Mitigating seismic risk for critical facilities is crucial for governments, decision-makers, researchers, society, and the economy in earthquake-prone regions in Europe and worldwide. The paper discusses some essential concepts and methods for developing and implementing a real-time risk assessment methodology through a specific testbed example in light of an engineering-based seismic risk reduction approach for critical buildings. The goal is to demonstrate that real-time seismic risk assessment of a target building could be feasible by combining a calibrated earthquake early warning system (EEWS) with the knowledge of structure-specific fragility curves evaluated with the aid of well-designed structural monitoring arrays. The whole approach is illustrated for a school building located in Thessaloniki city center. The target school is instrumented with permanent and temporary monitoring arrays using commercial accelerometric/velocimeter stations and special in-house developed low-cost Micro-Electro-Mechanical Systems (MEMS). Structural health monitoring (SHM) allows identifying the dynamic characteristics of the building and, finally, generate structure-specific fragility functions, which may differ from generic ones. Past and current seismic events recorded on the regional seismic network and locally on sensors installed at the school building are used for the calibration and validation of the regional EEWS in order to reduce the rate of false or missed alarms. The refined structure-specific fragility functions are incorporated into the central database and used by the developed real-time risk assessment software for the promptly prediction of seismic damages and losses. The performance of the whole system is effectively checked for a strong seismic event by reproducing the Mw 6.5, 1978 Thessaloniki destructive earthquake based on 3D physics-based numerical simulations.

On October 30th 2020, a M7.0 earthquake occurred in the Aegean Sea close to Samos Island (Greece) and the western coast of Turkey that had a strong impact on the city of İzmir, about 70 km away from the epicenter. 116 people were killed, several hundreds were injured and about 4% of the structures sustained damages, including buildings that totally collapsed. Strong amplification of ground motion within Bornova basin (İzmir) where severe damages were observed, has been reported among the detrimental factors that may have contributed to such earthquake losses. This study is devoted to providing evidence that the reason of the observed intensity of ground motion in Bornova basin is mainly related to site effects and in particular to what is referred to as basin effects. The evaluation of observations for basin effects is based on a analysis with empirical techniques in frequency domain as well as with time–frequency analyses to reveal the contribution of different wave types in the response. Empirical findings are correlated with the particular geological structure of the Bornova basin, the latter being enhanced with a representative deep V S model using surface wave inversion on strong motion recordings. Furthermore, empirical findings are validated with 2D numerical analyses of a simplified basin model. Building characteristics and structural damages are also discussed in an attempt to comprehend the relation between them and basin effects. Noting that basin effects are not explicitly included in any modern seismic code, an approach is proposed to introduce the extra amplification due to basin effects in design practice.

We present a detailed site effects study at a site (TYF) close to the Thermaikos gulf coast in Thessaloniki, northern Greece. Different types of data recorded by different instruments are analyzed. Empirical amplification is estimated using spectral ratios relative to a reference station (SSR) and horizontal to vertical spectral ratios (HVSR) using earthquake data. In addition, seismic noise records from different arrays were analyzed using HVSR. Our results show that earthquake data SSR fails for our data. The reason is the poor signal to noise ratio of our records. Better results were obtained using HVSR for earthquake data. Seismic noise HVSR were not useful due to the particular soil profile at TYF, with the exception of HVSR of seismic noise recorded in one of our arrays that were able to reflect a significant change in the coast line at our site. Although amplification at site TYF is relatively small, it is large enough to originate a difference of one intensity unit relative to firm ground motion. Amplification at TYF is caused by a deep soil structure (over 350 m in depth) and therefore cannot be captured using measures like Vs30.

A temporary seismic array of short-period seismometers was installed in the 8story AHEPA hospital, located in the city of Thessaloniki, N. Greece. The scope of the survey was to assess the dynamic characteristics of the RC-building by processing ambient vibration recordings of more than 40 seismic stations installed at different positions in the building. Part of the instruments was used in a soil experiment, outside of the hospital, to study possible Soil Structure Interaction phenomena. In addition to above experiments, a site-specific survey was performed in the Volvi basin, 30km ENE of the city of Thessaloniki. The scope of this experiment was to investigate the soil properties and the geometry of the subsurface geology. Coordinates: for the AHEPA experiment: 40.62°N, 22.96°E, for the Volvi experiment: BRGM-array 40.65°N, 23.24°E, GER-array 40.61°N, 23.21°E.

