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The Samos Island (Aegean Sea) Earthquake occurred on 30 October 2020. It produced a tsunami that impacted coastal communities, ground shaking that was locally amplified in some areas and that led to collapse of structures with 118 fatalities in both Greece and Turkey, and wide-ranging geotechnical effects including rockfalls, landsliding, and liquefaction. As a result of the global COVID-19 pandemic, the reconnaissance of this event did not involve the deployment of international teams, as would be typical for an event of this size. Instead, following initial deployments of separate Greek and Turkish teams, the reconnaissance and documentation efforts were managed in a coordinated manner with the assistance of international partners. This coordination ultimately produced a multi-agency joint report published on the 2-month anniversary of the earthquake, and this special issue. This paper provides an overview of the reconnaissance activities undertaken to document the effects of this important event and summarizes key lessons spanning topic areas from seismology to emergency response. [ABSTRACT FROM AUTHOR]

The Greek National Annex for current Eurocode 8 has adopted the seismic hazard zonation map published in 2003 as part of the modifications to the Greek Seismic Code EAK 2000 (EAK 2003). This map, which followed the catastrophic earthquakes that hit the country between 1978 and 2001, includes three seismic hazard zones with peak ground acceleration (PGA) ranging between 0.16 and 0.36 g. In this paper, following the significant progress that has been made worldwide in the last two decades towards the improvement of the definition of seismic actions and the seismic hazard maps using fully probabilistic models, we make a complete proposal for the Greek National Annex of the ongoing revision of Eurocode 8, which includes a new seismic hazard zonation map for Greece, as well as a novel site categorization scheme and related site amplification factors. To this end, we use the results of the European Seismic Hazard Model, ESHM20, as reported by Danciu et al. (The 2020 update of the European Seismic Hazard Model: Model Overview, 2021) which will be adopted as informative reference for the seismic hazard at European level in the forthcoming revision of Eurocode 8 (CEN/EC8). The herein proposed ground shaking zonation for rock conditions includes five zones with PGA values ranging between 0.13 and 0.37 g. For each zone, two newly proposed ground motion parameters, i.e., Sα,475 and Sβ,475, are provided, which are the two parameters used for anchoring the elastic response spectrum as defined in CEN/EC8, along with all the other necessary parameters for the definition of the elastic response spectrum, including site amplification. The proposal for the new seismic zonation is supported by a preliminary investigation of the impact of its adoption on the seismic design of new structures and on the seismic risk of the current building stock in Greece, to help gain a better insight on how important the differences imposed by the new zonation might be for the end-users and the administration. [ABSTRACT FROM AUTHOR]

The response of a two-storey RC school building in the town of Argostoli, Cephalonia Island, Greece, during the seismic sequence of January and February 2014, is examined. The structure was built following an older generation of seismic codes dating from the 1950s, which provide limited strength and ductility against lateral loads. Despite the severity of ground shaking and the two successive events, the building suffered relatively minor damage, like most of the RC buildings in the town. Following a short presentation of some basic seismological, structural and geotechnical aspects of the seismic sequence, the paper focuses on the seismic performance of the structure at hand. To this end, a series of detailed non-linear static and time-history dynamic analyses are reported, which highlight the interplay of soil, foundation and superstructure in modifying the seismic demand. It is demonstrated that SSI had an unexpectedly important (detrimental) role in the behaviour of the structure, increasing its natural period by about 25% and aggravating ductility demand in almost all columns, despite the moderately soft soil conditions (average VS = 180 m/s). The results shed light on the seismic performance of the building and help drawing conclusions on the engineering effects of the 2014 Cephalonia earthquake sequence. [ABSTRACT FROM AUTHOR]

