Local seismic response in the historical centre of Nafplio (Greece) as a tool for seismic risk management

doi:10.21203/rs.3.rs-5277459/v1

Local seismic response analysis is a crucial tool for assessing site-specific seismic hazards, particularly in urban areas of cultural and historical significance. However, these analyses often overlook the complexities of near-surface geological and topographical conditions, especially in regions with medium to high seismic activity. This study, funded by the H2020RISE-Marie Curie Action in the framework of the STABLE (STructural stABiLity risk assEssment) project, focuses on the local seismic response of the Nafplio (Greece), an urban area rich in cultural heritage. By adopting an integrated methodology, we aim to enhance the understanding of seismic risk in complex subsoil environments. The approach involves: i) constructing a 3D geological model of the area's subsoil setting, including it in an engineering geological modelling; ii) estimating the resonance frequency of the soft soils using ambient seismic noise measurements and earthquake-based geophysical techniques; iii) generating response spectra for three return periods of 50, 475, and 2000 years using both 1D and 2D numerical modelling approaches. The combined geophysical and numerical modelling results provide a more robust framework for evaluating local seismic amplification due to both stratigraphic and topographic features, offering valuable insights for disaster risk mitigation and resilience planning in seismic-prone urban areas.

Local seismic response

Engineering geological model

Geophysical investigations

Seismic numerical modelling

3D geological reconstruction

The severity of earthquake damage scenarios in urban areas is typically a result of a combination of local seismic amplification effects (Sextos et al. 2018) and the inherent seismic vulnerability of the built environment (Sorrentino et al. 2019). Local seismic response analysis plays a key role in understanding and mitigating these hazards, particularly in the context of seismic microzonation efforts. The accuracy of the local seismic response analysis largely depends on the reliability of the engineering geological model, as it represents the reference for conceptualising seismic responses and designing numerical simulations, as well as on the properties of the materials involved, including both soils and rocks. In addition to stratigraphic amplification, complex geological conditions of the subsoil can obscure or influence the interpretation of local seismic response, with effects arising from processes that differ from stratigraphic amplification alone (Martino et al. 2020).

Over the past few decades, there has been increasing recognition in the literature of the need for an integrated approach to seismic hazard assessment. Such approaches combine engineering geological modelling, geophysical investigations, and numerical simulations to evaluate seismic shaking at specific sites (ICMS Working Group 2008; Gaudiosi et al. 2014; Martino et al. 2015; Macerola et al. 2019; Moscatelli et al. 2020; Pergalani et al. 2020; Varone et al. 2021). These approaches are particularly relevant for evaluating seismic risks in areas with complex geological conditions (Kham et al. 2013; Salameh et al. 2017; Semblat et al. 2020; Hakimov et al. 2021, 2024).

The accuracy of local seismic response analysis is closely tied to the reliability of the engineering geological subsoil reconstruction, which allows the conceptualisation of seismic behaviour and the design of numerical models. In literature, several studies present 2D numerical approaches as a particularly valuable tool in a local seismic response analysis. By simulating the propagation of seismic waves from the seismic bedrock to the surface, these models account for the lateral and vertical variations of the physical and mechanical properties of soils as well as their stratigraphy, including lateral heterogeneities and complex topographical features (Martino et al. 2015; Sonmezer et al. 2019; Tunar Özcan et al. 2019; Antonielli et al. 2021; Moscatelli et al. 2021; Tallini et al. 2020).

Dealing with the cultural heritage protection topic, the H2020 - RISE “STABLE - Structural stABLity risk assEsment” project (www.stable-project.eu; Serpetti et al., 2020) aims at assessing both seismic action, through site-specific local seismic analysis, and the vulnerability of historic buildings to evaluate the damage scenario expected for the following historical centres of the Mediterranean area: Rieti (Italy) (Antonielli et al. 2023), Nafplio (Greece) (Saroglou et al. 2021) and Strovolos (Cyprus) (Iannucci et al. 2022).

This study focuses on assessing the local seismic response of Nafplio town (Greece), an important cultural and tourist hub located in the Argolic Gulf in northeastern Peloponnese, Greece (Fig. 1). The town’s complex geological setting, characterised by two steep-slope reliefs (called Acronafplia and Palamidi) and by a coastal plain, presents a good opportunity for seismic hazard analysis involving an historical town located in an area affected by medium to high seismic hazards. Because of the complex geological conditions, a high-resolution 3D geological model was developed to produce detailed zonation for the area in terms of seismic response, a significant advancement in the field. Recent advances in modelling technologies have enabled the reconstruction of subsurface structures through simple cross-sections and by creating 3D models which envelope the volumes of interest. These large-scale models have already been applied in local seismic response studies (Bala et al. 2023; Giallini et al. 2024).

The local seismic response analysis of the Nafplio historical town includes several key steps: i) assessment of the basic seismic hazard for different return periods, ii) reconstruction of an engineering geological model of the subsoil, iii) execution of single-station seismic ambient noise measurements, iv) 2D numerical simulations, v) standard spectral ratio (SSR) analysis using recorded earthquakes data; vi) seismic microzonation mapping. The results aim to provide a comprehensive understanding of seismic risk in the historical town of Nafplio and offer valuable insights for risk mitigation and resilience strategies in seismic-prone areas with cultural heritage significance.

The Nafplio town is in the Argolis Peninsula, which is part of the greater Dinaric-Hellenic orogenic system (Fig. 1) involving tectonic-sedimentary thrust-belt of NW-SE trend and with northwest vergence, generated during the Alpine-Himalayan orogenesis by the collision between the Adria plate and the south sector of the Eurasia Macroplate (Auboin 1959, 1970; Bortolotti et al. 2013). The Hellenides surround the southern part of the mountain chain and the area of Nafplio consists of a succession of 5 tectonic units, which in sequence, from bottom to top, are: i) Trapezona unit (middle/lower Triassic – Upper Jurassic); ii) ophiolitic unit of Dhimaina (middle Triassic - upper Jurassic / lower Cretaceous), composed of the ophiolitic complex of Migdalista and the mesoautoctonous series of Lygourio; iii) Iliokastron melangitic unit (middle/upper Jurassic - lower Cretaceous, outcrop in northern Argolis); iv) Adheres melangitic unit (Cretaceous - Paleocene, crop out in Southern Argolis); v) Faniskos unit (upper Jurassic - Eocene). These 5 units were involved in compressive deformation with folding and northwest thrusting, resulting also in an inverted stratigraphic sequence.

