Konstantia Makra

Site effects at Euroseistest—I. Determination of the valley structure and confrontation of observations with 1D analysis

This paper describes the process of construction of the 2D model of Volvis geological structure and results of empirical and theoretical approaches to the evaluation of site response at Euroseistest. The construction of the 2D model is based on a re-interpretation of the available geophysical and geotechnical data in an effort to improve the definition of the subsoil structure at Euroseistest in terms of the most important parameters needed to model site response. The results of this re-interpretation are compared with a previous published 2D model of the same alluvial valley. Different analysis of the measurements and different criteria in the synthesis of data have led to a different model, even if both studies had access to the same field measurements. This underscores the fact that a model results of an interpretation and is not uniquely determined by the data, no matter how detailed they are. The well known subsoil structure opened the possibility to correlate the geometry and the dynamic properties of the 2D model with the results of site response determined from a detailed analysis of two events in frequency and time domains and 1D numerical modeling. The study of site response shows the important effect of the lateral variations on the ground motion and suggests that the contribution of locally generated surface waves to the resonant peak may be important. In the case of Volvis graben, the limitations of the 1D approximation to simulate ground motion under complex soil conditions in both frequency and time domains are also shown. This paper lays the ground for a companion article dealing with 2D site effects in this basin.

Site effects at euroseistest : II. Results from 2D numerical modeling and comparison with observations

This paper presents results of numerical modeling of site response for Euroseistest. Ground motion across a very detailed model of the subsoil of this valley has been simulated for vertically incident SH waves. The predominance of locally generated surface waves is very clear in the synthetic seismograms. These results are then compared with published studies of observed site effects at this basin and with a detailed analysis of two events in the time domain. It is discussed in which sense it is possible to obtain a good fit between observations and 1D models, even though the real behavior involves locally generated Love waves. For this reason, it can be misleading to rely on an incomplete observation such as empirical transfer functions. Finally, it is stressed that in order to predict ground motion in alluvial valleys the information contained in the phase cannot be neglected.

Site Effects and Design Provisions: The Case of Euroseistest

Modern seismic codes usually include provisions for site effects by considering different coefficients chosen on the basis of soil properties at the surface and an estimate of the depth of bedrock. However, complex local geology may generate site amplification on soft soils significantly larger than what would be expected if we assume that the subsoil consists of plane soil layers overlaying a homogeneous half-space. This paper takes advantage of the large number of previous studies of site effects done at Euroseistest (northern Greece). Those studies have supplied a very detailed knowledge of the geometry and properties of the materials filling this shallow valley. In this paper we discuss the differences between site effects evaluated at the surface using simple 1-D computations and those evaluated using a very detailed 2-D model of the subsoil structure. The 2-D model produces an additional amplification in response spectra that cannot be accounted for without reference to the lateral heterogeneity of the valley structure. Our numerical results are extensively compared with observations, which show that the additional amplification computed from the 2-D model is real and affects by a significant factor response spectra, and thus suggests that some kind of aggravation factor due to the complexity of local geology is worthy of consideration in microzonation studies and seismic codes.

Euroseistest 3D Array for the Study of Complex Site Effects

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 effect s in complex geologic structures, including the increase of spectral amplification compared to 1D site amplification.

A LiDAR-aided urban-scale assessment of soil-structure interaction effects: the case of Kalochori residential area (N. Greece)

We present a methodology for a large-scale assessment of soil-structure interaction (SSI) effects on the vibrational characteristics and the seismic loading of structures in a real urban fabric by combining airborne monitoring techniques, field surveys and simple calculations in the realm of structural and geotechnical dynamics. The proposed procedure combines geometrical features of a building stock acquired from a LiDAR-based 3D city model, material and typology data of structures from in situ inspections in representative building blocks, geotechnical data from field surveys and strong ground motion data from seismic hazard and site effects analyses. The integrated data are employed to compute at a first stage the fundamental period of each building in the urban area by means of code-defined formulas for both fixed- and flexible-base foundation conditions, thus treating soil-structure interaction effects in a simplified manner. The seismic action in terms of spectral acceleration at the fundamental period of structures is then computed, following a series of 1D site-specific analyses of soil response under different seismic scenarios. Spatially distributed ratios of structural fundamental periods and spectral accelerations, corresponding to flexible- over fixed-base foundation conditions, are mapped in GIS environment as an index of SSI effects at large-scale. The methodology is implemented in the urban area of Kalochori located west of Thessaloniki in Greece. The examined case study showed that SSI may be significant even for low-amplitude motions and may lead to higher seismic forces compared to the fixed-base case, depending on the dynamic characteristics of the structures, the soil conditions and the shape of the response spectrum. The above may be of importance in microzonation and seismic vulnerability studies at urban-scale when a building-by-building assessment is not feasible and SSI effects are too important to be ignored.

Assessment of seismic loading on structures based on airborne LiDAR data from the Kalochori urban area (N. Greece)

Airborne LiDAR monitoring integrated with field data is employed to assess the fundamental period and the seismic loading of structures composing an urban area under prescribed earthquake scenarios. Α piecewise work-flow is adopted by combining geometrical data of the building stock derived from a LiDAR-based 3D city model, structural data from in-situ inspections on representative city blocks and results of soil response analyses. The procedure is implemented in the residential area of Kalochori, (west of Thessaloniki in Northern Greece). Special attention is paid to the in-situ inspection of the building stock in order to discriminate recordings between actual buildings and man-made constructions that do not conform to seismic design codes and to acquire additional building stock data on structural materials, typologies and number of stories which is not feasible by the LiDAR process. The processed LiDAR and field data are employed to compute the fundamental period of each building by means of code-defined formulas. Knowledge of soil conditions in the Kalochoti area allows for soil response analyses to obtain free-field at ground surface under earthquake scenarios with varying return period. Upon combining the computed vibrational characteristics of the structures with the free-field response spectra, the seismic loading imposed on the structures of the urban area under investigation is derived for each one of the prescribed seismic motions. Results are presented in GIS environment in the form of spatially distributed spectral accelerations with direct implications in seismic vulnerability studies of an urban area.

Field monitoring of strong ground motion in urban areas: the Kalochori Accelerometric Network (KAN), database and Web-GIS portal

The Kalochori Accelerometric Network (KAN) has been operating since 2014 in the urban area of Kalochori, 7 km west of Thessaloniki in Northern Greece. KAN is composed of seven accelerometric stations: three ground stations installed in distinct urban zones (i.e., residential, industrial, and tanks zone); three stations on top of a selected structure within each urban zone; and one free-field station away from the built environment. The stations are documented with installation and operating features, available characteristics of the housing structures, and geotechnical data. A set of 78 earthquakes that have been recorded by KAN between 16 January 2014 and 31 December 2016 are reported, allowing investigation of local site effects on seismic motion, variation of ground surface motion within different urban environments, and evaluation of dynamic response features of the instrumented structures. KAN stations monographs, processed acceleration recordings, and metadata of the recorded earthquakes are available online through a Web-GIS platform. Indicative example applications are discussed for possible data use in the field of soil and structural dynamics. The DOI linked to the complete set of KAN data is 10.6084/m9.figshare.5044804.