studies conducted by Cotton et al. (2006) on rock velocity profiles. In Figure 3D, the V<sub>S</sub>, V<sub>P</sub> and ρ profiles for the sediments within both Thessaloniki and Mygdonia basins and the outcropping bedrock layer are shown.

## 4 Validation and sensitivity tests

This Section presents the validation and sensitivity tests performed on the 3D numerical model of the study area. Table 1 lists the PBS performed as propaedeutic analyses to check the performance of the numerical model especially in relation to the updates of the numerical model (see Section 3):

- *Type of source.* Two real earthquakes were considered for validation, the M<sub>W</sub>4.4 12 September 2005 event and the M<sub>W</sub>6.5 20 June 1978 earthquake. Provided the considerably different magnitude of the validation events, a point-source and a finite-fault kinematic source model is adopted for the medium and large magnitude earthquake, respectively. While Smerzini

et al. (2017) focused on validating the model against the M<sub>W</sub>6.5 1978 event, in this work emphasis is placed on the M<sub>W</sub>4.4 event, which allows for a verification of the accuracy of the updated velocity model, neglecting the high uncertainties typically associated with the finite-fault modelling. For the 1978 event, the same kinematic source model (slip distribution and source time function) as in Smerzini et al. (2017) is adopted.

- *Basin models.* Numerical results with/without the Mygdonia basin and with the Thessaloniki basin are compared for the same source model, to check the influence of the added basin in the simulated ground motions in the city of Thessaloniki. While the influence of the basin is evident for the soft sites in the Mygdonia graben itself, the evaluation of the impact on sites at more considerable distances, such as in Thessaloniki at around 25 km from the basin, deserves some considerations.

- *Outcropping bedrock.* The effect of the new velocity profile for the outcropping bedrock is analyzed by comparing, for the same source model, results obtained with hard rock (V<sub>S</sub> = 2,000 m/s) and with softer rock profile (see profiles in Figure 3) at shallow depths.

- *Linear Vs. non-linear elastic soil constitutive law.* While for the M<sub>W</sub>4.4 event, a linear visco-elastic model is assumed as a reasonable assumption because of the low seismic excitation, for the M<sub>W</sub>6.5 event, a non-linear elastic constitutive model is adopted using the shear modulus and damping curves adopted in Smerzini et al. (2017). For the non-linear elastic modelling approach, we refer to Stupazzini et al. (2009).

- *Anelastic attenuation model.* Sensitivity tests are performed for the anelastic attenuation properties. This work assumes a frequency-proportional quality factor for all soil layers, Q<sub>S</sub> = Q<sub>0</sub>f/f<sub>0</sub>, with Q<sub>0</sub> values given in Figure 2G and reference frequency f<sub>0</sub> varying between 0.67 and 1 Hz.

<footnote><sup>a</sup>LE, linear visco-elastic.</footnote>

<footnote><sup>b</sup>NLE, non-linear visco-elastic.</footnote>

<footnote><sup>c</sup>f0 = reference frequency for frequency-proportional damping: Q(f)=Q₀f/f₀.</footnote>
<figure><img src="image_141.png" /><figcaption>FIGURE 4
(A) Location of recording stations. (B) Comparison between recorded (black) and simulated (red, model: 4.4-SR-TM-E1) velocity time histories for EW, NS, UP components at the stations: E03 ($R_{\text{epi}}$=5.4 km), GRA ($R_{\text{epi}}$=6.9 km), TST ($R_{\text{epi}}$=7.9 km), W02 ($R_{\text{epi}}$=9.8 km) and W03 ($R_{\text{epi}}$=10.4 km).</figcaption></figure>

## 4.1 Validation on the M<sub>w</sub> 4.4 12 September 2005 earthquake

The numerical model was applied to simulate a real M<sub>w</sub>4.4 earthquake event (strike: 281°, dip: 52°, rake: -98°) which occurred on 12 September 2005 near the Mygdonia basin with hypocenter at (40.7255°N, 23.3408°E) with a focal depth of 10 km. Owing to the small magnitude, the finiteness of the fault rupture area is neglected, and a point-source model is considered. Because the accuracy of simulations is controlled by uncertainties in the source properties, propagation path, and shallow layer structure, selecting a point-like and relatively deep event (>8 km) allows one to focus on the validation of the propagation path and local site response.

The earthquake was recorded by a total of nine stations of the EUROSEISTEST strong motion network (http://euroseisdb.civil.auth.gr/) which are considered herein for comparison between simulated and recorded ground motions. The location of the stations is shown in the map of Figure 4A.

Figure 4B shows the comparison between the recorded (black) and simulated (red, model: 4.4-SR-TM-E1 of Table 1) velocity

waveforms at five representative stations, namely E03, GRA, TST, W02, and W03. For the comparison, horizontal (EW and NS) and vertical (UD) components are considered and a low-pass filter at 1.5 Hz is applied. A comprehensive comparison in both time and frequency domain is available in Supplementary Image, where the recorded and simulated velocity time series and corresponding Fourier Amplitude Spectra (FAS) are shown for all nine stations which recorded the earthquake. In Supplementary Table S1, the complete list of station metadata is also provided.

The qualitative comparison of Figure 4 indicates that a satisfactory agreement is found between simulated and recorded waveforms, in terms of timing of first arrivals, amplitudes and duration. Horizontal components, especially on the NS, show a better agreement than the vertical ones. For the E03 station located at basin center, the direct arrivals on all components are well captured by the simulations but the numerical model lacks some complexity in late arrivals associated with reverberations in the basin. At GRA station, simulations overpredict the recorded peak velocity values, especially on NS and UD components, most likely because of the assumptions in the focal mechanism.
<figure><img src="image_142.png" /><figcaption>**FIGURE 5**
(A) Map of simulated peak ground velocity (PGV) for GMH and UD components. (B) Goodness-of-Fit according to Anderson (2004) evaluated on AI, PGV, PGD, SA (1s), SA (2s) and CAV for both GMH and UD components.</figcaption></figure>

To provide a quantitative evaluation of the performance of the PBS, the Goodness-Of-Fit (GoF) criteria by Anderson (2004) were evaluated, considering six different ground motion intensity measures representative of both peak and integral parameters: Peak Ground Velocity (PGV), Peak Ground Displacement (PGD), Cumulative Absolute Velocity (CAV), Arias Intensity (AI), SA (1s), SA (2s). The scores, shown in Figure 5B, were computed on both the geometric mean of horizontal components (GMH) and the UD component. For all stations, except E03, GRA and STC, horizontal GoF scores are fair-to-excellent. In particular, on the same component, for stations TST, PRO, W01, W02, and W03, good-to-excellent scores are found for all ground motion intensity measures. UD scores are slightly worse than the ones for the horizontal components, but they remain in the fair-to-excellent range for a majority of stations and ground motion parameters. In the same figure (Figure 5A) the maps of PGV (GMH and UD components) are also shown, to appreciate the spatial distribution of the ground shaking, with clear evidence of site amplification effects, especially on horizontal ground motion, in the Mygdonia basin.

## 4.2 Sensitivity analyses

### 4.2.1 Sensitivity to the velocity model

In this section, with reference to the simulation of the Mw6.5 1978 earthquake, the impact of the 3D velocity model adopted in

the simulation is investigated, by comparing the results obtained from models 6.5-HR-T-E (hard outcropping bedrock, with Thessaloniki basin only), 6.5-SR-T-E (softer outcropping bedrock, with Thessaloniki basin only) and 6.5-SR-TM-E (softer outcropping bedrock, with both Thessaloniki and Mygdonia basins). For all three simulations, a linear visco-elastic model is assumed.

Figure 6 shows the comparisons of NS velocity time histories and corresponding FAS (up to 1.5 Hz) obtained from the three PBS at two selected receivers close to the city of Thessaloniki, namely receiver A located on soft sediments in the vicinity of Thessaloniki shoreline and receiver B sited on outcropping bedrock, as indicated in Figure 6A. Results suggest that, at both receivers, softening of the mechanical rock properties produces a moderate-to-significant increase in the ground motions (peak velocities increase by a factor between 40% and 80% for rock and basin receivers, respectively). Enhancement of high frequency components is associated mainly with the site amplification features related to the softer rock layer, particularly evident at receiver B (Figure 6C). In contrast, the enrichment of lower frequencies is most likely related to the coupling of rupture propagation with softer layers. This effect is observable at both stations, confirming its correlation with the seismic source. Furthermore, the updated outcropping bedrock profile implies a more considerable impedance contrast between the sedimentary soil layers and the underlying bedrock at depths
<figure><img src="image_143.png" /><figcaption>**FIGURE 6**

Sensitivity of results to the velocity models: 6.5-HR-T-E (black), 6.5-SR-T-E (blue) and 6.5-SR-TM-E (red). (A) Map indicating the location of receivers A and B and the slip distribution of the 1978 earthquake. (B-C) Comparison of velocity time histories and FAS computed at receiver A and B from the three models under consideration.</figcaption></figure>

<figure><img src="image_144.png" /><figcaption>**FIGURE 7**

Comparison of velocity time histories and corresponding FAS (NS component) with respect to anelastic attenuation model ($f_0$) for $M_{W}$4.4 event.</figcaption></figure>

larger than 100 m (see Figure 3D), yielding significant amplification effects at low frequency at about 0.4–0.6 Hz. As expected, the presence of the Mygdonia basin has a limited impact on the ground motions at the Thessaloniki sites because of the large distances involved. However, the signals are further enriched in the higher frequency range because of the higher complexity of the source-to-site propagation path traveling across the Mygdonia soft sediments.