Specifically, the Nafplio study area was initially involved in a compressive tectonic phase during the Upper Jurassic, which brought the Migdalista ophiolitic complex above the Triassic-Jurassic limestones, this latter belonging to the Trapezona Unit.

Subsequently, during the Cretaceous, the Hellenid accretionary prism experienced intensive erosive episodes in the subaerial environment, with coarse clastic breccias rich in basalts and limestone deposition. Afterwards during the Upper Cretaceous and Paleocene, pelagic pink and red limestones followed by siliciclastic flysch sequences sedimented (Photiades 2010). This sequence was defined by Bortolotti et al. (2003) meso-autochthonous series of Lygourio.

A further compressive tectonic phase occurred during the late Eocene, according to Photiades (2010), which resulted in the simultaneous overthrust of the Adheres unit and the Faniskos unit above the flysch.

The outcropping geological formations around Nafplio town (Apostolidis and Koutsouveli 2010) include (from the oldest to the most recent using the codes presented in the geological map of Fig. 2):

- Limestone (lower Cretaceous - upper Cretaceous; coded: Ki-s.k): white to grey massive limestone with medium-thick layers and good geomechanical characteristics;

- Limestone with chert (Campaniano – Maastrichtiano; coded: Ks.k2): white thin layers of limestone with cherts;

- Limestone with chert (upper Cretaceous - Paleocene, coded: Ks.k1): white or pink to reddish thinly stratified limestone with chert; good geomechanical characteristics;

- Flysch (post-Ypresiano, coded: fg1): alternations of sandy marls, quartzite sandstones, siltstones and conglomerates with limestone;

- Conglomerates (upper Pleistocene, coded: Pt.st.): pebbles of fossiliferous limestone of different origins and a red clayey cement. Comparable to soft rock with sufficient geomechanical characteristics;

- Gravels and breccias (Pleistocene, coded: Pt.sc1): from cohesive to semi-cohesive coarse clasts (mainly calcareous) and reddish cement. Comparable to soft rock with poor geomechanical characteristics;

- Gravel (Quaternary - Holocene, coded: Q.sc): pebbles and limestone fragments in a sandy-clayey matrix;

- Filling materials (Holocene) which can be divided into: i) recent loose deposits and man-made fill (coded: har), consisting of sandy clays, sands and gravels; variable thickness from 1 to 3.5 m (in some points, it exceeds 4 m); ii) historical and younger deposits (coded: urb), consisting of construction materials and sandy-clayey soils; thicknesses less than 1 m.

The above-mentioned formations are represented in a revised geological map of the Nafplio study area, starting from the 1: 5000 scale geological map of the Nafplio region (Fotiadis and Mitropoulos 2006), the information contained in Georgiou and Galanakis (2010), and the engineering geological surveys carried out on-site in the summers of 2020 and 2021.

The study area is characterised by the mountainous landscape prevailing in the south and the flat landscape dominating to the north. The prominent topographic features are Palamidi ridge on the southeast (223 m), the Acronafplia ridge (85 m) on the south and Bourtzi Island 600m northwest of the harbour. The steep slopes of these ridges formed following the combination of both the horst and graben tectonics and the glacio-eustatic fluctuations of the sea level, which caused the abrasion and retreat of the sea cliffs, as well as the formation of marine terraces, and the deposition of the marine and continental conglomerates along the southern slope of the Acronafplia ridge (Georgiou and Galanakis 2010). Here, the surface run-off water caused material erosion from the slopes and the consequent transport and accumulation at the bottom.

The geomorphological evolution of the Nafplio area was deduced by analysing landforms and sequences of morpho-evolutionary episodes were reconstructed. The tectonics generated a horst and graben morpho-structural setting, which influenced the river network and subsequent marine sedimentations (Mitropoulos and Zananiri 2010). The tectonics was also a determining factor in forming the steep slopes characterising the south and north-west sides of the Acronafplia peninsula and the Palamidi ridge. The water, whose action through the sea is subordinated to glacio-eustatic fluctuations, caused the abrasion and retreat of the high coast that characterises the south and north-west side of Acronafplia ridge, as well as the formation of marine terraces, such as testified by the conglomerates of marine and continental origin, found along the Arvanitia trailon the southern slope of the Acronafplia ridge (Georgiou and Galanakis 2010). Furthermore, the surface run-off water caused material erosion from the slopes and the consequent transport and accumulation at the base.

Seismic hazard analysis represents a widely applied tool for estimating the strong motion amplitude at a specific return time; therefore, such an analysis provides a fundamental input in earthquake risk mitigation planning (McGuire 1993).

Seismic hazard is a quantitative estimation of ground shaking, defined by morphometric analysis. It includes peak ground acceleration (PGA), velocity (PGV), displacement (PGD), or response spectrum ordinates. Therefore, seismic hazard analysis involves the estimation of the most crucial parameters related to seismic hazard that is expected to occur at a given site during a predefined time interval (the return period - probabilistic approach, PSHA) or for specific earthquake scenarios (deterministic approach, DSHA). In the present study, both the probabilistic and the deterministic approaches are used for the seismic hazard assessment of the Nafplio Historical Centre. In particular, the PSHA methodology was first carried out to calculate the level of ground shaking for specific return periods. Using the calculated PGA values as a basis, we then used the DSHA methodology to obtain synthetic time series for earthquake scenarios from the local active faults, aiming to obtain these PGA values.

