Dimitrios Raptakis

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.

GEOTECHNICAL AND GEOPHYSICAL DESCRIPTION OF EURO-SEISTEST, USING FIELD, LABORATORY TESTS AND MODERATE STRONG MOTION RECORDINGS

EURO-SEISTEST is a European test site established in the Mygdonian basin, near Thessaloniki, in Northern Greece. Its aim is to allow detailed and well documented studies in many fields of engineering seismology, earthquake engineering, soil dynamics and seismology. Theoretical and experimental seismic ground response analysis along the valley is one of the most important tasks of the project. The very detailed knowledge of the geological and geotechnical structure of the site as well as numerous data from a continuous seismological and geodetical survey and dense semi-permanent and permanent instrumentation, offer the possibility of conducting detailed empirical and theoretical studies on the effects of surface geology on seismic ground motion. EURO-SEISTEST is a unique test site for checking the reliability of existing theoretical models of site effects, to develop new ones, and to interpret the physics of seismic ground response. The paper presents: (a) the general layout of the test site, (b) the main results of the extensive geophysical and geotechnical surveys, to determining the geometry and the dynamic properties of soil formations, and (c) the validation of valleys structure with instrumental estimates of the site effects along the valley. Emphasis is given to comparisons and correlations between results derived from different methods of seismic prospection (refraction, surface wave inversion, borehole seismics) and geotechnical in situ tests. In order to better constrain the structure of the valley and the dynamic properties of the soil formations, specific criteria and procedures are developed for soil characterisation.

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.

Thessaloniki’s Detailed Microzoning: Subsurface Structure as Basis for Site Response Analysis

The city of Thessaloniki is located in northern Greece, close to a large seismogenic area and it has experienced several destructive earthquakes during the present century. In this paper, we focus on the definition of the subsurface structure of the city from a site response analysis perspective. The paper presents, together with a summary of geology and seismicity, the results of a large-scale geophysical and geotechnical survey, in order to determine and validate geometry and dynamic properties of the main soil formations. The synthesis and combination of recent results regarding the dynamic properties with those obtained from the elaboration of a large database of classical geotechnical tests led to the design of a detail geotechnical map and of various 1-D profiles, 2-D cross sections and 3-D thematic maps for the main soil formations. These soil profiles and maps are oriented to site effect studies and provide a comprehensive picture easily adapted to geographic information systems (GIS) for planning and design purposes. Moreover, the results of this study were correlated with macroseismic observations reported in many earlier published microzonation studies of Thessaloniki. These comparative observations revealed the complexity of surface geology of the urban area, a fact which is expected to provoke additional amplification with respect to 1-D resonance.

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.