## 4.2.2 Sensitivity to anelastic attenuation

As indicated previously, a frequency-proportional quality factor has been assumed in this work and different values of the reference frequency $f_0$ were tested. The target event is the $M_{W}$4.4 earthquake and the results of the simulations 4.4-SR-TM-E0.67 and 4.4-SR-TM-E1 at E03 station are analyzed and compared. Note that the simulations consider softer

outcropping bedrock, with both Thessaloniki and Mygdonia basins, linear viscoelastic model, and differ only for the reference frequency $f_0$ which is changed from 0.67 to 1 Hz.

In Figure 7, the recorded NS velocity waveform and corresponding FAS are compared with those simulated using the two models with $f_0$=0.67 Hz (blue) and $f_0$=1 Hz (red, same as in Figure 4). The PBS with a lower reference frequency implies excessive reverberations in the coda of the signal because of lower damping values in the low-frequency range below around 1 Hz.

## 4.2.3 Effect of non-linear visco-elastic soil behavior

To simulate the seismic wave propagation more realistically for moderate-to-large magnitude events, the effect of non-linear visco-elastic soil behavior is investigated, referring to the
<figure><img src="image_145.png" /><figcaption>**FIGURE 8**

(A) G/G<sub>max</sub>-γ and D-γ curves (from Smerzini et al., 2017). (B) A set of receivers (triangles) along a representative cross-section across Thessaloniki. (C) Comparison of velocity time histories (NS component) computed using a linear and non-linear elastic soil model for the set of receivers in (B). (D) FAS of velocity waveform for a selected receiver (red triangle in Figure 8B).</figcaption></figure>

1978 event. Non-linear soil behavior is implemented in SPEED by introducing and generalizing to the 3D case the shear modulus (G/G<sub>max</sub>-γ) reduction and damping (D-γ) curves adopted routinely in 1D equivalent-linear approaches. Following Smerzini et al. (2017), a single set of G/G<sub>max</sub>-γ and D-γ curves (see average curve in Figure 8A—black line) was employed for the shallowest 100 m thick soil deposits within both the Thessaloniki and Mygdonia basins.

In Figure 8C the horizontal (NS) velocity time histories computed under the assumption of linear (simulation 6.5-SR-TM-E, in blue) and non-linear (simulation 6.5-SR-TM-N, in red) soil behavior are shown for a set of receivers along a representative cross-section, passing through the city center (as indicated in Figure 8B). Note that not all receivers along the considered cross-section are on soft soils. Instead, the three receivers on the NE portion of the cross-section are located on outcropping bedrock.

Non-linearity features in ground shaking are found at the soft soil sites but such effects are limited and predominantly affect the coda of the signals due to the moderate level of ground shaking and the relatively narrow range of frequencies propagated by the model (<1.5 Hz). In Figure 8D, the FAS of the velocity motions simulated by the linear and non-linear model at a selected receiver (red triangle in the map of Figure 8B) is also shown.

The analysis of the FAS of the two models confirms that, at this receiver and for this earthquake, non-linear effects slightly decrease the amplitude of frequencies above 0.7 Hz.

# 5 Earthquake ground motion for the M<sub>w</sub>6.5 1978 earthquake

This section provides a general overview of the ground shaking simulated for the M<sub>w</sub>6.5 1978 earthquake, emphasizing the comparison with empirical GMMs and the spatial correlation structure of ground motion. Note that the issues related to the validation of the PBS of this earthquake were already investigated by Smerzini et al. (2017), with particular reference to the finite-fault source model, which is recognized to play a major role in determining the ground motion in the epicentral area of a strong earthquake, as well as to the site response model in the Thessaloniki urban area. For the source model, a single rupture event was assumed as a reasonable simplification, although according to Papazachos et al. (1980) and Soufleris and Stewart (1981) it was a double event.

Low-frequency PBS are then enriched in the high-frequency range using the ANN2BB approach proposed by Paolucci et al. (2018; 2021). Essentially, broadband ground motions are
<figure><img src="image_146.png" /><figcaption>**FIGURE 9**
(A) Map of PGV (GMH), model 6.5-SR-TM-N. (B) Comparison of PGV (GMH) as a function of $R_{rup}$ between 3D PBS and Cauzzi et al. (2015)-CEA15.</figcaption></figure>

generated by combining the results of long-period PBSs with predictions of an Artificial Neural Network (ANN) trained on a set of strong motion records (in our case SIMBAD v6.0, Smerzini et al., 2014). Compared to standard hybrid approaches, one of the main advantages of the ANN2BB procedure is the capability to establish a correlation between short and long periods and, hence, to preserve a physically consistent spatial correlation structure also at high frequencies. Hereafter broadband ground motions will be considered.

## 5.1 Comparison with empirical ground motion models

Figure 9A shows the ground shaking map of PGV (GMH) obtained from the 3D PBS of the 1978 earthquake (model 6.5-SR-TM-N in Table 1). Maximum PGV-GMH values of 1.2 m/s are found inside the Mygdonia graben because of the coupling between the seismic source rupture with basin amplification effects. Within the urban area, PGV values between 0.025–0.25 m/s are found. Notably at the location corresponding to the station THE, a peak velocity amplitude of 6 cm/s is simulated, in fair agreement with the recorded one (equal to about 8 cm/s).

To check the overall consistency of the simulation, in Figure 9B the rate of attenuation of simulated PGV-GMH with the rupture distance ($R_{rup}$) is shown and compared with the one from the empirical GMM by Cauzzi et al. (2015), referred to as CEA15. Comparison is shown for three site categories (Site A, B, C) involved by the model according to EC8 (CEN-European Committee for Standardization, 2004) site classification. A satisfactory agreement is found between simulations and empirical predictions, especially in terms of site A (rock site) and site C (soft site), which are the predominant site classes in the model.

## 5.2 Spatial correlation of spectral accelerations

As indicated in the introduction, the 3D PBS presented in this work will represent the basis for conducting physics-based seismic risk evaluations in the Thessaloniki urban area. It is well-known that accounting for a realistic spatial correlation structure of ground motion intensity measures is relevant for risk assessment of spatially extended urban systems. To evaluate the spatial correlation, the procedure proposed by Infantino et al. (2021) is applied to the 3D PBS simulation of the 1978 Volvi earthquake. Specifically, under the hypotheses of stationarity and isotropy, the semivariogram $\gamma$ is computed as a function of inter-station distance $h$ on the within-event residuals of simulated spectral accelerations SA(T) with respect to the median trend using standard geostatistical tools. Referring the reader to Infantino et al. (2021) for the geostatistical approach, we limit herein to focus on the results of the semivariogram analysis for the 1978 earthquake. In Figure 10A, the sample semivariogram (grey dots) and the least-squares best-fitting exponential model (black line) are shown for PGA, SA (0.5 s), and SA (2.0 s), for the GMH component. In Figure 10B, the range, i.e., the distance above which the ground motion is assumed to be uncorrelated, and sill, i.e., the variance, are shown as a function of vibration period. As expected, range and sill are positively correlated. The range shows an increasing trend as a function of the period up to around 1 s. A maximum range of approximately 64 km is found, consistently with previous knowledge on this subject (e.g., Zerva and Zervas, 2002; Infantino et al., 2021), and, beyond this period, the range drops to values of about 20 km.
<figure><img src="image_147.png" /></figure>

<figure><img src="image_148.png" /><figcaption>FIGURE 10</figcaption></figure>

(A) Semivariogram for PGA, SA (0.5 s), and SA (2.0 s) for GMH component. (B) Range and sill of the best-fitting exponential model as a function of period.

A

<table><thead><tr><th></th><th>GER-LAN</th><th>ANTH</th></tr></thead><tbody><tr><td>Mw=6.5</td><td>10</td><td>10</td></tr><tr><td>Mw=6.7</td><td>10</td><td>10</td></tr><tr><td>Mw=6.9</td><td></td><td>10</td></tr><tr><td>Mw=7.0</td><td>10</td><td></td></tr></tbody></table>

B

<figure><img src="image_149.png" /></figure>

C

<figure><img src="image_150.png" /><figcaption>FIGURE 11</figcaption></figure>

(A) List of earthquake scenarios rupturing from the GER-LAN and ANTH faults. (B) Epicenter distribution of the simulated earthquake scenarios.