3.1. Seismotectonic and seismicity

The study area is located on the Hellenic subduction zone, which is characterised by the subduction of the African plate under the overriding Aegean plate. The subduction of the African plate is located several tens of kilometres under the broader area of Nafplio and thus, mostly intermediate to deep events occur in the region directly surrounding the town. According to the earthquake catalogue of Papazachos and Papazachou (2003), several strong events have occurred in the broader Eastern Peloponnese region, with magnitudes up to M7.0 (Fig. 3). The actual depth of these historical events cannot be determined accurately, however, by examining the recently recorded seismicity (Fig. 3), it can be deduced that those occurred in the close vicinity of Nafplio are probably of intermediate to large depth, related to the subduction zone. Shallow fault zones have been documented, however, to the east and north of the study area (Fig. 3), which can be attributed to some past strong events such as:

The Iria fault zone (Papazachos and Papazachou 2003; Karakaisis et al. 2010), a North dipping normal fault possibly related to several historical events with M > 6.0, with the most notable being the M6.4 earthquake of 1769, which caused significant damage to the town of Nafplio;
The Epidaurus fault zone (Karakaisis et al. 2010), which can be related to an M6.3 event that occurred in 1837;
The Athikia fault zone (Karakaisis et al. 2010), a series of north-dipping normal faults, related to an M6.5 event in 1858;
The Kechriae fault zone (Noller et al. 1997), also a north-dipping normal fault related to several M > 6.0 events in the area around Corinth;
The Xylokastro fault zone (Ferentinos et al. 1988; Armijo et al. 1996) located in the southeastern part of the Gulf of Corinth, attributed to the large M6.7 earthquake of 1742.

3.2. Basic seismic hazard assessment

The basic seismic hazard assessment for the area of interest was performed in probabilistic and deterministic contexts. The probabilistic seismic hazard assessment (PSHA) was carried out on the basis of the following steps (Reiter 1990; Baker 2008): i) identification of all regional active sources, ii) determination of the local seismicity parameters for the seismic sources (magnitude of completeness (Mc), b-value, seismicity rate), iii) establishing a source distance distribution model, which refers to the distribution of the sources within the study region, relative to the Nafplio city, iv) calculating the distribution of ground motion on the basis of the Ground Motion Prediction Equation (GMPE) and v) final calculation of the seismic hazard by combining the above-mentioned elements. Regarding the input sources in our case, we followed a fault-based approach, using the active sources described previously. The calculation of the local seismicity parameters is directly linked to the completeness of the implemented earthquake catalogue. In fact, a homogeneous catalogue should be implemented to reliably estimate the seismicity rate in any given region. The estimation of those parameters was based on the earthquake catalogue of Papazachos and Papazachou (2003) for the historical events, combined with the updated earthquake catalogue of Makropoulos et al. (2012) and the recently recorded seismicity by the National Observatory of Athens (NOA). The combined catalogue that resulted spans a period between 1968 up to 2020. The actual magnitude of completeness for the implemented catalogue was evaluated at M2.6, using the Maximum Curvature method proposed by Wiemer and Wyss (2000). This value is mostly related to the completeness of the early instrumental period that is covered by this catalogue. The parameters related to the seismicity rate were calculated based on the Modified Gutenberg-Richter law (Cornell and Vanmarke 1969; Sornette and Sornette 1999), whereas the selection of the maximum magnitude was based on the scaling laws of Wells and Coppersmith (1994), for each of the individual active sources. The GMPE proposed by Danciu and Tselentis (2007) for the Greek region was applied and as the last step, the PGA distribution for different return periods was obtained by applying the total probability theorem. Similarly, the Pseudo Spectral Acceleration (PSA) was also calculated for the spectral periods specified in the above GMPE. The resulting hazard curves and PSA spectra for return periods of 475, 950 and 2000 years for the town of Nafplio can be seen in Fig. 4, along with the response spectrum provided by the Greek seismic-resistant regulation for the same soil class and for historical buildings. The corresponding calculated PGA values for the three return periods are 65, 85 and 120 cm/s², respectively.

For the Deterministic Seismic Hazard Assessment (DSHA), we used the previously described seismic sources and simulated several seismic scenarios for them, to obtain synthetic acceleration waveforms with PGA values like those obtained by the PSHA technique, as well as for the worst-case earthquake scenario (maximum possible magnitude for each fault, based on the scaling law of Wells and Coppersmith 1994). Our purpose was to obtain accelerograms that were characteristic of the local faults and, at the same time, took into consideration the seismic hazard that was calculated for different return periods, according to the PSHA methodology. The simulation methodology that was used was the stochastic simulation of ground motion (Boore 1983) expanded for finite sources by Beresnev and Atkinson (1997). More specifically, we employed the EXSIM algorithm (Motazedian and Atkinson 2005), which implements a dynamic corner frequency, further improving the results (Table 1). According to the stochastic simulation methodology, the acceleration time series A(t) that is observed in a specific site is the result of the convolution of the source S(t), the path P(t) and the local site effect G(t). Each individual source was implemented as a planar surface, subdivided into a series of point sub-sources, distributed relative to the location and the geometrical characteristics of the fault (length, width, strike, dip). The final synthetic time series is obtained by convolving the individual calculated time series for each point sub-source.

Since the aim of the application of the above methodology was the calculation of synthetic accelerograms with specific PGA values, the simulations were carried out for a range of magnitudes and stress drop values for each fault. Consequently, the fault dimensions varied, depending on the earthquake magnitude that was simulated. The VS velocity and density at the depth of each fault were set at 3.4 km/s and 2.8 kg/m3 respectively, which are common values for depths between 5–15 km for Greek continental crust in the broader region (e.g., Latorre et al. 2004; Makris et al. 2013). The rupture velocity was set at 80% of the VS (e.g., Atkinson and Silva 1997; Beresnev and Atkinson 1998). Regarding the path effect, a 1/R geometrical spreading model related to propagation in homogeneous space was used since the source-to-site distances for all scenarios are relatively small (< 80 km), and thus, crustal heterogeneity is not expected to play any major role. The anelastic attenuation model that was used was the one proposed by Polatidis et al. (2003), which was based on real recordings of back-arc events in the Greek region and finally, the duration model of Atkinson and Boore (1995) was also implemented. The site effect is represented by an amplification versus frequency curve. In our case, we used the attenuation coefficients proposed by Klimis et al. (1999) for soil class A (rock) in combination with the proposed value of 0.035 for the K0 value by the same authors. The final synthetic time series were calculated after several iterations for random hypocentre locations and slip distributions.