Site effect studies are currently mostly based on seismic noise measurements, processed using methods like horizontal-to-vertical spectral ratios (HVSR) to estimate dominant frequencies and maximum amplifications, or methods based on cross-correlation between stations like spatial autocorrelation (SPAC) and seismic interferometry. However, HVSR and SPAC assume that the underlying medium has a 1D structure and that seismic noise consists mostly of surface Rayleigh waves. We present a detailed analysis of seismic noise records at two sites in Euroseistest (Mygdonian basin, northern Greece) where two normal faults have created lateral heterogeneity in the subsoil structure. Site GRA is located near fault F3, with a larger throw than fault F2, close to site FRM. Our results show the effects of these irregularities. Dominant frequency results for both sites become spatially incoherent close to the fault. In the case of site GRA, a range of wavelengths is missing in the recorded seismic noise field. These effects extend up to some fraction of the affected wavelengths in the case of fault F3. The effect of lateral variations on seismic noise measurements will be lost in gross spatial averaging of the results or simply the failure of HVSR or SPAC methods will be observed.

Παρουσιάζουμε μια λεπτομερή μελέτη πολλών VS προσομοιωμάτων στη θέση TST του νπειραματικού πεδίου Euroseistest. Επωφελούμαστε από 62 διαθέσιμα προσομοιώματα που προέκυψαν στην ίδια θέση από μετρήσεις και μεθόδους όπως σεισμική συμβολομετρία, ανάλυση τάσεων-παραμορφώσεων και προσομοίωσης ανόπτησης σε σεισμικές καταγραφές, συμβατικές σεισμικές διασκοπήσεις (αντιστροφή επιφανειακών κυμάτων, crosshole and downhole), και σεισμικό θόρυβο σε δίκτυα. Όλα τα προσομοιώματα ομαδοποιήθηκαν σύμφωνα με τις τεχνικές ανάλυσης σε 5 μέσους όρους, Τα αποτελέσματα έδειξαν ότι διαφέρουν μεταξύ τους καθώς εξαρτώνται από τις υποθέσεις κάθε τεχνικής. Οι διαφοροποιήσεις τους σε σχέση με τον μέσο όρο τους, αν και μικρές, είναι σημαντικές στις κατακόρυφες ασυνέχειες που ορίζουν τους σχηματισμούς διαφορετικής δυστμησίας, εισάγοντας σχετική αβεβαιότητα. Αποτελέσματα της σεισμικής απόκρισης με βάση τα προσομοιώματα έδειξαν ανάλογη αβεβαιότητα με σημαντική υποεκτίμηση της ενίσχυσης, γεγονός που πρέπει να διαχειρίζεται με προσοχή στον αντισεισμικό σχεδιασμό των κατασκευών.

Strong motion data that have been recorded during the 20-years of operation of the permanent network of EUROSEISTEST (Mygdonia basin, Northern Greece) have been homogenized and organized in an easily accessible, via the web, database. The EUROSEISTEST web portal and the application server running underneath are based solely on free and open source software (F/OSS; MySQL v5.5; RubyOnRails,SAC, Gnuplot and numerous GNU supporting utilities). Its interface allows the user to easily search strong motion data from approximately 200 events and 26 strong motion stations using event-related, record-related or station-related criteria. Further investigation of the data is possible in a graphical environment which includesplots of processed and unprocessed acceleration waveforms, velocity and displacement time histories, amplitude Fourier and response spectra and spectrograms. A great effort was directed toward the inclusion of accurate and most updated earthquake metadata, as well as a wealth of stations related information such as geotechnicaland geophysical site characterization measurements, subsoil structure and site effects. Acceleration data can be easily downloaded in either SAC or ASCII format, while all stations metadata are also available to download.

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.