(C) Example of rupture realizations (M<sub>w</sub>6.5 on GER-LAN fault) in terms of co-seismic slip, rise time and rupture time.
<figure><img src="image_151.png" /><figcaption>**FIGURE 12**
(A) Broadband simulated maps of median horizontal PGA, SA (0.3 s) and SA (0.5 s) for the 10 M<sub>W</sub>7 GER-LAN scenarios. (B) Simulated spectral accelerations as a function of distance (R<sub>JB</sub>) in comparison with the empirical GMM by Kotha et al., 2020 (K20) at both rock (blue) and basin (magenta) sites.</figcaption></figure>

## 6 Variability of ground motions as a function of seismic rupture realizations

After the successful validations and sanity checks discussed in previous sections, PBS were carried out to simulate 60 different normal-type seismic rupture realizations with M<sub>w</sub> in the range from 6.5 to 7.0 along the GER-LAN and ANTH fault systems. For all simulations, the reference model is “M”-SR-TM-N (see Table 1), where “M” (variable) is the magnitude of the scenario earthquake. As indicated in Figures 11A,B, for each fault, three magnitude levels (GER-LAN: M<sub>w</sub>=6.5/6.7/6.9; ANTH: M<sub>w</sub>=6.5/6.7/7.0) were simulated and, for each magnitude, a suite of 10 rupture realizations is generated. Globally, four magnitude levels from M<sub>w</sub>=6.5 to 7.0 were considered. An example of rupture realization along the GER-LAN fault with M<sub>w</sub>=6.5 is given in Figure 11C, in terms of the spatial distribution of co-seismic slip, rise time, and rupture time. Rupture realizations are generated within the SPEED engine assuming the kinematic source model proposed by Herrero and Bernard (1994) and applying some spatially correlated random perturbations to the rise time and rake angle following Smerzini and Villani (2012). Random perturbations of rupture times are disregarded to avoid potential super-shear issues.

Figure 12 shows an overview of the broadband simulated ground motions for the set of 10 M<sub>w</sub>7.0 earthquake scenarios along the GER-LAN fault. Figure 12A shows the map of median horizontal (GMH) PGA, SA (0.3 s), and SA (0.5 s), assuming a

lognormal statistical distribution of ground motion at each site [i.e., median = exp (μ<sub>ln</sub>) where μ<sub>ln</sub> is the average of natural logarithmic values]. Superimposed on the maps, the epicenters (colored stars) and corresponding effective fault areas (according to the definition by Mai and Beroza (2000) and Thingbaijam and Mai (2016)) of the 10 scenarios are displayed. In Figure 12B, for each intensity measure [PGA, SA (0.3 s), SA (0.5 s)], the entire set of 10 simulated spectral accelerations on both rock-stiff and soft sites (V<sub>S30</sub>< 360 m/s) is shown as a function of Joyner-Boore distance (R<sub>JB</sub>) in comparison with the GMM by Kotha et al. (2020), referred to as K20, for shallow crustal events (blue: rock, with V<sub>S30</sub>=1,400 m/s; red: soft soil, with V<sub>S30</sub>=300 m/s), in its ergodic formulation. V<sub>S30</sub> values of empirical predictions are selected to match the average V<sub>S30</sub> values implemented in the numerical model. For K20, median and corresponding dispersion bands (±σ) are shown. Short-period SA is selected herein because the Thessaloniki building stock is characterized by fundamental vibration periods approximately in this range (Riga et al., 2021). A satisfactory agreement is found between PBS and empirical predictions for all considered spectral accelerations both in terms of median values and variability.

In Figure 13, the comparison between PBS and K20 is extended to all simulated scenarios, for horizontal SA (1 s). Specifically, each graph of Figure 13 shows the decay with R<sub>JB</sub> distance of horizontal SA (1 s) obtained from the entire set of 3D PBS, in comparison with K20 (median ±σ), for each target earthquake scenario. The latter is identified by the causative fault and magnitude. As commented previously, a general
<figure><img src="image_152.png" /><figcaption>**FIGURE 13**
Comparison of simulated SA (1 s) as a function of distance ($R_{JB}$) with K20 at both rock (blue) and basin (magenta) sites for all earthquake scenarios considered in this work: (A) ANTH with $M_{W}=6.5$ (left), 6.7 (center), 6.9 (right) and (B) GER-LAN fault with $M_{W}=6.5$ (left), 6.7 (center), 7.0 (right).</figcaption></figure>

<figure><img src="image_153.png" /><figcaption>**FIGURE 14**
Frequency histograms and best-fitting lognormal distribution of PGV-GMH at Thessaloniki city center from the GER-LAN (A,B) ANTH earthquake scenarios with varying magnitude.</figcaption></figure>

consistency is found between PBS and empirical predictions, although for lower magnitudes (M6.5) simulations tend to be lower, on average, than empirical GMM.

Finally, we analyzed the statistical distribution of simulated ground motions at selected sites within the urban area of Thessaloniki. To this end, Figure 14 shows the histograms of the frequency distribution of PGV-GMH values simulated at 69 soft sites within the Thessaloniki city center (at similar source-to-site distances and soil conditions), along with the best-fitting lognormal probability density function. The first statistical

moments of the lognormal fit are reported in the figure legend. For a given target magnitude, the first statistical moments (median values and standard deviation) associated with GER-LAN (Figure 14A) scenarios tend to be systematically higher (or at least equal) than the ones for ANTH (Figure 14B) scenarios. This means that, for the city of Thessaloniki, earthquake scenarios from Mygdonia graben are more hazardous than those from the Anthemountas. This may be explained as a consequence of the relative location between the city of Thessaloniki and the causative faults, combined with the
radiation pattern features. The geometry and focal mechanism (normal) of both fault systems is such that directivity effects are mainly in the up-dip direction, i.e., along the path pointing to the South with respect to the hypocenter. For the ANTH case, significant ground motion amplification effects are found southward, in the opposite direction with respect to the city of Thessaloniki.

Standard deviation values ($\sigma_{1n}$) range between 0.45 and 0.7, with a central value of about 0.5, which is lower than the one associated with ergodic empirical GMMs. This is reasonable as the standard deviation from PBS simulations should be compared with non-ergodic $\sigma_{1n}$ (e.g., according to Atik et al. (2010), it is about 0.4 for PGV).

# 7 Conclusions

In this research, a 3D spectral-element numerical model of the broader Thessaloniki region is constructed by including new features with respect to the previously published model. Specifically, the 3D seismic wave propagation model was updated by including, in addition to the Thessaloniki basin, the Mygdonia basin and by modifying the velocity of the outcropping bedrock of the crustal model. For model calibration, different sensitivity tests were performed to analyze the role played by the velocity model updates, the Q factors assumed in the simulation and the non-linear visco-elastic soil behavior. To validate the model, the PBS of two real earthquakes, a small $M_{W4.4}$ event and the destructive $M_{W6.5}$ 1978 earthquake, both originating from the fault system bordering the Mygdonia basin, has been carried out. The validation is conducted at two levels, first by comparing the simulated velocity waveforms with the available recordings (for the $M_{W4.4}$ event) and with empirical GMMs (for the $M_{6.5}$ event) and second, by computing the spatial correlation structure of spectral accelerations. These comparisons successfully validate the 3D model, making it suitable for calculating realistic ground motion scenarios in seismic risk studies.

After the validation of the numerical model, PBS were performed to simulate a broad set of earthquake scenarios with $M_{W}$ from 6.5 to 7.0 rupturing the two fault systems (GER-LAN and ANTH), which are the most relevant for seismic hazard in the city of Thessaloniki. These scenarios will be used as input ground shaking scenarios for seismic risk analyses of the building portfolio in Thessaloniki in a future work. 10 different rupture realizations were considered to capture the aleatory variability associated with the source for each target magnitude level and causative fault. The variability of the simulated ground motions is analyzed with the twofold aim of 1) verifying that the median and scatter values of predicted response spectral accelerations are realistic, in comparison with the trends obtained from GMMs calibrated on recordings, and 2) quantifying the effect of source variability on the statistical distribution of ground motion parameters at the site. A satisfactory agreement is found

between PBS and empirical GMMs for the different vibration periods, including the short periods predicted by the ANN-based technique, and for the different earthquake scenarios under consideration, both in terms of median values and standard deviation. The resulting ground motion variability at selected sites is comparable to the standard deviation values associated with non-ergodic GMMs. The analysis of the statistical distribution of PGV at sites located in the central area of Thessaloniki highlights the critical role played, for a given magnitude and similar source-to-site distances, by the relative position between the receiver and the causative fault as well as by the features of the fault rupture realization (source directivity, radiation pattern). It is found, in particular, that for the city of Thessaloniki, rupture scenarios originating from the GER-LAN fault tend to be more hazardous than those from the ANTH fault system because of the relative position with respect to the fault and the focal mechanism. The set of validated 3D PBS produced in this work may represent the basis for further studies focused on seismic risk assessment in Thessaloniki, such as 1) to develop non-ergodic hybrid (i.e., from recordings and simulations) GMMs for generating region- and site-specific seismic shaking scenarios; 2) to calibrate region-specific, anisotropic and non-stationary spatial correlation models; 3) to provide ground motion time series for constraining numerical seismic fragility studies especially at high levels of ground shaking intensity, for which recordings are still too sparse.

# Data availability statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

# Author contributions

JL: Modeling, Simulation, Formal Analysis, Investigation, Data curation, Writing—original draft, Writing—review and editing. CS: Conceptualization, Methodology, Supervision, Writing—Review and editing. All authors have read and agreed to the published version of the manuscript.