Table 1

Main parameters used with the EXSIM algorithm for the calculation of the various synthetic accelerograms.
Stress drop		30–80
Hypo loc		Random
Dt		0.002 s
Vs		3.4 km/s
rho		2.8 kg/m3
Vrup		0.8VS
Geometrical spread		1/R
Q model	Q0	55
Q model	n	0.91
window
Pulsing percentage		50
Slip distribution		random
K0		0.035

Based on the obtained synthetic acceleration time series, a set of 7 recordings of real earthquakes was selected for each return period (i.e., 50, 475, and 2000 years) using the REXELweb tool (Sgobba et al. 2021) by the National Institute of Geophysics and Volcanology INGV (http://itaca.mi.ingv.it/ItacaNet_31/#/data_and_services/tools/rexel). The selection was performed using the mean elastic response spectrum computed on the synthetic accelerograms for each return period as a target for the tool, therefore ensuring the spectro-similarity between the target response spectrum and the mean elastic response spectrum of the 7-time histories selected for each return period from the Engineering Strong Motion Database (ESM) and the Italian Accelerometric Archive (ITACA). The selected time histories were used in the subsequent steps of this work to perform seismic wave propagation simulations aimed at calibrating the subsoil models as well as assessing the expected seismic shaking of the site.

The conceptual workflow (Fig. 5) that leads to the evaluation of the expected seismic action at Nafplio town started with the construction of the preliminary geological model based on existing literature and the stratigraphy of the previously available surveys. The interpreted geological cross-sections of the historical town centre represented the geological model of reference that has been validated using a large set of in-situ geophysical survey results, i.e., by fitting site resonance frequency (f₀) with the main peak of the amplification function obtained by the 1D linear elastic modelling of the corresponding site stratigraphy.

The validated engineering geological model has been represented in 3D, which allows observing the spatial relationships between different geological bodies with marked differences in physical and mechanical properties. The reliability of local seismic response analyses is indeed directly related to the accuracy of the engineering geological reconstruction of the near-surface subsoil.

A 2D numerical modelling of seismic motion has been performed with an equivalent linear approach (finite elements) with LSR 2D (by Stacec) on 4 selected engineering geological cross-sections to extract the time histories and elastic response spectra (5% damping) on 124 control points distributed on the cross sections.

By analysing the values of the amplification index along the modelled cross-sections and the corresponding engineering geological model of the subsoil, a total of 6 zones with peculiar effects of expected seismic amplification were recognised and mapped. An average elastic response spectrum has been computed for each zone and each analysed return period (i.e., 50, 475 and 2000 years).

4.1. Engineering-geological modelling

The reconstruction of geological and engineering geological models of the Nafplio area has been achieved using several previous investigations and the data collected from the on-site geological and engineering geological surveying, as well as previous scientific literature (Karastathis et al. 2010).

Regarding the previously available data, the following types of investigations were considered:

Geophysical investigations: data of a seismic reflection campaign conducted by the Hellenic Survey of Geology & Mineral Exploration (HSGME) with the morphological and stratigraphic reconstruction of the seabed of the Gulf of Nafplio, thanks also a bathymetric map at a scale of 1:5000. Through the seismic reflection, the geological structures below the seabed were also outlined with the identification of 3 unconformities, which define four seismic units. HSGME also conducted, both in the coastal area and the hinterland of Nafplio, a series of seismic reflection surveys to detect the depth of the bedrock and the possible presence of faults. HSGME also carried out 4 cross-hole investigations, 2 of which are close to the town of Nafplio. Results show the presence of a loose deposit, consisting of clay and silt, with S-wave velocities of less than 400 m/s. The velocity of the P-waves is instead higher than 1500 m/s, probably also due to the saturation of the deposit for marine intrusion;
Boreholes: a total of 21 boreholes, 17 offshore, in the port area, and 4 on land, (performed by Triton Consulting Engineers S.A., Geomichaniki, HSGME) have been consulted. The same stratigraphic sequence characterised the analysed boreholes, with differences in the thickness of the lithotypes: off the harbour to the northwest, between the coast and Bourtzi island, the terrigenous component is prevalent, while in the northern sector of the offshore of the ancient town, the terrigenous component is lower than the lithoid one. All the stratigraphies obtained from the boreholes were used to better constrain the preliminary geological model.

Together with the information derived from previous studies and the technical reports used to reconstruct the geological model of the plain and the gulf in front of Nafplio, two detailed geological and geotechnical field surveys were carried out in July 2020 and July 2021, mainly focused on the reconstruction of the geological model of the Acronafplia and Palamidi reliefs and distribution of the recent deposits. The on-site survey at Acronafplia revealed that the hill is composed of heavily fractured Cretaceous limestone, with flysch outcropping in the depression towards Palamidi ridge. The survey along the ridge’s southern and western sides confirmed susceptibility to rock falls and coastal erosion leading to shoreline retreat. The N/NW side of Palamidi features a thrust with an NW vergence, where an inverted stratigraphic succession crops out. The thrust zone shows complex deformation, with fractured limestone, flysch, and striated calcite surfaces. The area between the ridges exhibits repeated outcrops, and a normal fault has lowered the Acronafplia peninsula. The northern slope of Palamidi is surrounded by a Pleistocene gravel deposit, which is in erosive contact with Eocene flysch and extends to the coast along the W-SW slope.

The analysis of the bibliographic sources, the available technical reports and the data collected during the two field surveys allowed the definition of a detailed and updated geological map concerning the versions available in the literature (Fig. 2). Furthermore, 9 geological cross-sections were conceptualised to constrain the geological model in depth and for better reconstructing the lateral relations of the various geological bodies (Fig. 6).

The critical analysis of both previous data and field surveys has allowed the reconstruction of an engineering geological map (Fig. 7) and nine engineering geological sections (Saroglou et al. 2021), highlighting that outcropping geological formations can be regrouped into three main lithotechnical units as bedrock, recent soft soil materials, and man-made ground.

