Variance reduction and signal-to-noise ratio: Reducing uncertainty in spectral ratios

This paper uses an unusually large dataset to study scatter in site-effect estimation, focusing on how the events that increase uncertainty can be removed from the dataset. Four hundred seventy-three weak motion earthquake records from the surface and bedrock of a 178-m-deep borehole in Aegion, Gulf of Corinth, Greece, are used to evaluate spectral ratios. A simple statistical tool, variance reduction (VR), is first used to identify two groups of events that lie closest and farthest from the average, which is considered here as the initial best estimate of the site response. The scatter in the original dataset is found to be due to the group of events with smallest VR. These events can be removed from the dataset in order to compute a more reliable site response. However, VR is not normally used to choose records for site-effect studies, and it cannot be applied to the usual small datasets available. The signal-to-noise ratio (SNR) is normally used to this end, for which reason we investigate whether SNR can be used to achieve similar results as VR. Signal-to-noise ratio is estimated using different definitions. Data selection based on SNR is then compared to that using VR in order to define an SNR-based criterion that discriminates against events that, according to VR, increase scatter. We find that defining the SNR of a surface record as the mean value over a frequency range around the resonant peak (here, 0.5–1.5 Hz) and using a cutoff value of 5 may be used in this case to exclude most events for which VR is small. This process is also applied to the downhole station, where we obtain similar results for a cutoff value of 3

Seismic wave amplification: Basin geometry vs soil layering.

International audienceThe main purpose of the paper is to analyze seismic site effects in alluvial basins and to discuss the influence of the knowledge of the local geology on site amplification simulations. Wave amplification is due to a combined effect of impedance ratio between soil layers and surface wave propagation due to the limited extent of the basin. In this paper, we investigate the influence of the complexity of the soil layering (simplified or detailed layering) on site effects in both time and frequency domain. The analysis is performed by the Boundary Element Method. The European test site of Volvi (Greece) is considered and 2D amplification in the basin is investigated for various soil models. Seismic signals are computed in time domain for synthetic Ricker signals as well as actual measurements. They are analyzed in terms of amplification level as well as time duration lengthening (basin effects) for both SH and SV waves. These results show that the geometry of the basin has a very strong influence on seismic wave amplification in terms of both amplification level and time duration lengthening. The combined influence of geometry/layering of alluvial basins seems to be very important for the analysis of 2D (3D) site effects but a simplified analysis could sometimes be sufficient. In the case of Volvi European test site, this influence leads to (measured and computed) 2D amplification ratios far above 1D estimations from horizontal layering descriptions

Seismic risk in the city of Al Hoceima (north of Morocco) using the vulnerability index method, applied in Risk-UE project

The final publication is available at Springer via http://dx.doi.org/10.1007/s11069-016-2566-8Al Hoceima is one of the most seismic active regions in north of Morocco. It is demonstrated by the large seismic episodes reported in seismic catalogs and research studies. However, seismic risk is relatively high due to vulnerable buildings that are either old or don’t respect seismic standards. Our aim is to present a study about seismic risk and seismic scenarios for the city of Al Hoceima. The seismic vulnerability of the existing residential buildings was evaluated using the vulnerability index method (Risk-UE). It was chosen to be adapted and applied to the Moroccan constructions for its practicality and simple methodology. A visual inspection of 1102 buildings was carried out to assess the vulnerability factors. As for seismic hazard, it was evaluated in terms of macroseismic intensity for two scenarios (a deterministic and probabilistic scenario). The maps of seismic risk are represented by direct damage on buildings, damage to population and economic cost. According to the results, the main vulnerability index of the city is equal to 0.49 and the seismic risk is estimated as Slight (main damage grade equal to 0.9 for the deterministic scenario and 0.7 for the probabilistic scenario). However, Moderate to heavy damage is expected in areas located in the newer extensions, in both the east and west of the city. Important economic losses and damage to the population are expected in these areas as well. The maps elaborated can be a potential guide to the decision making in the field of seismic risk prevention and mitigation strategies in Al Hoceima.Peer ReviewedPostprint (author's final draft

The web portal of the EUROSEISTEST strong ground motion database.