# Funding

This work has been funded by the URBASIS Project “New challenges for Urban Engineering Seismology,” H2020-MSCA-ITN-2018, Grant Agreement No. 813137.

# Acknowledgments

The authors wish to thank Manuela Vanini, Sri Sangaraju, Ilario Mazzieri and Chiara Amendola for their
support in the development and processing of the physics-
based simulations. The authors thank also Fabrice Hollander
for providing the 3D model of the Mygdonia basin. Authors
are grateful to Alessandro Vuan and Basil Margaris for their
constructive comments that helped improve the quality of
the paper.

Conflict of interest

The authors declare that the research was conducted
in the absence of any commercial or financial
relationships that could be construed as a potential
conflict of interest.

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Supplementary material

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<figure><img src="image_154.png" /></figure>

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EDITED BY
Matteo Picozzi,
University of Naples Federico II, Italy

REVIEWED BY
Stefania Tarantino,
Istituto Nazionale di Geofisica e
Vulcanologia, Italy
Eugenio Chioccarelli,
Università Mediterranea di Reggio
Calabria, Italy

*CORRESPONDENCE
Bojana Petrovic,
bpetrovic@ogs.it

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RECEIVED 29 April 2022
ACCEPTED 01 August 2022
PUBLISHED 11 October 2022

CITATION
Petrovic B, Scaini C and Parolai S (2022),
Applying the damage assessment for
rapid response approach to the august
24 M6 event of the seismic sequence in
central Italy (2016).
Front. Earth Sci. 10:932110.
doi: 10.3389/feart.2022.932110

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# Applying the damage assessment for rapid response approach to the august 24 M6 event of the seismic sequence in central Italy (2016)

Bojana Petrovic*, Chiara Scaini and Stefano Parolai

National Institute of Oceanography and Applied Geophysics - OGS, Italy

Seismic monitoring networks are increasingly being used in urban areas to record and locate earthquakes. Recordings in the proximity of buildings also allow assessing, as a first approximation, the expected building damage. The DARR (Damage Assessment for Rapid Response) method provides local-scale information on expected damage patterns. The potential of this approach is discussed here for the August 24 M6 event of the Central Italy seismic sequence (2016–2017). We focus only on the first event of the sequence because cumulative damage is outside the scope of this study. The earthquake recordings are available from two Italian monitoring networks: the Italian Accelerometric Archive (ITACA) and the OSS (Osservatorio Sismico delle Strutture), which collects data from monitored buildings and bridges in Italy. We selected four target areas (Amatrice, Norcia, Visso and Sulmona) characterized by different epicentral distances and building typologies, that suffered different levels of damage during the M6 event on 24 August 2016. Using recordings either in the free field or in the basement of buildings, the expected relative displacement of building typologies common in the studied areas is calculated with the DARR method. Using predefined damage thresholds from literature, the obtained results allow quantifying the expected damage for dominant building typologies in the surroundings of the recording sites. We investigate and discuss the potential use and applicability of the DARR method in different areas depending on the epicentral distance and building characteristics. The results indicate that the DARR approach is useful for supporting and improving rapid response activities after a seismic event.

KEYWORDS

seismic damage assessment, rapid response, DARR method, seismic building monitoring, Central Italy 2016 earthquake sequence, simplified building models, dynamic behavior of buildings, seismic retrofitting
Introduction

Most casualties caused directly by earthquakes are due to
damage in residential buildings (So and Spence, 2013). For this
reason, the rapid assessment of expected damage can support
effective response actions and prioritize interventions, thereby
reducing human losses. Assessing damage to buildings depends
on multiple factors, such as the characteristics of the ground
shaking, the building's vulnerability (and its response to shaking)
and the occurrence of local amplification (site effects).

Ground motion recordings allow extracting the ground
shaking characteristics and, in particular, the peak parameters
(e.g., peak ground acceleration, PGA). The engineering
community has devoted considerable effort to identifying the
critical values of ground motion that lead to building damage and
collapse. This was done based on both empirical (Rota et al.,
2008; Masi et al., 2019) and analytical (Borzi et al., 2008; Donà
et al., 2020) approaches. Fragility and vulnerability curves have
been defined for different building types and are currently used to
estimate expected damage in case of seismic events (e.g., Borzi
et al., 2008; Poggi et al., 2020).

Measuring and analyzing the ground motion is therefore of
paramount importance for estimating expected damage to
buildings. However, using peak ground motion parameters to
assess expected damage does not account for the frequency
content of the recorded signal. For this reason, the coverage
of seismic monitoring networks is increasing worldwide and
includes seismic stations installed both in the field and in
buildings or infrastructure (Mori et al., 1998; Trifunac et al.,
2001; Okada et al., 2004; Espinosa-Aranda et al., 2009; Gorini
et al., 2010; Satriano et al., 2011; Wu et al., 2013; Parolai et al.,
2017; Bragato et al., 2021). In Italy, ground motion recordings are
collected in the Italian Accelerometric Archive (ITACA, Russo
et al., 2022). In addition, the Italian monitoring network, OSS
(Osservatorio Sismico delle Strutture, Dolce et al., 2017)
comprises more than 120 public buildings, and some bridges
and dams continuously monitored by low-cost seismic sensors.

The recorded signal in buildings allows assessing their
response to earthquakes and monitoring changes to their
structural health (e.g., Rahmani et al., 2015; Rahmani and
Todorovska, 2021). Past studies have demonstrated the
relevance of assessing the building's fundamental period (Goel
and Chopra, 1997), which is a key parameter for estimating the
expected performance of buildings during earthquakes (Michel
et al., 2010). Some authors (e.g., Crowley and Pinho, 2010; Michel
et al., 2010) pointed out discrepancies between the simplified
period-height relationships used in most building codes (e.g.,
Eurocode, CEN 2004) and the fundamental period estimated
experimentally using ambient noise measurements (e.g., Gallipoli
et al., 2009). Thus, several authors proposed period-height
relationships based on experimentally estimated fundamental
periods (e.g., Gallipoli et al., 2009; Michel et al., 2010; Gallipoli
et al., 2022). It is also relevant to compare the fundamental

frequency range of buildings and soils (e.g., Gallipoli et al., 2020)
to assess the possible occurrence of soil-building resonance (e.g.,
Bard et al., 1996; Mucciarelli et al., 2004).

Since 2009, the OSS network has provided significant
information on the dynamic response of single buildings
during the main Italian earthquakes (Spina et al., 2010). The
occurrence of damage is estimated by comparing the observed
interstory drift values with thresholds defined in literature (e.g.,
Rossetto and Elnashai, 2003 for reinforced concrete, RC,
buildings). However, the procedure requires at least two
recordings (one at the top and another at the bottom of the
building) in order to estimate the interstory drift ratio.

Based solely on recordings at the bottom of buildings (or in
the free field nearby), it is possible to rapidly estimate the
occurrence of damage by taking the simplified buildings linear
dynamic response into account (Scaini et al., 2021; Petrovic et al.,
2022). In fact, the DARR (Damage Assessment for Rapid
Response) method uses the entire recording and simulates the
maximum relative displacement (drift) for a specific building
type (defined by fundamental frequency and damping ratio)
based on simplified oscillators (single or multi-degree-of-
freedom) using the Z transform (Lee, 1990; Jin et al., 2004;
Parolai et al., 2015). The method has produced successful results
for selected building types, including unreinforced masonry
(URM) buildings (Petrovic et al., 2022), a common building
typology in Italy and in particular in Central Italy (Sorrentino
et al., 2019).

The DARR method can be extended to estimate expected
damage to specific building typologies in the surrounding area of
a recording (Scaini et al., 2021). In addition to interstory drift
limits (e.g., Borzi et al., 2008; Rossetto et al., 2016), relative
displacement limits (e.g., Lagomarsino and Giovinazzi, 2006) are
available for different building types. These include both RC and
URM typologies associated with different characteristics (height,
age of construction, seismic design level). The occurrence of
damage is assessed by comparing the estimated relative
displacement or interstory drift (ratio) for each building type
with the limits available in literature. However, DARR is based on
a number of assumptions, in particular on an average building
height (when considering building types instead of specific
buildings) and the simplified dynamic behavior of building
typologies (dominated by the fundamental mode/modes,
obtained from period-height relationships from literature), the
choice of thresholds for damage occurrence, and the
homogeneous soil conditions in the target area.