4.2. Single-station seismic ambient noise measurements

A total of 99 single-station seismic ambient noise measurements were carried out to validate the geological model between 2020 and 2021. The survey area covered the different zones identified in the Nafplio area (Fig. 7), each representing peculiar geological characteristics as identified by engineering geological modelling. The single-station seismic ambient noise measurements were performed using two different kinds of sensors: i) LE-3D/5 s three-component seismometers (0.2 Hz eigenfrequency) together with REFTEK 130-01 dataloggers; ii) SL06 24-bit digitisers with built-in SS20 three-component velocimeter (2.0 Hz eigenfrequency) by SARA Electronic Instruments. Seismic ambient noise was recorded for 1 hour at each measurement site at a sampling frequency of 250 Hz. and elaborated by GEOPSY (Wathelet et al. 2020) with the following processing parameters:

- a 40 s windows with overlaps of 25% for each time series;

- tapering to 5% and smoothing of the spectrum for each window by the algorithm of Konno and Omachi (1998) with the b parameter = 40;

− 0.2–20 Hz frequency range of the analysis.

The horizontal-to-vertical noise spectral ratio (HVNR) method, here utilised to elaborate the seismic ambient noise measurements, is commonly employed to assess the 1D stratigraphic resonance of locations where there is a significant contrast between the low S-wave velocity subsoil and the seismic bedrock beneath it (Haghshenas et al. 2008). In HVNR processing, the Fourier transforms (FFTs) of the horizontal ground motion components are averaged and subsequently divided by the FFT of the vertical component. Additionally, Geopsy software was employed to analyse the polarization of the HVNR peak. The average HVNR function was computed in 10° increments from 0° to 180° and reconstructed on the horizontal plane as a function of azimuth.

The HVNR functions, obtained from the measurements carried out on the recent deposits of the Nafplio coastal plain, are characterised by a marked peak having frequency values of 2–5 Hz in the historical town centre (Fig. 8, left panel) and 1–2 Hz in the modern town area (Fig. 8, middle panel). These values can be associated with the stratigraphic resonance within the coastal plain deposits since the HVNR peaks present the characteristic “eye-shape” (Castellaro and Mulargia 2009) of the Fourier amplitude spectra, i.e., horizontal components increase, and vertical component decreases, and no prevalent direction of polarization is noticed. In addition, the increase of the frequency resonance found from the measurements performed on the coastal plain (Fig. 7) can be referred to a decrease in the deposit thickness moving from the modern town to the historical centre area in agreement with the reconstructed subsoil model (Fig. 6). On the other hand, the HVNR functions obtained from the seismic ambient noise measurements performed on the Acronafplia relief do not show any significant peak (Fig. 8, right panel), strengthening the hypothesis of the presence of the limestone seismic bedrock.

4.3. Engineering geological model validation

A first phase of numerical modelling of seismic wave propagation was performed to validate and calibrate the reconstructed engineering geological model. An iterative procedure (Moscatelli et al. 2021; Iannucci et al. 2022) based on the comparison between the amplification functions obtained by 1D numerical simulations of seismic wave propagation performed with the Strata software (Kottke and Rathje 2008), and the HVNR curves, obtained from the single-station seismic ambient noise measurements, were used as explained in the following.

A seismostratigraphy was derived from the engineering geological cross-sections of Nafplio in correspondence with the sites where the measurements were performed. Each seismostratigraphy contains a log of lithotechnical units (subsoil column) composed of a soft soil multilayer, with relative thickness supposed in the site, a seismic bedrock infinite half-space placed at a validated depth and a value, or a range of values associated with each unit based on geotechnical data already available. The numerical simulations were performed for each soil column by a linear elastic approach, varying firstly the values of unit weight volume (γ_n) and shear wave velocity (Vs) within the ranges defined for each lithotechnical unit and secondly the thickness of the lithotechnical units up to obtain a correspondence between the amplification function (i.e., the ratio of the FFT amplitude spectra at the subsoil surface and at the outcropping seismic bedrock) obtained by the numerical modelling and the HVNR peak from the seismic ambient noise measurements. Finally, the cross-sections were modified according to the thickness of the lithotechnical units obtained by the validation and calibration process.

Table 2 reports the physical and geotechnical parameters of each lithotechnical unit obtained by the validation and calibration procedure, which were used in the subsequent 2D numerical modelling to assess the expected seismic shaking. Based on available literature data, dynamic parameters of the soils, such as decay curves for damping and shear stiffness modulus, were attributed to the multilayer.

Table 2

Parameters of the lithotechnical units obtained by the validating and calibrating process and used for the subsequent 2D numerical modelling.
Litothecnical unit	𝛾_n (kN/m³)	Vs (m/s)	Decay curve
Man-made fill	18	150	elastic linear D = 2%
Clayey sands	20	170–240	Anastasiadis et al. (2001)
Marly-clayey flysch	19	300	National Seismic Service of Italy (2003)
Loose gravels	19	250	Lo Presti et al. (2002)
Sandy gravel	20	420	Lo Presti et al. (2002)
Conglomerates	23	750	elastic linear D = 2%
Bedrock (limestones)	23	1100	elastic linear D = 0.5%

4.4. 3D engineering geological modelling

The validated geological engineering model was exploited for the extrapolation of virtual stratigraphic boreholes at 358 points located along the 9 cross-sections previously validated. The spacing of the virtual boreholes, set equal to 50 m, locally descends to 10 m to increase the resolutive accuracy at complex geological structures. The geophysical-stratigraphic data collection belonging to a total of 406 points was stored in a geodatabase and processed for the generation of a 3D solid model (Song et al. 2019). The relational geodatabase constitutes a digital record collection model of georeferenced data that is stored in a multiple Excel worksheet (Ciampi et al. 2021). Stratigraphic parameters associated with the drillings (elevation and lithotechnical units) were interpolated using the inverse distance weighting technique, employing 4 neighbouring points and an exponent of 2, for generating a voxel-based solid model (Liu et al. 2021). A high-fidelity filter was adopted to honour the value of the control point variable (Ciampi et al. 2022). The domain of the data-driven block model consists of a 3D mesh with a node resolution of 10 m (x) x 10 m (y) x 0.2 m (z). The model features 153 x 123 x 106 voxels in three dimensions and spans from − 66 to 144 m a.s.l., covering an area of about 130 ha. A 4D modelling, performed based on the temporal evolution of coastal deposits inferred from 1960 and 2020 aerial photos, provides a volumetric assessment of geomorphological processes and erosional dynamics over time. The resulting digital models paint data-driven 3D geologic structures and capture lateral-vertical variations of lithotechnical units, employing a vertical exaggeration factor of 1.5 to highlight lithologic transitions (Fig. 9).