Οι καταγραφές της εδαφικής επιτάχυνσης του πολυδύναμου πεδίου δοκιμών EUROSEISTEST (Μυγδονία λεκάνη, Βόρεια Ελλάδα), που έχουν συλλεχθεί κατά τη διάρκεια των 20 χρόνων λειτουργίας του, ομογενοποιήθηκαν και οργανώθηκαν σε μια εύκολα προσβάσιμη, μέσω του διαδικτύου, βάση δεδομένων. Η δικτυακή πύλη του EUROSEISTEST και όλες οι λειτουργίες του διακομιστή της έχουν σχεδιαστεί με αποκλειστική χρήση ανοιχτού λογισμικού (MySQL v5.5; RubyOnRails, SAC, Gnuplot, διάφορα άλλα GNU εργαλεία). Το γραφικό περιβάλλον της δικτυακής πύλης παρέχει τα κατάλληλα εργαλεία για την εύκολη αναζήτηση δεδομένων από τους 26 σταθμούς του μόνιμου δικτύου του EUROSEISTEST και τους 200 περίπου σεισμούς που έχουν καταγραφεί μέχρι σήμερα. Παρέχεται επίσης δυνατότητα επισκόπησης των δεδομένων μέσω εικόνων με χρονοϊστορίες της επιτάχυνσης, ταχύτητας και μετάθεσης, φάσματα πλάτους Fourier, απόκρισης, καθώς και φασματογραφημμάτων. Ιδιαίτερη βαρύτητα έχει δοθεί στην υψηλή ποιότητα πληροφορίας των μετα-δεδομένων (σεισμών, σταθμών καταγραφής και εδαφικής δομής) που έχουν συμπεριληφθεί στη βάση. Όλα τα δεδομένα επιτάχυνσης του EUROSEISTEST και τα μεταδεδομένα που αφορούν τον γεωτεχνικό-γεωφυσικό χαρακτηρισμό και την απόκριση του εδάφους στις θέσεις των σταθμών καταγραφής είναι διαθέσιμα μέσω της σχετικής δικτυακής πύλης.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.

Understanding single-station ground motion variability and uncertainty (sigma) – Lessons learnt from EUROSEISTEST

Accelerometric data from the well-studied valley EUROSEISTEST are used to investigate ground motion uncertainty and variability. We define a simple local ground motion prediction equation (GMPE) and investigate changes in standard deviation (σ) and its components, the between-event variability (τ) and within-event variability (φ). Improving seismological metadata significantly reduces τ (30-50%), which in turn reduces the total σ. Improving site information reduces the systematic site-to-site variability, φS2S (20-30%), in turn reducing φ, and ultimately, σ. Our values of standard deviations are lower than global values from literature, and closer to path-specific than site-specific values. However, our data have insufficient azimuthal coverage for single-path analysis. Certain stations have higher ground-motion variability, possibly due to topography, basin edge or downgoing wave effects. Sensitivity checks show that 3 recordings per event is a sufficient data selection criterion, however, one of the dataset’s advantages is the large number of recordings per station (9-90) that yields good site term estimates. We examine uncertainty components binning our data with magnitude from 0.01 to 2 s; at smaller magnitudes, τ decreases and φSS increases, possibly due to κ and source-site trade-offs Finally, we investigate the alternative approach of computing φSS using existing GMPEs instead of creating an ad hoc local GMPE. This is important where data are insufficient to create one, or when site-specific PSHA is performed. We show that global GMPEs may still capture φSS, provided that: 1. the magnitude scaling errors are accommodated by the event terms; 2. there are no distance scaling errors (use of a regionally applicable model). Site terms (φS2S) computed by different global GMPEs (using different site-proxies) vary significantly, especially for hard-rock sites. This indicates that GMPEs may be poorly constrained where they are sometimes most needed, i.e. for hard rock