In this work, we present an application of the DARR method
to the first shock of the Central Italy seismic sequence, which
occurred on 24 August 2016 (Rossi et al., 2018). Recordings are
available at several locations (both in free field and in structures)
and at different distances from the epicenter, from both the
ITACA and OSS databases. We focus our study on four target
areas (Amatrice, Norcia, Visso and Sulmona) with different
epicentral distances for which different damage levels have
<table><caption>TABLE 1 Relative displacement limits (in centimeters) for selected building typologies according to Lagomarsino and Giovinazzi (2006).</caption><thead><tr><th rowspan="2">Building typology</th><th rowspan="2">Seismic code</th><th colspan="3">Relative displacement limits (cm) for selected damage levels</th></tr><tr><th>No damage (ND)</th><th>Extensive damage (ED)</th><th>Complete damage (CD)</th></tr></thead><tbody><tr><td>Simple stone URM, low-rise (1–2 story)</td><td>n.a.</td><td>&lt;0.19</td><td>&gt;0.85</td><td>&gt;1.40</td></tr><tr><td>Simple stone URM, mid-rise (3–5 story)</td><td>n.a.</td><td>&lt;0.42</td><td>&gt;1.35</td><td>&gt;2.10</td></tr><tr><td>Regular URM, RC-floors, low-rise (1–2 story)</td><td>n.a.</td><td>&lt;0.28</td><td>&gt;1.38</td><td>&gt;2.36</td></tr><tr><td>Regular URM, RC-floors, mid-rise (3–5 story)</td><td>n.a.</td><td>&lt;0.62</td><td>&gt;2.19</td><td>&gt;3.50</td></tr><tr><td>RC frame, low-rise (1–3 story)</td><td>Low code</td><td>&lt;1.67</td><td>&gt;4.78</td><td>&gt;7.16</td></tr><tr><td>RC frame, mid-rise (4–7 story)</td><td>Low code</td><td>&lt;2.60</td><td>&gt;7.42</td><td>&gt;11.14</td></tr><tr><td>RC frame, low-rise (1–3 story)</td><td>Moderate code</td><td>&lt;1.96</td><td>&gt;6.48</td><td>&gt;10.15</td></tr><tr><td>RC frame, mid-rise (4–7 story)</td><td>Moderate code</td><td>&lt;2.79</td><td>&gt;10.19</td><td>&gt;16.39</td></tr><tr><td>RC frame, low-rise (1–3 story)</td><td>High code</td><td>&lt;1.84</td><td>&gt;7.47</td><td>&gt;12.30</td></tr><tr><td>RC frame, mid-rise (4–7 story)</td><td>High code</td><td>&lt;2.23</td><td>&gt;10.59</td><td>&gt;17.99</td></tr></tbody></table>

Extensive and complete structural damage (ED, CD respectively) are associated with the exceedance of the respective limits. No damage (ND) is expected for displacement lower than the ND limit, while non-structural damage (NSD) is expected in all other cases. n.a. - not applicable.

been observed. The analysis of the available recordings of the event in the four locations shows that, based on the earthquake recordings and the building characteristics, it is possible to quickly estimate whether structural damage is expected for previously characterized building typologies dominant in the study area. Outcomes are compared with damage evidence collected during field surveys (visual inspection and subsequent damage assessment) performed in these areas after the 24 August 2016 event (e.g., Fiorentino et al., 2018; Sorrentino et al., 2019; D'Ayala et al., 2019).

## DARR method and damage thresholds

For the damage assessment, we use DARR, a method proposed by Scaini et al. (2021) and Petrovic et al. (2022). The linear dynamic behavior of buildings is simulated as simple single-degree-of-freedom (SDOF) oscillators, describing the buildings in a first order approximation by their fundamental frequency and the damping ratio. The fundamental frequencies are estimated from building-soil specific period-height relationships from Gallipoli et al. (2022). An average story height of 3.5 m is assumed for the ground floor (e.g., Chieffo et al., 2019), for the upper storys 3 m are considered (e.g., Sorrentino et al., 2019). The latter is compatible with the minimum required height of 2.7 m in Italy and some other European countries (Appolloni and D'Alessandro, 2021) and accounts for the floor thickness. Additionally, 1.5 m are added for

the roof. For the damping ratio, a standard value of 5% (e.g., Eurocode 8, CEN 2004) is used. The relative displacement between top and bottom (drift) is calculated with the Z transform (e.g., Lee, 1990; Jin et al., 2004; Parolai et al., 2015). The total displacement at the top can be obtained as the sum of the displacement at the bottom and the relative displacement. The interstory drift ratio is estimated by dividing the relative displacement (drift) by the building height.

The occurrence of damage can be estimated using either interstory drift (e.g., Rossetto et al., 2016) or relative displacement limits from literature (e.g., Lagomarsino and Giovinazzi, 2006), based on the characteristics (construction materials) and height of the studied buildings. In this study, we consider the relative displacement damage limits for low to mid-rise URM (simple stone and regular) and RC buildings, representative of the Italian building stock and dominant in the study area. The relative displacement thresholds for different damage states are adopted from Lagomarsino and Giovinazzi (2006, Table 1) for different building typologies. The limits for low, moderate and high-code RC frames are adopted for the zone of higher seismicity in the Italian official seismic zonation (Italian Seismic Hazard map, Meletti et al., 2006; Stucchi et al., 2011 and following modifications). The description of damage levels for both masonry and reinforced concrete buildings is based on the EMS-98 (Grünthal, 1998) macroseismic scale. The method focuses on the expected occurrence of structural damage (extensive or complete). Extensive damage corresponds to level-3 damage of EMS-98 (moderate structural damage, heavy non-structural damage). Complete damage includes
<figure><img src="image_155.png" /><figcaption>**FIGURE 1**
Map showing the locations of the four test sites Amatrice, Norcia, Visso and Sulmona (red rhombi) and the epicenter of the M6 event (yellow star).</figcaption></figure>

<figure><img src="image_156.png" /><figcaption>**FIGURE 2**
(A–D): Location of the earthquake recordings and the selected buildings for the four target areas (from left to right, Amatrice, Norcia, Visso and Sulmona). (E–H): Results of DARR for 3-story simple stone URM buildings in the target areas of Amatrice, Norcia, Visso and Sulmona. (I–K): Documented damage collected from post-event surveys after the 24 August 2016 event in Amatrice, Norcia and Visso. The damage patterns are extracted from OpenStreetMap (OpenStreetMap contributors, 2015) and are in good agreement with the documented post-event damage (e.g., Gaudiosi et al., 2016; Stewart and Lanzo, 2018; D’Ayala et al., 2019). In Sulmona no damage was observed in Sulmona.</figcaption></figure>
both level 4 and 5 of the EMS-98 scale (heavy and very heavy structural damage and/or collapse). The absence of damage is associated with a relative displacement lower than 70% of the yielding displacement (Table 1), which corresponds to level-1 damage of the EMS-98 following Lagomarsino and Giovinazzi (2006). In all other cases, non-structural damage is assumed.

## Damage assessment for building typologies in selected target areas

Our study makes use of the recordings collected by the OSS network (Osservatorio Sismico delle Strutture, Dolce et al., 2017) and the ITACA database (Russo et al., 2022), in order to estimate the expected damage for selected locations for the 24 August 2016 M6 event. We focus on the first event of the Central Italy 2016 sequence, since the applied DARR method (Scaini et al., 2021; Petrovic et al., 2022) does not account for cumulative damage.

Based on the analysis of the damage patterns after the first event of the Central Italy seismic sequence (24 August 2016, Figure 1), four areas (Figures 2A–D) are selected for the application of DARR: Amatrice, Norcia, Visso and Sulmona. Two of the selected areas were very close to the epicenter (Amatrice and Norcia, respectively 9 and 15 km away). Nonetheless, the observed damage patterns (Stewart and Lanzo, 2018; D'Ayala et al., 2019) are very different due to the characteristics of the buildings (e.g., presence or absence of retrofitting) and the occurrence of site effects. Moreover, the different damage patterns could be due to the near-source effect (Luzi et al., 2016; Tinti et al., 2016; Chiaraluce et al., 2017). The third area (Visso) is located at a medium distance of 28 km from the epicenter. Damage was mostly observed in the central-western part of the town and hardly in the historical center located in the southeastern part (Gaudiosi et al., 2016). The fourth area (Sulmona) is located at a greater distance from the epicenter (approximately 90 km): here, the shaking was perceived by inhabitants but did not cause significant damage to the buildings.

Since for Amatrice no recordings were available from the OSS network, a free field recording from the ITACA database located close to the historical center was used to estimate the expected damage for selected building typologies. For Norcia, Visso and Sulmona, recordings of sensors of the OSS, installed in the basement of a building located in the historical center (Norcia: 3 story RC school building, Visso: 2 story simple stone URM school building, Sulmona: 4 story RC hospital building) were used for the damage assessment in the target areas which correspond to the historical centers of the three towns. In the case of Visso, the target area also includes an urbanized area located in the central-western part of the town.

The building stock of the historical centers of the four considered towns is/was composed mainly of low- and mid-rise simple stone buildings mostly constructed before 1919

(Munari et al., 2010; Sorrentino et al., 2019). In addition, low-to mid-rise regular masonry and RC frame buildings were considered due to their presence in the studied areas. These buildings were constructed typically between 1950 and 1980.

## Amatrice

The ground motion recordings in Amatrice are available from the ITACA database. The considered station AMT (Figure 2A, red square) is located on sandy-silty lithofacies (Todrani and Cultrera, 2021), which can be classified as soft soils. The minimum and maximum distances between the recording and the buildings in the old town are of 200 and 600 m, respectively (Figure 2A). Unfortunately, no recording in the historical center of Amatrice is available.