Infrastructures are critical for the functioning of society. Due to globalization, damages between different components of infrastructure systems can cross international boundaries, resulting in broad economic and social impacts. Hence, it is fundamental to develop powerful tools for the assessment of infrastructure risk, considering a wide spectrum of uncertainties. Past studies covering infrastructure risk assessment are limited to a few countries, partly because for assessing the infrastructure risk, issues due to the complexity of the systems, like paucity and heterogeneity of the data and methods, the consideration of dependencies between components and systems, modeling of ground shaking in terms of scenario and probabilistic approach, taking into account site effects, spatial variability and cross correlation of ground motion at the urban scale, as well as consideration of risk metrics tailored to infrastructure, still remain partially or not properly answered. Moreover, the presently available infrastructure risk assessment tools are not sufficiently illustrative, user-friendly, and comprehensive to meet actual needs. To this end, the paper making an overview of these issues proposes a comprehensive approach that leverages the main strength of existing infrastructure risk methodologies, integrating them into a powerful open-source tool and providing common platform from hazard to risk analysis that will serve for global and easy usage. The methodology and its implementation are illustrated through a test-bed study of the water supply network of the city of Thessaloniki in Greece, considering an Mw 6.5 scenario of the 1978 Thessaloniki earthquake and an event based probabilistic approach and, simultaneously evaluating the sensitivity of cross spatial correlation. [ABSTRACT FROM AUTHOR]

Two strong earthquakes hit Thessaly region on March 3rd, 2021 (Mw = 6.3) and on March 4th, 2021 (Mw = 6.1). The epicentres of the earthquakes were located at approximately 23 and 29 km respectively NW of Larissa, one of the most populous cities in Greece. Several aftershocks followed thereafter. Although no injuries were recorded, several structures suffered significant damage close to the epicentre, while some others collapsed. Approximately 300 residents of the village of Damasi were transferred to temporary settlements and tents. The event occurred during the COVID19 lockdown and created significant stress and disruption to residents. This paper focuses on the earthquake swarm itself as well as the damages observed in residential buildings, schools, and churches in the earthquake-stricken region. The earthquakes mainly impacted low-rise domestic masonry buildings, while the more modern reinforced concrete structures built following the recent seismic regulations were almost unaffected. The typology of buildings in the region, together with photographs demonstrating the extent of damage are presented herein. Despite the rather satisfactory performance of modern buildings in recent earthquakes in Greece, the preliminary investigations from the Thessaly Earthquakes showed that there is still a significant level of vulnerability in existing masonry building stock constructed using traditional methods and materials. This issue could re-emerge in future earthquakes striking other rural areas of Greece, something that needs to be addressed systematically in the future. [ABSTRACT FROM AUTHOR]

The present work constitutes the first part of a multi-parametric research for the representation of damage scenarios in Lefkada old town. The study area lies in the most seismically active zone of Greece and has suffered several devastating earthquakes. Most of its old buildings were built by local practices incorporating elements of earthquake resistance and have been designated as particular earthquake resistant constructions by the European Council Cultural Heritage Unit. The most recent earthquake of $$\hbox {M}_\mathrm{w}=6.2$$ that occurred on 14 August 2003, located about 10 km from the town, caused moderate damage to its buildings and substantial geotechnical failures. Investigations on the coseismic effects yielded extensive literature information, which initially motivated our investigations. This paper deals with the empirical vulnerability assessment of the buildings in Lefkada old town based on basic structural characteristics such as the type and material of the structural system, the period of construction, the number of storeys, etc. Those characteristics, primarily available from the most recent available census were further exploited by an in-situ survey, during which all 1,420 buildings in the old town were inspected. Following, each building was indexed with an EMS-98 vulnerability class. Expert decisions on the damage scale due to the 14 August 2003 event were reassessed and converted into damage grades of EMS-98. The full data set was also analysed in ArcGIS producing detailed maps of the 14 August 2003 earthquake damage distribution by structural typology and vulnerability class in Lefkada old town. [ABSTRACT FROM AUTHOR]