Systemic seismic vulnerability and risk assessment of urban infrastructure and utility systems

The seismic vulnerability and risk assessment of infrastructure and utility systems are essential to prevent or mitigate sufficiently the negative consequences, implement resilience management strategies, and recover efficiently after a major earthquake. In a complex urban environment, having multiple interacting and interdependent infrastructures becomes even more important. Earthquake hazards not only affect a single asset, but also their impact is much greater because of the inter- and intra-dependences among various infrastructure, utility systems, and lifelines. Therefore, we urgently need efficient tools to quantify and assess the systemic vulnerability and risk of urban infrastructure and utility systems. This is a challenging topic that is nowadays receiving more attention from the research community, the industry domain, and the policymakers. This paper aims to review the available modelling approaches and tools for the seismic risk analysis of interconnected systems, including advantages and limitations. It focuses in particular on the European funded SYNER-G project that encompasses interdependencies, delivers a holistic methodology, and implements a comprehensive framework based on the Object-Oriented Modelling paradigm. The capacities of the SYNER-G framework are illustrated through a selected application regarding the seismic risk analysis of interconnected infrastructure and utility systems in the city of Thessaloniki, Greece. Among other aspects, the paper discusses hazard modelling issues of the two common approaches, the probabilistic and the scenario-based procedure and illustrates in a specific example the impact of mitigation strategies, based on their effect on the performance of the interconnected systems and the overall loss reduction. The integration of interdependencies into the risk analysis and resilience strategies facilitates a better understanding of critical infrastructure operation and enables well-informed proactive and reactive decision-making and efficient disaster risk management, by infrastructure owners and operators, insurance companies, consulting agencies, and local authorities.The present work has been done in the framework of grant agreement No. 813137 funded by the European Commission ITN-Marie Sklodowska-Curie URBASIS-EU project. Also, we would like to acknowledge all the contributors to the SYNER-G project that was funded from the European Community’s 7th Framework Program under grant No. 244061

Numerical simulation of soil-structure interaction experiments on shallow founded structures for different mass configurations

Soil-Structure Interaction (SSI) phenomena and foundation rocking can modify the structural response signifi- cantly with respect to the response predicted adopting the fixed-base assumption. The importance of SSI and rocking depends, among other factors, on the structural mass and the distribution of static stresses at the soil-foundation interface. Within this context, an experimental campaign was carried out aiming to investigate the SSI effects on the response of a 3m x 3m x 5m steel- framed structure. The prototype structure, called EUROPROTEAS, was founded on a shallow footing at the well-characterised Euroseistest site, while its mass was either 18Mgr or 9Mgr. The present study simulates free vibration experiments, placing particular emphasis on soil nonlinearity and soil-foundation interface. A novel approach to simulate gaps at the soil-foundation interface, foundation rocking and to manipulate interface stresses under static conditions is presented. The three aspects are shown to significantly affect the response, while they are found to be more important for the lighter structure

Seismic Response of Hagia Sophia Church in Thessaloniki Including Soil-Foundation-Structure Interaction

This study investigates the behavior of “Hagia Sophia” church in Thessaloniki under seismic loading. It is one of the greatest Byzantine churches in the city and it is inscribed on the World Heritage List. The main scope of this work is to estimate the seismic response of the historic structure accounting for the actual foundation and soil flexibility at its base, to find the locations in need for retrofit and finally, to propose possible intervention methods. We simulate numerically the soil - foundation -structure system, and for the properties of the building materials we estimate their strengths with the use of two codes; the EC6 and the Greek Regulation for the structural intervention of masonry (KADET). We simulate soil-foundation flexibility using impedance functions under the foundation according to NIST (2012) provisions. The influence of soil–foundationstructure interaction is investigated. As a reference case, we also consider a fixed-base model to compare the output of the two analyses and highlight the influence of the soil and masonry foundation flexibility on the dynamic response of the church. Finally, we further analyze the intervention method of micropiles as a possible method of enhancement for the foundation of the monumen