According to Vignaroli et al. (2019) and Milana et al. (2018), the old town of Amatrice was built on sands or conglomerates. Amplification effects affect the ground shaking on the Amatrice terrace (Milana et al., 2018). We assume that the recording from the AMT station can be used to assess the expected damage in the old town, but the ground acceleration on the Amatrice terrace might be larger than at the recording site due to amplification effects (Gaudiosi et al., 2021).

Following the DARR method, the relative displacement (drift) for different building typologies representative for the building stock of Amatrice (2 and 3 story URM buildings in the historical center and 3–6 story RC buildings in the town) are estimated (Table 2). The maximum absolute value of both components (aligned with the main perpendicular directions of the town) is reported in Table 2.

The studied building typologies were prevalent in the town of Amatrice. The building stock in the historical center was predominantly simple stone URM buildings (95%), most of them with 2 or 3 stories (Sorrentino et al., 2019). The fundamental periods T of the different building typologies are calculated using the period-height relationships proposed by Gallipoli et al. (2022) for URM and RC buildings on soft soils (URM: T = 0.0170H, RC-MRF: T = 0.0164H). The recordings of station AMT were first rotated to be aligned with the main perpendicular directions of the historical center, to take the orientation of the studied buildings (as a first order approximation, see Figure 2) into account. Based on the relative displacement limits in Table 1 (low-rise simple stone URM, extensive damage: 0.85 cm, complete damage: 1.40 cm; low-rise regular URM, extensive damage: 1.38 cm, complete damage: 2.36 cm), non-structural damage should be expected for 2 story simple stone and regular URM buildings (Table 2, relative displacement: 0.63 cm). Following the damage limits in Table 1, extensive damage should be expected for 3 story simple stone URM buildings (Table 2, relative displacement: 1.57 cm) and non-structural damage for 3 story regular URM buildings.
<table><caption>TABLE 2 Damage assessment for Amatrice for the most common building typologies (2-3 story URM and 3-6 story RC buildings).</caption><thead><tr><th>Building typology</th><th>Average building height (m)</th><th>Period T* (s)</th><th>Frequency f (Hz)</th><th>Relative displacement (cm)</th><th>Expected damage</th></tr></thead><tbody><tr><td>2 story URM</td><td>8</td><td>0.136</td><td>7.35</td><td>0.63</td><td>Simple stone and regular URM: NSD</td></tr><tr><td>3 story URM</td><td>11</td><td>0.187</td><td>5.35</td><td>1.57</td><td>Simple stone URM: ED, Regular URM: NSD</td></tr><tr><td>3 story RC</td><td>11</td><td>0.180</td><td>5.54</td><td>1.32</td><td>Low and moderate code RC frame: ND</td></tr><tr><td>4 story RC</td><td>14</td><td>0.230</td><td>4.36</td><td>2.92</td><td>Low and moderate code RC frame: NSD</td></tr><tr><td>5 story RC</td><td>17</td><td>0.279</td><td>3.59</td><td>3.31</td><td>Low and moderate code RC frame: NSD</td></tr><tr><td>6 story RC</td><td>20</td><td>0.328</td><td>3.05</td><td>4.88</td><td>Low and moderate code RC frame: NSD</td></tr></tbody></table>

<figure><img src="image_157.png" /><figcaption>FIGURE 3</figcaption></figure>

Photos of Amatrice. (A) View of the historical center of Amatrice on 26 February 2012 (Wikimedia Commons, Silvio Sorcini, licensed under CC-BY 4.0 and is available at: https://commons.wikimedia.org/wiki/File:2012-02-26_Corso_Amatrice.jpg); (B) Historical center on 29 August 2016, damaged by the M6 event of 24 August 2016 (Wikimedia Common, Diego Bianchi, licensed under CC-BY 2.0 and is available at: https://commons.wikimedia.org/wiki/File:Terremoto_centro_Italia_2016_-_Amatrice_-_corso_Umberto_I_(29242968591).jpg); (C) 5 story RC building, constructed following a low code, showing extensive damage photographed on August 31, 2017 (Mapillary, https://www.mapillary.com/, valentina_p, licensed under CC BY SA 4.0).

Our results for the expected extensive damage of 3 story simple stone URM buildings (Table 2; Figure 2E) are in accordance with the information on observed damage in the Amatrice historical center (e.g., Figure 2I; Stewart and Lanzo, 2018; D'Ayala et al., 2019). Following these reports, most of the low and mid-rise URM buildings were either highly damaged or totally destroyed during the 24 August 2016 event (see Figures 3A,B for a comparison of the main road before and after the considered M6 event). According to Sorrentino et al. (2019) most damage occurred to buildings with more than 2 stories. A video of the damage undergone by the historical center is available at: https://www.youtube.com/watch?v = 3-UDfhIH70M. Outside the historical center, in Amatrice a number of low (1-3 story) and mid-rise (4-7 story) RC buildings (of which 37 surveyed by Masi et al., 2019) were constructed in different time periods, based on different building codes (low and moderate). For these

buildings, different damage levels were observed after the analyzed M6 event (Stewart and Lanzo, 2018; Masi et al., 2019; D'Ayala et al., 2019). In our study, we considered the damage thresholds for both low and moderate code RC frame buildings. Results are presented for 3-6 story RC buildings which are the most frequent RC buildings in the study area. Following both the low and moderate code thresholds, non-structural damage should be expected for 4-6 story RC frame buildings. Here, we present two examples of RC buildings for which information on the damage state is available from literature (D'Ayala et al., 2019). The first one is a 6 story RC building constructed in the late 1980s, following a moderate building code (Figure 2A, yellow triangle), only non-structural damage was inspected after the event. The second is a 5 story RC building constructed in the late 1970s or early 1980s following a low building code (Figures 3C, 2A, red triangle). This building was

<footnote>*Calculated using T=cH from Gallipoli et al. (2022) for URM and RC buildings on soft soil.</footnote>

<footnote>ND, No structural damage; NSD, Non-structural damage; ED, Extensive damage; CD, Complete damage.</footnote>
<table><caption>TABLE 3 Damage assessment for different building typologies for Norcia (1-3 story URM and 3-5 story RC buildings).</caption><thead><tr><th>Building typology</th><th>Average building height (m)</th><th>Period T* (s)</th><th>Frequency f (Hz)</th><th>Relative displacement (cm)</th><th>Expected damage</th></tr></thead><tbody><tr><td>1 story URM</td><td>5</td><td>0.085</td><td>11.76</td><td>0.20</td><td>Simple stone and regular URM: NSD</td></tr><tr><td>2 story URM</td><td>8</td><td>0.136</td><td>7.35</td><td>0.98</td><td>Simple stone URM: ED, Regular URM: NSD</td></tr><tr><td>3 story URM</td><td>11</td><td>0.187</td><td>5.35</td><td>2.17</td><td>Simple stone URM: CD, Regular URM: NSD</td></tr><tr><td>3 story RC</td><td>11</td><td>0.180</td><td>5.54</td><td>2.26</td><td>Low and moderate code RC frame: NSD</td></tr><tr><td>4 story RC</td><td>14</td><td>0.230</td><td>4.36</td><td>2.20</td><td>Low and moderate code RC frame: ND</td></tr><tr><td>5 story RC</td><td>17</td><td>0.279</td><td>3.59</td><td>2.07</td><td>Low and moderate code RC frame: ND</td></tr></tbody></table>

extensively damaged during the M6 event (Figure 3C), suffering structural damage (partial collapse of the masonry infill panels and shear failure of some columns). Additional pictures of the damage (non-structural and structural) undergone by the two RC buildings are available in D'Ayala et al. (2019). Considering the damage thresholds for the two selected mid-rise RC frame buildings, from our results we expect non-structural damage for both buildings. This is in accordance with the observed damage for the moderate-code, but not for the low-code building.

## Norcia

For Norcia, the M6 earthquake recordings of the sensors (OSS, Figure 2B) installed in the basement of the San Benedetto school building (e.g., Comodini et al., 2018; Falcone et al., 2021) were used to estimate the expected relative displacements (Table 3, maximum absolute values of the two horizontal components are reported) using the DARR method. The target area corresponds to the historical center, which according to Sorrentino et al. (2019) was mostly constituted by low-rise URM buildings (in total 95%): 74% of them had 2 stories, 15% 1 story and 11% 3 stories. Hence, the building typologies of 1-3 story URM buildings were analyzed in this study. Moreover, low and mid-rise RC buildings were studied.

The historical center is located in an intermountain sedimentary basin (e.g., Bindi et al., 2011; Luzi et al., 2019; Pagliaroli et al., 2020), i.e., on soft soils with potential occurrence of site effects. Thus, the fundamental periods T of the building typologies are calculated using the period-height relationships for URM and RC buildings on soft soil from Gallipoli et al. (2022). Since the school building was aligned the same way as most of the buildings in the historical center, the traces were not rotated.