We present a detailed study of investigation of various shear wave velocity, VS, profiles at the TST site of the Euroseistest test-site. We benefit from the availability of 62 VS models derived from earthquake records, conventional seismic prospecting, and seismic noise array measurements. Five groups of models provided from many different non-invasive and invasive methods (seismic interferometry, stress–strain analysis, annealing simulation, surface wave inversion, cross-hole and down-hole tests, and seismic noise array measurements) lead to averaged VS profiles. The estimate of VS models that we obtain differs depending on the technique used. In such cases, it becomes clear that, it is better to understand the differences and not simply compute an average. The percentage of the observed disparity with respect to the average reference model albeit is small, becomes significant at certain depths and is associated with the existence of strong vertical discontinuities, thus introducing an uncertainty on the interface definition between the main formations. On the other hand, the use of VS profiles in ground simulation studies (especially 2D or 3D) usually implies the need to represent them as gradient functions. Testing representative generalized power law functions, we found that they fail to predict reliably the VS model for the total thickness of sediments. To overcome this, a third order polynomial function is proposed. Finally, we test the sensitivity of average VSz index widely used in soil categorization and site amplification studies and find that all VS models, either measured or proposed, are equivalent in terms of VSz vanishing any discrimination between layering as well as models.

In this study, we assess the dynamic characteristics of an 8-story RC-building composed by two units connected through a structural joint. This building, belonging to one of the largest hospitals in northern Greece, has been selected in the framework of an European funded project as test site for developing a structural health monitoring system and it is instrumented with a permanent strong motion network. The assessment of the dynamic characteristics is performed using ambient vibration recorded by a temporary seismic network installed inside the structure. Non-parametric identification methods, namely the peak picking and frequency domain decomposition, are applied to perform operational modal analysis and extract the natural frequencies and mode shapes of the structural system. Since the detection of changes in the shear wave velocity inside the building is relevant for health monitoring analysis, we use the ambient vibration recordings to perform a deconvolution interferometry. Moreover, a shear-beam model is considered to estimate the velocity in the first three floors, where the distribution of internal sources introduces complex patterns in the impulse response functions. The velocity for lowest part of the building is estimated by optimizing the match between the arrival times of the empirical and theoretical pulses. Finally, the velocities and quality factors estimated from ambient vibration analysis are consistent with preliminary results obtained analyzing earthquake data recorded in the same building.

We present the results of the analysis of array recorded microtremors at 14 sites, close to the edges of the Mygdonian basin in northern Greece (Euroseistest). These measurements were made in order to better constrain the geometry and velocity structure of the basin as the soil layers taper out close to rock outcrop, where geology is complex and we may expect significant changes of the subsoil structure over short distances. The data were analysed using the SPAC method and HVSR. The first interprets the measurements as Rayleigh waves (for the vertical component we analysed) and allows to invert a phase velocity dispersion curve from computed correlation coefficients. The second estimates a local transfer function directly, from ratios of Fourier amplitude spectra. A phase velocity dispersion curve could be derived for 12 of the 14 measurement sites, and at three of the sites no resonant frequency was observed in the HVSR. It is encouraging that we obtained good results at most of our sites, in spite of the lateral heterogeneity expected close to the edges of Euroseistest. Our results allow us to obtain shear wave velocity models at most of the measurement sites (12 out of 14). They are also useful to explore the relation between size of the array and wavelength range for which a dispersion curve may be estimated, which in our case has strong limitations. We identify the frequency of resonance of the sediments as a small loss of coherency in SPAC’s correlation coefficients. Finally, we also consider the applicability of the joint inversion of the resonance frequency determined using HVSR and the phase velocity dispersion curve obtained from SPAC.