The three-dimensional geological model reproduces the subsurface geological architecture as well as the spatial relationships between the various geological units. In contrast to conventional 2D representation techniques, which omit significant portions of the spatial domain, the 3D model empowers anatomical restitution of entire subsurface volumes through solid reconstruction, paving an opportunity for overall interpretation and qualitative/quantitative analysis of the geological bodies’ spatiotemporal distribution (Royer et al. 2015; Antonielli et al. 2023). A volumetric computation in post-processing operations reveals the deposition of about 0.8 million m³ of filling material in the last 60 years, testifying to advancing littoral dynamics driven by coastal anthropisation over time (Ouillon 2018). Besides, dynamic extraction of 2D engineering geological cross-sections from the data-driven solid model encourages numerical modelling of the local seismic response (Fig. 10).

4.5. Numerical modelling of the local seismic response

To evaluate the seismic response of the studied area, a subsequent phase of numerical modelling was conducted using the LSR 2D software (STACEC s.r.l.). This tool enables two-dimensional modelling of seismic motion propagation via an equivalent linear analysis, applying finite element methods in the time domain under total stress conditions and incorporating a viscoelastic rheological behaviour based on the Kelvin–Voigt model. Physical, geophysical, and decay parameters (such as shear stiffness and damping degradation curves), designated to the geological-technical units (refer to Table 1), were assigned to the multilayer structure, and the preselected seismic inputs were applied to the infinite half-space representing the seismic bedrock. Additionally, the model was discretised into triangular meshes with an approximately 2 m resolution grid. This grid was automatically generated by the software to achieve optimal resolution, considering the varying S-wave velocities of the layers and a maximum analysis frequency of 20 Hz. Free-field conditions (i.e., damping in the X and Z directions) were applied on the lateral borders, along with a kinematic constraint on vertical motion at the base of the domain within the infinite half-space. Numerical simulations were conducted for all reconstructed engineering geological cross-sections using one of the 3 sets of seven time histories. According to ICMS (2008) guidelines, these histories were selected for the number of inputs required for numerical simulations, with 50, 475, and 2000-year return periods. The ESM database and the Rexel software were used, with a spectro-compatibility period range of 0.15-2s and tolerances of 10% lower and 30% upper.

This numerical analysis made it possible to determine the following for each control point along the cross-sections (indicated by red dots in Fig. 11) and for each analysed return period: the Fourier amplitude spectrum, the amplification factor (AF) according to ICMS (2008) standards, and the elastic response spectrum (with 5% damping).

The results from the 2D modelling, expressed as horizontal acceleration, indicate higher values in the soft soils along the valley edge, likely due to diffraction and reflection of seismic waves caused by a non-horizontal, shallow bedrock. An incremental rise in acceleration was observed at the top of elevations, such as Acronafplia Hill. This increase can be attributed to the ridge effect, where the focusing of seismic waves leads to amplification at the peak of the ridge.

4.6. Local seismic response from recorded earthquakes

For the present study, a temporary seismic net of 4 seismic stations composed of 3-component velocimeters (2.0 Hz eigenfrequency) by SARA Electronics Instruments was also installed in a continuous acquisition mode, distributed in peculiar places of the Nafplio town chosen based on the results derived from HVNR analysis (see section 4.2). This seismic net allowed the recording of regional and local earthquakes to be used to define the amplification functions by the spectral ratios to reference (SSR) (Borcherdt 1970) and horizontal-to-vertical earthquake spectral ratio (HVER). The stations (see Fig. 7 for location) were active in the period 17–25 September 2022 and recorded more than fifty events (1 < M < 5) out of about 160 recorded by the Hellenic Seismic Network (https://bbnet.gein.noa.gr/).

Table 3

Earthquake selected for the cluster 1, 1 < M < 2.5 until to 150 km with main sources W-NW7
DATE	TIME (UTC)	LAT	LONG	DEPTH (Km)	Ml
17/09/2022	12:50:51	38.166	21.6463	19.2	2.1
17/09/2022	17:51:55	36.9699	21.351	15.9	2.5
19/09/2022	8:58:33	37.3755	23.2983	79.4	2.1
20/09/2022	9:35:43	37.4725	22.2821	16	1.7
20/09/2022	11:12:59	38.3043	23.3153	7.8	2
20/09/2022	11:50:45	37.6204	23.2512	15.5	2
20/09/2022	17:58:29	37.3801	23.3395	22.1	1.7
20/09/2022	21:39:56	37.6749	22.6474	63.6	1.7
20/09/2022	23:12:45	38.1715	21.6463	20	2
21/09/2022	0:47:15	37.4162	23.3208	14.9	1.5
21/09/2022	4:37:16	37.4382	23.1404	16.1	1.6
21/09/2022	7:10:34	37.408	23.4206	19.1	1.6
21/09/2022	14:02:18	37.462	21.9868	10.4	2.5
24/09/2022	5:28:00	37.9555	22.5893	10.3	1.3
24/09/2022	6:08:19	37.9697	22.5851	10.9	1.5
24/09/2022	9:28:37	37.8223	22.7637	7.4	1.4
24/09/2022	19:59:39	37.5577	23.5703	11.2	1.3
25/09/2022	3:05:57	38.237	22.8543	12.8	1.1
25/09/2022	6:38:07	37.9628	22.5993	13.7	2.3