On October 30th, 2020, a magnitude 7.0 earthquake offshore off the northern coast of Samos, Greece, generated a tsunami that impacted the nearshore Greek islands and the Aegean coastline of Turkey. Here, we describe detailed results from several post-event field surveys, and report first wave arrival timing and polarity information as well as tsunami height/runup measurements, from five islands. In Chios, wave runup reached 1.38 m, in Samos ~ 3 m, in Fourni 1.57 m, in Thimena 1.46 m, and in Ikaria 1.18 m. This event marks two milestones. One, the General Secretariat for Civil Protection of Greece, disseminated a message through Greece's 1–1-2 Emergency Communications Service to all cell phones in the eastern Aegean geographical region, warning recipients to stay away from coastal areas. According to eyewitnesses, the message was received ~ 3–5 min prior to the second and largest flood in Vathi, as the first flood had not sufficiently alarmed the local authorities to evacuate residents. Two, we were able to infer complete tsunami hydrographs from measurements for the first two floods in Vathi, which suggests that the water level rose to about one meter overland flow depth in one minute. [ABSTRACT FROM AUTHOR]

The accuracy of a seismic risk model to predict future losses is a significant challenge as it depends on many parameters, each with important uncertainties, that are present in all components of the model. The aim of this study is to present a process that can be followed to validate the efficiency of a predictive risk model for seismic risk assessment at urban or regional scale, through comparison of estimated physical damages with those recorded at past seismic events, discussing at the same time the uncertainties involved in the different steps of this process. The proposed process is applied for the verification of the open-access uniform European Seismic Risk Model (ESRM20), considering two major past seismic events in Greece, i.e. the Athens 1999 Mw 5.9 and the Thessaloniki 1978 Mw 6.5 earthquakes, for which we possess relatively good damage reports for a representative portion of the two metropolitan agglomerations. Despite the numerous uncertainties and the unavoidable simplifications, a good comparison is found between the estimated and observed damage, especially for the city of Thessaloniki. Regarding the distribution of damage, it is anticipated that the most heavily damaged typologies for both case studies are found to be the low-to-medium-rise, non-ductile masonry buildings. The analyses also revealed the importance of the credibility of the adopted exposure model and of the adequate selection of ground motion models for seismic hazard assessment. Τhis challenging work and the positive results acquired so far aim to contribute to the efforts towards increasing the resilience of cities. [ABSTRACT FROM AUTHOR]

The stochastic estimation of coherency is perceived, in some studies, to be significantly influenced by the time window length under consideration. For the engineering purposes, usually, coherency is estimated from the strong motion, i.e., the shear (S-) wave segment of the recorded time series. However, the identification of a purely S-wave dominated window could be challenging in case of complex wave mixing on the seismograms. Moreover, there has been relatively little research on how much variation, in reality, is introduced in the coherency estimates owing to the choice of signal length. Therefore, the debate about the procedure for selecting a representative time window has remained inconclusive: different authors keep their different practices. The current article is an effort to shed light on the sensitivity of the coherency estimates to the choice of various time windows. The research draws on the dataset gathered from a dense seismic-array deployed at the small size, shallow alluvial valley of Koutavos-Argostoli, situated in Cephalonia Island, Greece. The lagged coherency over interstation distance ranges from 5 to 80 m has been estimated from a set of 46 earthquakes having magnitude 2-5 and epicentral distance 3-200 km, considering different lengths of time windows. The findings revealed that the statistics of coherency estimates derived from many events is only weakly sensitive to the selection of time window lengths provided that the segments include the same energetic pulse. [ABSTRACT FROM AUTHOR]

Patras is the third largest city in Greece and an ideal candidate for earthquake early warning (EEW) applications due to its high seismic hazard, its existing research infrastructure and the presence of critical structures such as the Rion-Antirion bridge. Patras is located a few hundred kilometres from the Hellenic Arc, where very strong and potentially damaging earthquakes occur. This distance is large enough to allow a few tens of seconds of warning time prior to significant shaking, provided earthquakes are timely detected by a dense seismic network. Within the framework of the EC-funded project REAKT, the Virtual Seismologist (VS) EEW software was installed at Patras Seismological Laboratory. Its initial performance evaluation is presented here. In general VS provides magnitudes similar to the official, manually revised ones. Given the current station density and network telemetry, the average time that VS needs to deliver the first magnitude estimate is rather large, of the order of tens of seconds and not yet satisfactory for routine operational use of EEW. Even so, the system is able to provide up to 10 s of warning time prior to S-wave arrivals for events occurring on the Hellenic Arc. Our results indicate that the seismic networks in Greece need enhancements for regional EEW, either by adding stations or by upgrading the hardware to reduce delays. The application of an EEW system in the area is promising and, once operational, capable of mitigating earthquake risk. [ABSTRACT FROM AUTHOR]