Research Article| September 01, 2013 The EUROSEISTEST Strong‐Motion Database and Web Portal Kyriazis Pitilakis; Kyriazis Pitilakis aResearch Unit of Soil Dynamics, and Geotechnical Earthquake Engineering, Department of Civil Engineering, Aristotle University of Thessaloniki, P.O. Box 424, 54124 Thessaloniki, Greecezroum@auth.gr Search for other works by this author on: GSW Google Scholar Zafeiria Roumelioti; Zafeiria Roumelioti aResearch Unit of Soil Dynamics, and Geotechnical Earthquake Engineering, Department of Civil Engineering, Aristotle University of Thessaloniki, P.O. Box 424, 54124 Thessaloniki, Greecezroum@auth.gr Search for other works by this author on: GSW Google Scholar Dimitris Raptakis; Dimitris Raptakis aResearch Unit of Soil Dynamics, and Geotechnical Earthquake Engineering, Department of Civil Engineering, Aristotle University of Thessaloniki, P.O. Box 424, 54124 Thessaloniki, Greecezroum@auth.gr Search for other works by this author on: GSW Google Scholar Maria Manakou; Maria Manakou aResearch Unit of Soil Dynamics, and Geotechnical Earthquake Engineering, Department of Civil Engineering, Aristotle University of Thessaloniki, P.O. Box 424, 54124 Thessaloniki, Greecezroum@auth.gr Search for other works by this author on: GSW Google Scholar Konstantinos Liakakis; Konstantinos Liakakis bDepartment of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Search for other works by this author on: GSW Google Scholar Anastasios Anastasiadis; Anastasios Anastasiadis aResearch Unit of Soil Dynamics, and Geotechnical Earthquake Engineering, Department of Civil Engineering, Aristotle University of Thessaloniki, P.O. Box 424, 54124 Thessaloniki, Greecezroum@auth.gr Search for other works by this author on: GSW Google Scholar Dimitris Pitilakis Dimitris Pitilakis aResearch Unit of Soil Dynamics, and Geotechnical Earthquake Engineering, Department of Civil Engineering, Aristotle University of Thessaloniki, P.O. Box 424, 54124 Thessaloniki, Greecezroum@auth.gr Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2013) 84 (5): 796–804. https://doi.org/10.1785/0220130030 Article history first online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Kyriazis Pitilakis, Zafeiria Roumelioti, Dimitris Raptakis, Maria Manakou, Konstantinos Liakakis, Anastasios Anastasiadis, Dimitris Pitilakis; The EUROSEISTEST Strong‐Motion Database and Web Portal. Seismological Research Letters 2013;; 84 (5): 796–804. doi: https://doi.org/10.1785/0220130030 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search EUROSEISTEST is a European, multi‐purpose physical laboratory (test site) established in 1993 in the tectonically active graben of Mygdonia, a few tens of kilometers away from the city of Thessaloniki, in northern Greece (Fig. 1a; http://euroseis.civil.auth.gr; Pitilakis et al., 2011). It is one of the longest‐running test sites in the world, providing researchers with high‐quality multidisciplinary data and facilities for conducting innovative experimental (e.g., Raptakis et al., 1998; Jongmans et al., 1998; Pitilakis et al., 1999; Raptakis et al., 2000) and theoretical studies (e.g., Chávez‐García et al.,... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A field experiment with a temporary embankment loading gave the opportunity to evaluate changes of both soil properties and site response, due to a low stiffness shallow soil model. The Quarter Wave-Length approximation is mainly used to quantitatively indicate the expected changes. The original Vs models derived by seismic prospecting methods (Raptakis. Soil Dynamics and Earthquake Engineering 2012;34: 69–77), similarly performed prior to and after preloading, were used for site amplification estimates as a function of frequency. The quantitative comparisons of either results derived by field techniques combined to each other or those of each technique separately before and after preloading, reveal changes in site response showing a clear de-amplification after preloading but of lower order than stiffness increase. This fact may have beneficial manpower and cost consequences on the acquisition of the most “detailed” and “reliable” Vs input model for site response studies. Among others, the fact that a significant disparity between amplifications is observed only after preloading indicates that changes in the soil properties refer to the ground hypotheses for which the exploration tools present by principle limitations and drawbacks.

There are many publications on the investigation of soil properties using seismic prospecting. Among these properties, special attention has been given to shear wave velocity VS, using more than two different methods for soil and site characterization. In this study, the in-hole, non-invasive refraction and surface wave inversion methods to evaluate soil improvement are investigated. The investigation was conducted on the new Egnatia highway (Northern Greece). Wave velocity profiles have been measured before and after preloading for the construction of an embankment at a soft soil site. The purpose is to quantify the dynamic properties and to evaluate the efficiency of the applied tools in detecting their variation. Among others, an emphasis was given to the observed improvement at particular layers of high sand content.

Euroseistest is currently the longest running instrumented test site in the world. It was originally defined as the 2D (N-S) cross section of the Mygdonian basin, N-E from Thessaloniki Greece, epicenter area of the M6.4 1978 earthquake. In this paper, we present the effort to extend the test site to a larger portion of the whole sedimentary structure, i.e., from 2D to a 3D structure. To this end we have compiled available geological and geotechnical information. We have analyzed microtremor and earthquake data. We present the results of the analysis of all available information and data. The synthesis of all data allowed us to propose reliable image of the geometry and the properties of the basin. We have also obtained a reliable estimate of the site response throughout the basin and we have discussed several aspects of site effects in complex geologic structures, including the increase of spectral amplification compared to 1D site amplification.