Based on our results (Table 3, Figure 2F), non-structural, extensive and complete damage should be expected for 1, 2, and 3 story simple stone URM buildings, respectively. For 1-3 story regular URM buildings, non-structural damage should be

expected. During the first event of the sequence, only a few buildings were damaged, most of them not structurally (Figure 2J and D'Ayala et al., 2019), but some of them also suffered structural damage. After the 1979 (Val Nerina) and 1997 (Umbria-Marche) events, most of the buildings in Norcia have been strengthened and retrofitted (Sisti et al., 2019; Sorrentino et al., 2019). Structural damage was only observed for a few masonry buildings with poor or no retrofitting. The URM buildings in the historical center of Norcia are assumed to behave as regular URM buildings to account for the retrofitting. In that case, only non-structural damage would be observed, in accordance with the few damage observed in the historical center of Norcia after the M6 event.

One example of the retrofitting of the buildings in Norcia is the 3 story RC school building, from which the recordings in the basement were used for the damage assessment. The building was designed in the 1960s and retrofitted in 2003 and 2011 (Comodini et al., 2018). The second retrofitting included the installation of dissipative braces on the 1st to 3rd floors. Figure 4 shows the simulated and observed relative displacements for this building for an M3.9 event occurring on 30 November 2013 at approximately 20 km distance and for the M6 event of 24 August 2016. The observed accelerations at the top and bottom of the building and the corresponding Fourier Spectra are presented in Supplementary Figure S1, S2. The building is simulated as an SDOF oscillator in both directions, with fundamental frequencies of 4.3 Hz (longitudinal direction X) and 4.6 Hz (transverse direction Y) and a damping ratio of 2% for small magnitude events. The values characterizing the SDOF oscillator (frequency and damping) have been estimated using a small magnitude event and tested for several small magnitude events, giving satisfactory results in all cases. As presented in Figure 4A, the simulated and observed maximum absolute relative displacements are similar for the M3.9 event for both directions. For the y direction, the relative displacement was almost precisely reconstructed. When considering the M6 event, probably due to the activation of the dissipators, the damping increases in the y direction. Therefore, a 10% damping ratio

<footnote>*Calculated using T=cH from Gallipoli et al. (2022) for URM and RC buildings on soft soil.
ND, No structural damage; NSD, Non-structural damage; ED, Extensive damage; CD, Complete damage.</footnote>
<figure><img src="image_158.png" /><figcaption>**FIGURE 4**
Observed (blue) and simulated relative displacements (orange) at the top of the building for an M3.9 event of 30-11-2013 (A) and the M6 event of 24-08-2016 (B) for the two main directions of the building (X and Y).</figcaption></figure>

<table><caption>TABLE 4 Damage assessment for different building typologies for Visso (2-4 story URM buildings).</caption><thead><tr><td>Building typology</td><td>Average building height (m)</td><td>Period T* (s)</td><td>Frequency f (Hz)</td><td>Relative displacement (cm)</td><td>Expected damage</td></tr></thead><tbody><tr><td>2 story URM</td><td>8</td><td>0.136</td><td>7.35</td><td>0.23</td><td>Simple stone URM: NSD, Regular URM: ND</td></tr><tr><td>3 story URM</td><td>11</td><td>0.187</td><td>5.35</td><td>0.43</td><td>Simple stone URM: NSD, Regular URM: ND</td></tr><tr><td>4 story URM</td><td>14</td><td>0.238</td><td>4.20</td><td>1.14</td><td>Simple stone and regular URM: NSD</td></tr></tbody></table>

(Ferraioli and Lavino, 2019; Foti et al., 2020) is assumed for this direction. Although we do not precisely reconstruct the observed relative displacement at the top of the building, the simulated and observed maximum absolute relative displacements at the top of the building are similar. The simulated maximum absolute relative displacement was estimated as 1.7 cm. When considering the damage limits for a low-rise RC frame building constructed after a low or moderate code, non-structural or no damage should be expected, respectively. This is in accordance with the fact that only non-structural damage was observed.

## Visso

For the Visso target area, the recordings in the basement of the Pietro Capuzi school building (OSS, Figure 2C) have been used to estimate the expected relative displacements (Table 4, maximum absolute values of the two horizontal components are reported) and the corresponding expected damage. The main axes of the school building were approximately oriented as the dominant building directions of the Visso town. The case study of the Visso school building has been studied in Petrovic et al. (2022) and, following the limits for 1–2 story simple stone URM buildings in Table 1,

complete damage should be expected for the August 24 M6 event. After the event, moderate to severe damage has been observed (Brunelli et al., 2021). The Visso target area includes both the historical part (located in the southeast) and the central-western part of the town. The town of Visso is/was mainly composed of 2–4 story simple stone URM and 2–3 story regular URM buildings (Gaudiosi et al., 2016) constructed on soft soils (Brunelli et al., 2021). Thus, the T-h relationships for soft soils (Gallipoli et al., 2022) were used. Many buildings had been partially reconstructed or retrofitted after the 1997 Umbria-Marche sequence (Gaudiosi et al., 2016).

Following the considered damage limits (Table 1), non-structural damage should be expected for 2–4 story simple stone URM buildings (Table 4, Figure 2G). For 2–3 and 4 story regular URM buildings, no damage and non-structural damage should be expected, respectively. Our results are partially in accordance with the fact that damage has been observed only for a few buildings in the historical center (Gaudiosi et al., 2016).

However, the surveys performed after the 24 August event (Gaudiosi et al., 2016) show that both non-structural and structural damage occurred for many simple stone buildings outside the historical center (Figure 2K), including some partially reconstructed after the 1997 Umbria-Marche sequence (Gaudiosi et al., 2016). These differences might be due to a combination of factors including the variability of the dynamic behavior of

<footnote>*Calculated using T=cH from Gallipoli et al. (2022) for URM and RC buildings soft soil. ND, No damage; NSD, Non-structural damage; ED, Extensive damage; CD, Complete damage.</footnote>
<table><caption>TABLE 5 Damage assessment for different building typologies for Sulmona (2-3 story URM and 3-5 story RC buildings).</caption><thead><tr><th>Building typology</th><th>Average building height (m)</th><th>Period T* (s)</th><th>Frequency f (Hz)</th><th>Relative displacement (cm)</th><th>Expected damage</th></tr></thead><tbody><tr><td>2 story URM</td><td>8</td><td>0.136</td><td>7.36</td><td>0.01</td><td>Simple stone and regular URM: ND</td></tr><tr><td>3 story URM</td><td>11</td><td>0.187</td><td>5.35</td><td>0.01</td><td>Simple stone and regular URM: ND</td></tr><tr><td>3 story RC</td><td>11</td><td>0.180</td><td>5.54</td><td>0.01</td><td>Low and moderate code RC frame: ND</td></tr><tr><td>4 story RC</td><td>14</td><td>0.230</td><td>4.36</td><td>0.02</td><td>Low and moderate code RC frame: ND</td></tr><tr><td>5 story RC</td><td>17</td><td>0.279</td><td>3.59</td><td>0.03</td><td>Low and moderate code RC frame: ND</td></tr></tbody></table>

buildings within the same typology and the occurrence of site effects, also suggested by Gaudiosi et al. (2016).

## Sulmona

The recordings in the basement of the Orthopedic Surgery Pavilion of the Sulmona hospital (part of OSS, Figure 2D) have been used to calculate the expected relative displacements (Table 5, maximum absolute values of the two horizontal components are reported) in the target area of Sulmona (epicentral distance: approximately 90 km). The buildings in the historical center are mostly URM (simple stone) buildings constructed before 1800 (Munari et al., 2010). Thus, we considered here 2 and 3 story URM buildings for the historical city center, as well as low and mid-rise RC buildings for the recently constructed buildings. Sulmona was constructed on terraced fluvial and alluvial deposits (Di Giulio et al., 2015), therefore, the T-h relationships for soft soils (Gallipoli et al., 2022) were considered. The hospital building's main directions are aligned with the dominant building directions in the town of Sulmona.

No damage should be expected Figure 2H based on the obtained relative displacement (Table 5, Figure 2H) using the limits for different damage states (Table 1) for all building typologies. Our results are in accordance with the fact that no damage was reported in Sulmona after the M6 event.

## Discussion

In this study, we tested and verified the potential of the DARR method for providing local-scale information on structural damage for selected building typologies dominant in the studied areas. The estimation is based on a single recording in or close to be selected target area when the site conditions can be assumed homogeneous. Since only the recordings of one sensor are needed for each target area, it is a cost-effective and quick method for a rapid estimation of the expected damage both in single monitored buildings and target areas, and can support

rapid response actions of the civil protection. If more than one recording is present for a considered target area, the choice should be made depending on the proximity of the recording and the similarity of geological conditions.