Table 4

Earthquake selected for the cluster 2, 2.6 < M < 5 until 500 km with main sources W and S.
DATE	TIME (UTC)	LAT	LONG	DEPTH (Km)	Ml
17/09/2022	18:41:41	35.5728	25.8261	15.7	4.2
18/09/2022	1:49:07	35.554	25.8073	18.9	2.7
18/09/2022	14:45:29	38.156	21.6591	26.7	2.8
19/09/2022	3:31:37	35.0775	26.424	10.4	4.2
19/09/2022	10:17:38	37.9399	20.2263	17.5	2.8
19/09/2022	11:40:35	37.9065	20.0867	9.4	2.7
19/09/2022	22:43:20	37.8085	20.3641	6.8	3.1
20/09/2022	1:31:27	39.4048	20.7069	15.7	2.9
20/09/2022	7:02:02	37.8745	20.2469	10.7	2.6
20/09/2022	11:53:50	35.5733	25.8257	18.2	3.4
20/09/2022	16:42:40	37.131	21.0818	5.8	3.2
21/09/2022	6:27:04	37.9166	20.159	26.4	2.8
21/09/2022	9:59:58	38.3418	22.1169	11.4	2.7
21/09/2022	17:33:31	39.0202	23.5368	11.8	2.9
23/09/2022	0:00:03	37.847	20.0583	10.9	2.8
23/09/2022	3:43:14	36.2256	21.3721	13.2	2.7
23/09/2022	7:30:11	36.2622	21.2622	23.5	2.8
23/09/2022	14:15:52	35.6708	27.5226	25.7	3.9
23/09/2022	22:20:38	34.8898	26.7325	6.5	3.4
24/09/2022	13:21:46	36.2732	21.427	16.9	2.6
24/09/2022	15:41:34	36.2494	21.3959	14.3	2.6
24/09/2022	18:16:20	36.2439	21.4197	28.8	3.2
25/09/2022	2:04:08	39.537	25.145	13.7	2.6
Among the total recorded earthquakes, 42 signals only were considered suitable to be processed and were divided into two different clusters magnitude-distance (from Nafplio) (Fig. 12): cluster 1, 1 < M < 2.5 (19) until 150 km with main sources W-NW (Table 3) and cluster 2, 2.6 < M < 5 (23) until 500 km with main sources W and S (Table 4); for each cluster the following elaborations were firstly performed:

Trim of the continuous records using the onset time of events.
Fast Fourier Transform (FFT) calculation for each component of all selected earthquakes;
application of the smoothing function by Konno and Ohmachi (1998) on the FFT spectra (considering one single window for each earthquake);
computation of the HVER function as the ratio between the quadratic mean of the two horizontal (H) FFT spectra and the vertical (V) FFT spectrum for each earthquake;
computation of the mean HVER function, with associated standard deviation, for each site by averaging the HVER calculated for all earthquakes of the cluster (Fig. 13).

By comparing HVNR and HVER for the 4 sites distributed in Nafplio town, it has been confirmed that an absence of amplification effects results in the case of outcropping bedrock, i.e., for SS1 (A in Fig. 13) situated close to the town centre; for the SS2 (B in Fig. 13) station the HVER analysis reveals weak amplification effects at frequencies lower than 2 Hz, probably related to the location of the station at the top of a ridge. Moreover, in the case of sites SS3 (C in Fig. 13) and SS4, localised downtown, HVNR and HVER are in good agreement and confirm evidence of amplification effects at resonance frequencies of about 2.5 Hz for SS3 (C in Fig. 13) and 7 Hz for SS4 (D in Fig. 13). These results highlight the different local geological conditions of the subsoil and the deepening of the seismic bedrock moving towards the northern sector of the town.

Based on the HVER analysis, the SS1 station was chosen as the reference site and to better constrain the local seismic response analysis, the selected earthquake signals were used to define the amplification functions by computing the standard spectral ratio (SSR - Borcherdt 1970) according to the following steps:

standard spectral ratios SSR were computed for each measurement site as the ratio between homologues components (E-W and N-S) of the earthquake recorded in SS2, SS3, SS4 and in the reference site SS1;
the mean SSR function was computed, with associated upper and lower standard deviation limits, for each measurement site by averaging the SSR calculated for all earthquakes of the cluster (Fig. 14).

Results from SSR analysis were compared with the outputs from 1D and 2D numerical modelling under elastic conditions to validate the amplification effects observed in Nafplio town. In general, it is worthy to observe (Fig. 14):

a good agreement in the main amplification frequencies at all measurement sites has been obtained except for SS3, cluster 2 (D), where some problems in raw data were present;
Amplification amplitudes from SSR functions are generally higher than the ones from the modelled functions;
For the SS4 measurement site, the SSR functions also show amplification frequencies different from the main mode, not evidenced by the outputs of 1D and 2D numerical modelling, probably due to minor seismic impedance contrasts related to the local subsoil layering.

Following the results obtained from all integrated analyses of the local seismic response to better define and summarise the seismic amplification in the Nafplio studied area, an Amplification Index (AI) was defined as the ratio between output and input integral Fourier amplitude spectra. AI was computed from the modelling results obtained for the 475-year return period in the three frequency ranges associated with the site resonance frequency values observed by the previous analyses (HVNR, HVER, SSR and numerical modelling), i.e., 1–3, 3–5, and 5–10 Hz. By analysing the values trend of AI along the modelled cross-sections and the corresponding engineering geological model of the subsoil, different zones with a specific seismic response were delimited (Fig. 15). This procedure was repeated for all the cross-sections to construct a map representative of homogeneous seismic response zones (Fig. 16).

