Maria Manakou

Seismicity and seismic hazard parameters evaluation in the island of Crete and the surrounding area inferred from mixed data files

Abstract A study of the spatial distribution of seismicity and seismic hazard is undertaken for Crete and the surrounding area (CISA). For the estimation of seismicity parameters, CISA is divided in rectangular subregions. By applying the centroid of strain energy technique, we obtain seismicity parameter maps closely related to the tectonics of the area. The b-value from the magnitude–frequency distribution is calculated by a maximum likelihood technique ( Page, 1968 . Bull. Seism. Soc. Am. 58, 1131–1168), as well as by Gumbels first asymptotic distribution of extreme values technique. The b-value shows consistent spatial variation along CISA for both techniques. Low b-values are dominant in the western part of CISA, while higher b-values are present to the east side, forming a pattern like a ‘sleeping letter S’. This pattern may be attributed to the variation of the regional velocity of deformation. Finally, we performed seismic hazard analysis for shallow and intermediate events. From mixed (historical and instrumental) data files, we estimated the regional maximum magnitude Mmax, as well as the other parameters like the activity rate of seismic events λ and the well-known b parameter ( Kijko and Sellevoll, 1989 . Bull. Seismol. Soc. Am. 79, 645–654).

Earthquake Ground Motion in the Mygdonian Basin, Greece: The E2VP Verification and Validation of 3D Numerical Simulation up to 4 Hz

n a low‐seismicity context, the use of numerical simulations becomes essential due to the lack of representative earthquakes for empirical approaches. The goals of the EUROSEISTEST Verification and Validation Project (E2VP) are to provide (1) a quantitative analysis of accuracy of the current, most advanced numerical methods applied to realistic 3D models of sedimentary basins (verification) and (2) a quantitative comparison of the recorded ground motions with their numerical predictions (validation). The target is the EUROSEISTEST site located within the Mygdonian basin, Greece. The site is instrumented with surface and borehole accelerometers, and a 3D model of the medium is available. The simulations are performed up to 4 Hz, beyond the 0.7 Hz fundamental frequency, thus covering a frequency range at which ground motion undergoes significant amplification. The discrete representation of material heterogeneities, the attenuation model, the approximation of the free surface, and nonreflecting boundaries are identified as the main sources of differences among the numerical predictions. The predictions well reproduce some, but not all, features of the actual site effect. The differences between real and predicted ground motions have multiple origins: the accuracy of source parameters (location, hypocentral depth, and focal mechanism), the uncertainties in the description of the geological medium (damping, internal sediment layering structure, and shape of the sediment‐basement interface). Overall, the agreement reached among synthetics up to 4 Hz despite the complexity of the basin model, with code‐to‐code differences much smaller than predictions‐to‐observations differences, makes it possible to include the numerical simulations in site‐specific analysis in the 3D linear case and low‐to‐intermediate frequency range.

Les Ostracodes et le Quaternaire d’Aigion (golfe de Corinthe, Grèce)

Resume Le Pleistocene superieur et l’Holocene d’Aigion ont livre une microfaune abondante dont 35 especes d’Ostracodes. Le Pleistocene du forage d’Aigion contient generalement des Ostracodes de milieu oligohalin a Cyprideis torosa, Candona angulata et Tyrrhenocythere amnicola tandis que dans l’Holocene dominent les especes marines infralittorales a Cytheridea neapolitana, Carinocythereis whitei, Loxoconcha ovulata et Cytherois frequens. La sedimentation marine s’effectuait a des profondeurs de quelques metres a quelques dizaines de metres. Selon la coupe d’Aghios Constantinos, les niveaux, a caractere lagunaire, oligohalins, sont caracterises par la presence d’Euxinocythere schuldtae et d’une espece naine de Xestoleberis. Un changement radical d’environnement se produit donc a la limite Pleistocene–Holocene et pourrait concerner l’ensemble du golfe de Corinthe.

Seismic response of an 8-story RC-building from ambient vibration analysis

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.

Ambient vibration measurements on monuments in the Medieval City of Rhodes, Greece

Vulnerability assessment of masonry historical monuments relies on the accurate assessment of their real structural and foundation conditions, which is always a very difficult and challenging task to determine. Ambient noise measurements are among the non-destructive field techniques, which can provide useful information regarding the structural integrity, the dynamic response characteristics of the structure and the dynamic properties of the foundation soil. The performance-based design approach developed in the research project PERPETUATE for the vulnerability assessment of monuments uses all this information in a cost efficient way. Moreover, validation of the whole methodology has been made on several monuments in the Medieval City of Rhodes. To illustrate the use of ambient noise measurements in the frame of PERPETUATE methodology, we performed a number of ambient vibrations measurements on three selected monuments in the Medieval City of Rhodes (Greece) and the surrounding soil. The aim was to identify the dynamic characteristics of the selected structures and the dynamic properties of foundation-soil where these structures are founded. The Frequency Domain Decomposition technique is adopted for ambient modal identification using ARTeMIS software and the continuous wavelet transformation for calculating the wavelet energy to compare it with the pathology pattern of a typical masonry wall. The dynamic properties of the foundation soil, in terms of shear-wave velocity, were also explored using the Spatial Autocorrelation Coefficient method. The goal of the ambient noise measurement performed herein is to better simulate the complex masonry structures in view of estimating their vulnerability using the PERPETUATE methodology and support rehabilitation decisions. The improvement of the initial numerical model using the results of the ambient vibrations measurements illustrates the usefulness of the approach and provides some helpful practical guidelines.

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.

Structural monitoring and earthquake early warning systems for the AHEPA hospital in Thessaloniki

This study aims at presenting the analyses of monitoring data that have been used in the context of structural monitoring and Earthquake Early Warning (EEW) for a hospital building in Thessaloniki. Permanent and temporary instrumentation arrays, implemented under the responsibility of Aristotle University of Thessaloniki (SDGEE-AUTH) in close cooperation with German Centre for Geosciences (GFZ) are presented. The ambient noise data recorded at the temporarily installed networks are used for the dynamic characterization of the building based on both vibrational and waveform approaches. Moreover, long-term ambient noise recordings from the permanent array installed within the hospital are used for the investigation of the daily and seasonal wandering of the building resonance frequencies related to environmental effects. The modal identification results are used in a comprehensive framework for the computation of the up-to-date fragility curves representing the actual structural state considering aging effects of the construction materials, possible pre-existing damages and changes in the geometry and mass distribution. The building-specific fragility functions are integrated into two independent EEW systems and rapid damage assessment approaches, namely the PRESTo software and an onsite EEW algorithm on the instruments of the permanent array, to provide the expected level of damage after strong ground shaking at the monitored building. The implemented monitoring networks and the developed operational tools can be used in the context of seismic risk mitigation and preparedness for structural safety assessment under earthquake loading.

Site effects using methods based on lateral homogeneity and laterally heterogeneous media: An impossible marriage?

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.