The DARR method relies on several assumptions discussed in Scaini et al. (2021) and Petrovic et al. (2022). There is a wide number of studies on the dynamic response of specific buildings to earthquakes (e.g., Rahmani and Todorovska, 2021) that allow identifying damage patterns in a precise way. The DARR method only accounts for the linear dynamic response of buildings, assuming that it can support the identification of structural damage. With this approximation, the precise reconstruction of the traces during the non-linear behavior is not possible, but the overstepping of the thresholds and the subsequent damage is successfully assessed (Scaini et al., 2021; Petrovic et al., 2022). For the purposes of rapid damage assessment, this approximation is satisfactory. DARR can be used for a large number of buildings with prior information (dominant building typologies, average building heights, period-height relationships, soil conditions) supporting rapid post-event damage assessment. Nonetheless, to correctly define the damage thresholds and to assess the expected damage, precise information on the building typologies (including construction age for building code identification) is needed. In particular, the use of period-height relationships requires information on the average building or story height, which might vary among the typologies, leading to different values of the fundamental frequency and thus, different relative displacement and expected damage. In case of scarce information on the building typologies, the identification of the damage thresholds and thus, the assessment of expected damage might be erroneous. In our study, we estimated the frequencies representative for the considered building typologies from period-height relationships from literature (Gallipoli et al., 2022). These relationships were developed from ambient vibration measurements in residential buildings in southern and northeastern Italy. There might be a variation in the frequencies of the building typologies of the studied area due to variations in the construction (e.g., materials, story

<footnote>*Calculated using T=cH from Gallipoli et al. (2022) for URM and RC buildings on soft soil.</footnote>

<footnote>ND, No structural damage; NSD, Non-structural damage; ED, Extensive damage; CD, Complete damage.</footnote>
<figure><img src="image_159.png" /><figcaption>**FIGURE 5**
Displacement Response Spectra for (A) Amatrice (B) Norcia (C) Visso and (D) Sulmona for the two main directions of the buildings (gray and black lines) in the considered towns.</figcaption></figure>

heights etc.). Peak relative displacements increase drastically for mid to high-rise buildings with frequencies lower than 2 Hz (Norcia) and 5 Hz (Amatrice), as shown by the response spectra in Figure 5. Following the damage limits (Table 1), these buildings would suffer extensive or complete damage according to DARR.

The DARR method relies heavily on the choice of the relative displacement or interstory drift limits used for the damage assessment. Currently, the literature provides both interstory drift (e.g., Borzi et al., 2008; Shahzada et al., 2011; Chourasia et al., 2016; Minas and Galasso, 2019) and relative displacement limits (e.g., Lagomarsino and Giovinazzi, 2006; Frankie et al., 2013; Lestuzzi et al., 2016). Each set of limit values is derived based on different assumptions and for specific building typologies and study areas, and their use and validity depends on the typology dynamic behavior. DARR assumes that the relative displacement and interstory drift limits derived with different methods (e.g., numerical methods) are comparable with those found with the Z-transform for the purpose of a simplified damage assessment. However, further work is needed to validate the usability of the relative displacement and interstory drift limits derived with numerical methods. This would be strongly supported by the availability of more recordings both at the bottom and at the top of damaged buildings in correspondence of structural damage.

In this study, we considered different building typologies that are representative for the studied areas. The selection of damage thresholds for the analyzed building typologies is critical and further work is needed in order to estimate the expected behavior of specific

building types (e.g., retrofitted URM buildings) and the influence of different soil conditions (e.g., Gallipoli et al., 2020). In addition, different damage state definitions are available from literature, based either on empirical damage identification (e.g., EMS-98, Grunthal, 1998) or limit states identified by numerical modeling (e.g., Borzi et al., 2008). In order to compare the outcomes of different damage assessment, a strong unification effort is required, as discussed by e.g., Rosetto and Elnashai (2003), Faravelli et al. (2019).

Amatrice and Norcia had a similar epicentral distance (9 and 15 km, respectively) and the ground motion recordings of the August event in Norcia and Amatrice are comparable in terms of PGA and PGD. However, during the 24 August 2016 M6 event, masonry buildings in Norcia successfully resisted a ground motion that might have led to a structural damage state (D4-D5) as documented in Amatrice (Figure 2). The different damage patterns might be due to several factors including site effects, building typologies (e.g., retrofitting) and orientation, and the main direction of seismic wave propagation. In particular, DARR assumes that the ground motion in the target areas is homogeneous and can be represented by the recording. However, Figure 5 shows the different response spectra in Amatrice and Norcia, with large amplitudes at lower frequency related to local site effects. All these factors should be pondered when interpreting the recording from a single station and concur to identify the target area for applying DARR. For the Amatrice test site, the location of the recordings was outside the historical center (target area). Due to site effects, the relative displacement and thus, the expected damage might be slightly underestimated.
Apart from a number of studies on specific buildings (e.g., Ferraioli and Lavino, 2019; Gandelli et al., 2019; Foti et al., 2020), to our knowledge there are no studies that estimate the expected increase in relative displacement or interstory drift capacity for specific retrofitted building typologies. Additional studies on relative displacement or interstory drift limits for retrofitted building typologies might improve the results on the expected damage from the DARR method. In addition, no specific period-height relationships exist for retrofitted buildings which modify the stiffness of the buildings (e.g., Michel et al., 2018). Finally, retrofitting can influence the building's damping and can be different in the two main building directions, as shown for the case of San Benedetto school in Norcia. Further research on the effect of retrofitting on the dynamic response of the considered building typologies would be desirable.

For the case study of Visso, non-structural damage should be expected for the August 24 event for 2-4 story simple stone URM buildings. The school building in Visso was a 2 story simple stone URM building and was damaged during this event. However, the school building has to be considered as a different building typology, with much higher floor heights and a more complex T shape. Following the T-h relationships from Gallipoli et al. (2022), for a 2 story building of 8 m height, we assume an average fundamental frequency of 7.35 Hz. In contrast, the school building was 13.5 m high and had a fundamental frequency of 3.18 Hz (Ferrero et al., 2020). For this reason, the expected relative displacement is much higher (3.5 cm) than the one estimated for a standard 2 story URM building, resulting in complete damage. The response spectra in Figure 5 shows that for Visso the expected relative displacement increases for fundamental frequencies below 4 Hz. In the study of the school building in Visso (Petrovic et al., 2022), the damping ratio has been estimated as 15%. In this study, the standard value of 5% (EC8, CEN 2004) was used, resulting in a slightly higher relative displacement.

DARR supports the combined use of both relative displacement and interstory drift limits which might be appropriate for different case-studies (e.g., high-rise flexible buildings). Further work is needed to explore how these limits work and their performance for different building types. The validation of DARR is, at the time, limited by the difficulties of having simultaneous availability of the required data which comprise earthquake recordings, buildings characteristics (type, material, age, height, fundamental frequencies and damping) and knowledge of the soil conditions. Future efforts will be devoted to testing the method in other areas where the information is available.

There are several studies on the effect of different ground motion parameters on the expected structural building damage (e.g., Ghimire et al., 2021 for RC buildings). The optimum criterion depends on the expected collapse mechanism (which varies between building typologies), and can be defined based on multiple indicators, including duration (Hancock and Bommer, 2006). This criterion would help the rapid identification of areas where structural damage is expected and prioritize interventions.

## Conclusion

In this work, we present an application of the DARR method to the August 24 M6 event of the Central Italy seismic sequence (2016-2017). Expected damage is estimated in four selected towns in Central Italy (Amatrice, Norcia, Visso and Sulmona) where earthquake recordings were available. Results of the damage assessment, performed for dominant building typologies (URM and RC frames), were validated with post-event surveys. DARR successfully estimated expected damage for some building types as mid-rise URM (e.g. in the historical center of Amatrice) but failed to identify the damage occurred for low-rise URM buildings (e.g. in the town of Amatrice). The reasons for this are discussed in the article pointing out the aspects to be improved in future. Extensive and complete damage is obtained in accordance with the observed damage for 2 and 3 story simple stone URM buildings in Norcia and non-structural damage for regular URM buildings. It also correctly estimates the absence of structural damage in a target area located at a larger epicentral distance (Sulmona). Our results suggest that relative displacement limits are suitable for the damage assessment of low and mid rise building typologies considered in this work. DARR has the potential to provide a timely and cost-effective estimation of the expected damage, both for selected buildings and target areas, to support rapid response in the aftermath of a potentially destructive earthquake.

## Data availability statement

The datasets presented in the study are publicly available. All datasets presented/accessed in this study are cited in the article/ Supplementary Material. Earthquake recordings were extracted from ITACA (https://itaca.mi.ingv.it/) and OSS (https://oss.protezionecivile.it/osspublic/#/).

## Author contributions

BP: Conceptualization, data analysis and interpretation, Writing-original draft preparation; CS: Conceptualization, data collection and interpretation, Writing-original draft preparation; SP: Conceptualization, Writing-reviewing and editing.

## Funding

Bojana Petrovic is supported by a research fellowship from the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG, PE 2891/1-1, Projektnummer 428372009). This research was partially funded by the Autonomous Region Friuli Venezia Giulia, Veneto Region and the ARMONIA project (real-time acceleration network for monitoring sites and buildings in Italy and Austria, 2014-2020 INTERREG V-A Italy-Austria).
## Acknowledgments

The DARR method was developed for the Friuli Venezia-Giulia region within the research activities supported by the Autonomous Region Friuli Venezia Giulia, the Veneto Region and the ARMONIA project (real-time acceleration network for monitoring sites and buildings in Italy and Austria, 2014–2020 INTERREG V-A Italy–Austria).

## Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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## Supplementary material

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