We study site effects using 520 weak motion earthquake records from a vertical array in Aegion, Greece. The array is inside a basin, has four stations in soil, and one in bedrock (178 m depth). The site is marked by high seismicity and complex surface geology. We first use the records to establish the downhole accelerometer orientations and their evolution with time. Then we estimate site effects using empirical spectral ratios with and without a reference site (standard and horizontal-to-vertical spectral ratio). We find significant site amplification which cannot be accounted for by 1D model predictions, along with a significant difference in the amplification level between the two horizontal components. These are indications of 2D effects, namely surface waves generated at the basin edge. The difference in amplification between the horizontal components is maximised when these are rotated with respect to the orientation of the basin edge. The strongest amplification takes place in the direction parallel to the basin edge (SH, or out-of-plane motion), and is up to 2 times higher than in the perpendicular direction (SV, or in-plane motion). This directional effect on the amplification is corroborated by numerical 2D modelling using incident SH and SV waves, with the former possibly generating strong Love waves. In the records, the directionality is clear for windows containing the largest amplitudes of the records (S waves and strong surface waves), while it tends to vanish for coda-wave windows. This directionality is also observed when using response spectral ratios rather than Fourier ratios. We compute soil-to-rock amplification factors for peak ground acceleration (PGA) and find it is significantly higher than what is predicted by current design codes. We attribute this difference to the basin edge amplification, linear soil behaviour, and to the inability of simple scalar values like PGA to describe complex amplification effects. Finally, we analyse the earthquake records at a surface station near the slope crest and do not observe significant topographic amplification. [ABSTRACT FROM AUTHOR]

Crete is actively seismic and site response studies are needed for estimating local site conditions subjected to seismic activity. In order to collect basic data, we performed ambient noise recordings to estimate the site response of the surface and near subsurface structure of the small-scale Kastelli Basin in northwest Crete. The spatial horizontal to vertical spectral ratios (HVSR) resonance pattern of the investigated sites in the centre of the Basin consists of either one or two peaks divided into low to high frequency range in different sites as follows: (a) in some sites only one amplified peak at low frequencies (0.6-1.2 Hz), (b) in other sites only one amplified peak at medium frequencies (2.9-8.5 Hz) and (c) in yet other sites two amplified peaks in the low to high frequency range (0.6-15.5 Hz). The investigated sites are amplified in the frequency range 0.6-15.5 Hz, while the amplitude reaches to a factor of 4 in the spectral ratios. The one HVSR amplified peak at low frequencies is related to locally soft or thick Quaternary deposits. Microtremors were measured in the coastal northwest part of the Basin in a well-lithified Cretaceous limestone site characterized by fractures and faults striking predominantly in a sector NNE to NNW. Sites of one amplified peak at medium frequencies are extended from coastal northwest to southwest delineating a structure striking to NNW. The two amplified peaks are attributed to shallow subsurface heterogeneities/irregularities, locally induced by fault zones and to the overlying Quaternary deposits. Spatial HVSR variations in the frequency and HVSR shape delineate four structures striking NNE, NNW and in a sector NW to WNW, crosscutting the dense populated Basin suggesting that microtremors could be a valuable tool for providing a first approximation of fault zone delineation at least for the Kastelli-Kissamos Basin. The Basin is classified into the X soil category of the Greek Seismic Code 2000. [ABSTRACT FROM AUTHOR]