A total of 6 zones with peculiar types of amplification were recognised (Fig. 16):

NAF-Z1: composed of thick deposits (> 20 m), with resonance frequencies ranging from 1 up to 3 Hz, responsible for a stratigraphic amplification. It is located in the north-eastern part of the historic centre of Nafplio (i.e., SS3 station) and in the northern part of the most recent urbanised area;
NAF-Z2: consisting of sedimentary deposits with a moderate thickness (between 10 and 20 m), site frequency ranging from 3 up to 5 Hz, and relative amplification mainly attributable to a stratigraphic effect. It extends along a strip that crosses the town of Nafplio from east to west, at the south of zone NAF-Z1;
NAF-Z3: thin deposits (< 10 m) located at the bottom of the carbonate ridges of Palamidi and Acronafplia and in saddle-area which connects the two ridges. The area is characterised by resonance peaks at high-frequency values, ranging from 5 up to 13Hz (i.e., SS4 station); the amplifying phenomenon is connected both to a stratigraphic effect (main peak at 5 Hz about) and probably to valley edge effects, as better identified consistently by SSR functions and secondarily 2D modelling (i.e. 13 Hz about);
NAF-Z4: area mostly represented by soft soil deposits, characterised by the absence of amplification and, in some cases, by seismic deamplification; this zone is extended along a narrow strip that extends along the Arvanitia route, south of the Acronafplia ridge, up to the north-west of the aforementioned relief;
NAF-Z5: includes a wide portion of the rocky ridge of Acronafplia, and it is characterised by a weak amplification, probably linked to topographical effects, with frequencies of interest ranging from 3 up to 5 Hz like showed especially from SSR functions (i.e., SS2 station);
Z6: corresponds to the northern slope of the Palamidi hill, and it is characterised by seismic deamplification or absence of amplification.

In the end, elastic (5% damping) response spectra were derived for each homogeneous seismic zone for the three considered return periods (i.e., 50, 475, and 2000 years) by averaging the elastic response spectra of seven control points included in each zone. Figure 17 shows the response spectra for the 475-year return period obtained by the LSR study for each zone compared with the target response spectra obtained for the 475-year return period by the basic seismic hazard analysis and the design response spectra provided for the Greek normative (GAR, 2000) according to the soil class defined based on the local seismostratigraphic model of the subsoil.

Except for NAF-Z6 located on the Palamidi ridge that presents response spectra lower than the targets, all the obtained response spectra show a significant amplification for all period ranges with respect to the input spectra representative of basic seismic hazard obtained at the seismic bedrock (cf. 3.2 paragraph). In particular, NAF-Z2 e NAF-Z3 show the maximum seismic amplification, especially in the period band between 0.1 and 0.4 s, while NAF-Z1 is characterised by a lower amplification for a larger period band (0.1–0.7 s); NAF-Z4, at the bottom of the Acronafplia ridge, reveals a quite negligible seismic amplification. It is worth noting that also NAF-Z5, located on the Acronafplia ridge, is characterised by clear amplification, especially in the period range 0.1–0.5 s; this evidence can be referred to topographic amplification effects, not initially detected by seismic ambient noise measurements (Figs. 7 and 8) but evidenced as by HVER, SSR functions and 2D numerical modelling.

However, by comparing LSR spectra with Greek normative spectra for the different soil classes found for each zone, it is possible to observe that the normative spectrum is always too overestimated with respect to the LSR spectrum. Therefore, a cost-benefit analysis must be done in choosing the spectra to be used as the expected seismic action at the ground surface for building design or reinforcement in the frame of seismic risk mitigation strategies.

Over the last decades, the protection and preservation of historical and cultural heritage with respect to damages related to seismic hazards have become increasingly important for national and local authorities. In the framework of the European project STABLE (H2020 RISE-Marie Curie Action), this study was carried out to evaluate seismic site effects in the Nafplio town area, an important historical centre located in the northeastern Peloponnese, Greece, where a complex geological setting and a medium-to-high regional seismic hazard converge.

To assess the local seismic response, a comprehensive approach was adopted. This includes the reconstruction of the subsoil’s engineering geological profiles based on field surveys and previous site investigations, a seismic hazard study, and geophysical investigations utilising seismic ambient noise measurements. A 3D geological model of the subsoil was developed, and 2D numerical modelling was performed along selected cross-sections to carry out a detailed seismic microzonation of the area.

Results by seismic ambient noise analysis revealed that significant seismic resonance exists in the different zones of Nafplio. In particular, the HVNR and HVER functions obtained from the seismic ambient noise measurements and earthquake measurements carried out on the recent deposits of the Nafplio coastal plain, respectively, are characterised by a marked peak having frequency values of 2–5 Hz in the historical town centre and 1–2 Hz in the modern town area. These findings were further confirmed by 1D and 2D numerical models, which highlighted amplification phenomena with high horizontal acceleration values across all considered return periods (50, 475, and 2475 years), particularly in the alluvial plain, along ridge slopes, and at the top of the Acronafplia ridge. SSR functions, calculated from earthquake recordings at four seismic stations, corroborated the amplification frequencies identified in the numerical models.

By using the distribution of amplification index values derived from the numerical modelling across three frequency ranges (i.e., 1–3, 3–5 and 5–10 Hz), different zones were mapped, enveloping areas with expected similar seismic response. In each zone, an elastic response spectrum for the three selected return periods was derived; maximum values in terms of PSA, especially in the period range 0.1–0.5 s were observed, a period range that could be significant considering the typical buildings in Nafplio city centre (microzones NAF-Z1 and NAF-Z2).

These results output that a significant seismic amplification characterises the studied area which can be related to a complex geological and geomorphological setting (i.e., including stratigraphic and topographic peculiarities). These findings evidence the relevance of combining and integrating data from accurate field surveys, quick geophysical investigations, acquisition, and analysis of experimental data (earthquakes), as well as numerical modelling to derive response spectra for new building design as well as reinforcement interventions of already existing buildings in the framework of seismic risk mitigation strategies.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Fundings

This work was supported by H2020-MSCA-RISE-2018 STABLE (STructural stABiLity risk assEssment) Project.

Acknowledgement

The activities of M. Fiorucci and R. Iannucci were mainly performed during their postdoctoral research fellowship at the Department of Earth Sciences of Sapienza University of Rome.

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Local seismic response in the historical centre of Nafplio (Greece) as a tool for seismic risk management

Status:

Version 1

Abstract

Figures

1. Introduction

2. Geological and geomorphological settings

3. Seismic hazard/earthquake scenario