This article examines the contribution of Ottoman sources of historical information for the study of the seismicity of Greece, in particular of Athens during the period of the Ottoman domination, a period during which relatively few original Greek sources of data are available. An annotated list of a number of case histories of earthquakes in Greece from Ottoman sources is presented translating only a summary of the information for those who want to access the whole document. They contain not only a valuable record of earthquakes, but also, for the earthquake historian, a wealth of information about locations where earthquakes were not felt. The article discusses the effects of earthquakes, of the 1826-7 siege on the Acropolis its purpose being to provide information with such annotation as would facilitate its use by engineering seismologists with additional macroseismic data that could supplement the rather meagre information. [ABSTRACT FROM AUTHOR]

A large number of earthquake studies using both empirical and theoretical approaches clearly depict the strong correlation of resulting damage and local geology. This correlation forms the scientific basis of the subsurface soil structure studies for site response evaluation, since adequate knowledge of the subsoil geotechnical and geophysical properties can lead to realistic seismic hazard estimation through appropriate modeling of strong seismic motion. In this framework, ambient noise analysis and one dimensional numerical simulation modeling have been performed for the town of Grevena (Northwestern Greece) in order to study both the subsurface soil structure, as well as its expected effect on seismic motions. The horizontal to vertical spectral ratio (HVSR) method was implemented on an almost uniform grid of 60 single station ambient noise measurements inside the urban area, while the noise array technique was applied at four selected sites of the study area. The HVSR curves show complex patterns, occasionally with double HVSR peak frequencies, with the higher one HVSR frequency showing a good correlation with the recent Holocene clay-dominant formation. Using these results, as well as information on surface geology and existing geotechnical data, a microzonation of the Grevena town is attempted, indicating zones of similar site response to ground motion. Numerical simulation of ambient noise with synthetic recordings suggests that the complex features observed in several HVSR curves could be attributed to the impedance contrast due to the quite shallow (Holocene clays to Pliocene-Pleistocene sands) and a deeper (Pliocene-Pleistocene sands to Oligocene-Miocene bedrock) sedimentary formation. [ABSTRACT FROM AUTHOR]

On the south flank of the Acropolis of Athens there are two 10-m high columns belonging to the monument of Thrasyllos, free-standing since the third century BC. Obviously they have survived earthquakes for more than 2,300 years responding as natural seismoscopes with their damage 'recording' all strong earthquakes during their long life. Their existence naturally raises the questions of whether their mere survival can contribute to our scientific understanding of the long-term seismicity of Old Athens by addressing the various interrelated aspects of the problem combining earth science, seismology, archaeology, history, and engineering. Starting from the literary survey and the seismotectonic investigation of the region we find that although earthquakes were in the City, rarely causing concern, there is no evidence that Old Athens was ever seriously damaged. However, the historical literary record is incomplete having gaps at long periods of time; the columns, therefore, may be used to enhance our knowledge on the long-term seismicity, not only by estimating the worst-case ground shaking that could topple them-which has not happened yet-but in particular to assess the degree of shaking which was responsible for the damage that can be seen today on them. The decoding of this information is not a straightforward procedure and, in general, does not lead to a unique solution. The main reasons for this is on one hand the non-linearity and the sensitivity of the seismic response of multidrum columns, and on the other the lack of information on the evolution of the damage and the uncertainty on the real cause of it: it is not easy to determine how much of the damage to the columns that can be seen today must be attributed to earthquakes alone and how much of it is due to other reasons. In this research, from the detailed investigation of the current state of the columns, the damage caused by earthquakes was separated from the one caused by rusting of dowels and temperature effects. Back analyses were performed using suitable surrogate ground motions, aiming at determining the worst-case earthquake that could cause the collapse of the columns, and the maximum level of shaking that could cause the observed damage. The numerical analyses were performed using the Distinct Element Method and models that were exact reproductions of the columns based on the available data, while the joint properties were calibrated against ambient and forced vibration measurements. The results are consistent with the seismotectonic analysis and show that the mean response spectrum of the worst-case earthquakes is generally compatible with the response spectrum of EC 8 for the corresponding return period. It is possible, though, that the maximum experienced shaking is less. More information is needed on the real effect of the dowels of the drums to draw conclusions on the latter based on the observed damage. [ABSTRACT FROM AUTHOR]