PART II: Comparison of Theoretical and Experimental Estimations of Site Effects

— To check the reliability and the quality of the theoretically estimated ground responses obtained from the 2-D simulation by the application of the hybrid method in PART-I, we compare some of them with those obtained at the same sites from observed data using the Standard Spectral Ratio (SSR). The comparison validates our synthetic modeling and shows that in cases of complex geometries, the use of at least 2-D numerical simulations is required in order to reliably evaluate site effects and thus facilitate the microzonation of the city of Thessaloniki.     

Particle distributions and X-ray spectra in single or multiple solar current sheets

The acceleration of charged particles in a site of magnetic reconnection is analysed by detailed numerical simulations. Single or multiple encounters of the particles with Harris-type reconnecting current sheets (RCSs) are modelled as an overall stochastic process taking place within an active region. RCS physical parameters are selected in a parameter space relevant to solar flares. Initially, the charged particles form a thermal (Maxwellian) distribution corresponding to coronal temperature  ≃2 × 10⁶ K  . Our main goal is to investigate how the acceleration process changes the shape of the particles' kinetic energy distribution. The evolution of the kinetic energy distribution, calculated numerically after one encounter of the particles with a single RCS, is found to be in good agreement with our previously published analytical formulae. In the case of consecutive encounters, we find that the kinetic distribution tends to converge to a practically invariant form after a relatively small number of encounters. We construct a discrete stochastic process that reproduces the numerical distributions and we provide a theoretical interpretation of the asymptotic convergence of the energy distribution. We finally compute the theoretical X-ray spectra that would be emitted by the simulated particles in a thick target model of radiation.     

Regular and chaotic dynamics in 3D reconnecting current sheets

We consider the possibility of particles being injected at the interior of a reconnecting current sheet (RCS), and study their orbits by dynamical systems methods. As an example we consider orbits in a 3D Harris type RCS. We find that, despite the presence of a strong electric field, a 'mirror' trapping effect persists, to a certain extent, for orbits with appropriate initial conditions within the sheet. The mirror effect is stronger for electrons than for protons. In summary, three types of orbits are distinguished: (i) chaotic orbits leading to escape by stochastic acceleration, (ii) regular orbits leading to escape along the field lines of the reconnecting magnetic component, and (iii) mirror-type regular orbits that are trapped in the sheet, making mirror oscillations. Dynamically, the latter orbits lie on a set of invariant KAM tori that occupy a considerable amount of the phase space of the motion of the particles. We also observe the phenomenon of 'stickiness', namely chaotic orbits that remain trapped in the sheet for a considerable time. A trapping domain, related to the boundary of mirror motions in velocity space, is calculated analytically. Analytical formulae are derived for the kinetic energy gain in regular or chaotic escaping orbits. The analytical results are compared with numerical simulations.     

Complex Site Effects in Thessaloniki (Greece): I. Soil Structure and Comparison of Observations with 1D Analysis

This paper presents a 2D model of the geological structure of Thessaloniki city and results of empirical and theoretical approaches for the evaluation of site response due to complex site effects. The construction of the 2D model is based on the available geophysical and geotechnical data in terms of the most important parameters needed to model site response. The well-known subsoil structure, despite the existence of some local uncertainties, gave the possibility to correlate the geometry and the dynamic properties of the 2D model with the results of site response determined from the analysis of one event in frequency and time domains and 1D numerical modelling. The study of site response shows the effect of the lateral variations on ground motion and suggests that the contribution of locally generated surface waves to the resonant peak may be important. In this case history, 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.     

Complex Site Effects in Thessaloniki (Greece): II. 2D SH Modelling and Engineering Insights

This paper presents results of numerical modelling of site response for Thessaloniki, obtained with two different 2D methods; a finite difference and a finite element method. Ground motion across a 2D model of the subsoil of the city has been simulated for vertically incident SH waves. The predominance of locally generated surface waves is very clear in the synthetic seismograms of a weak event and of stronger ones. These results are then compared with the observations in time domain and frequency domain. The role of the soil formations with high attenuation in the lateral propagation and the effect of the differential motion close to the lateral variations are also pinpointed. The stronger events were finally used to compute strong ground motion in order to reveal and to discuss practical engineering aspects such as peak ground acceleration value, the most familiar indicator in seismic norms, the soil to rock spectral coefficients for the period bandwidth of interest, and the aggravation factor in terms of 2D to 1D response spectra as a useful ruler to account for complex site effects.     

New code site classification, amplification factors and normalized response spectra based on a worldwide ground-motion database

A global dataset of more than 3,000 ground motion records from 536 sites from Greece, Italy, Turkey, USA and Japan is compiled and used to propose code-oriented elastic acceleration response spectra and soil amplification factors for a new site classification system, which, besides the classical geotechnical parameters $N_{SPT}, S_{u}$ and PI, uses also the fundamental period of the site, the thickness of soil deposits and the average shear wave velocity to the seismic bedrock, instead of $V_{s,30}$ . We propose a new classification system with the associated amplification factors and normalized response spectra for two seismicity levels, i.e. $M_{s}\le 5.5$ and $M_{s}>5.5$ . Uncertainties in the estimation of soil amplification factors are captured using a logic-tree approach, which allows the efficient use of alternative models and methods. The aim of this work is to improve the present EC8 soil classification. The effectiveness of the proposed classification system is compared to that of EC8 classification system using an error term, which represents the average dispersion of data within all categories of a given classification scheme. Error terms for the new classification system are lower than the error terms for EC8 classification system at all periods.     

Cross-scale: multi-scale coupling in space plasmas

Most of the visible universe is in the highly ionised plasma state, and most of that plasma is collision-free. Three physical phenomena are responsible for nearly all of the processes that accelerate particles, transport material and energy, and mediate flows in systems as diverse as radio galaxy jets and supernovae explosions through to solar flares and planetary magnetospheres. These processes in turn result from the coupling amongst phenomena at macroscopic fluid scales, smaller ion scales, and down to electron scales. Cross-Scale, in concert with its sister mission SCOPE (to be provided by the Japan Aerospace Exploration Agency—JAXA), is dedicated to quantifying that nonlinear, time-varying coupling via the simultaneous in-situ observations of space plasmas performed by a fleet of 12 spacecraft in near-Earth orbit. Cross-Scale has been selected for the Assessment Phase of Cosmic Vision by the European Space Agency.      

Small-Strain Shear Modulus and Damping Ratio of Sand-Rubber and Gravel-Rubber Mixtures

This study examines the small-strain dynamic properties of mixtures composed of sandy and gravelly soils with granulated tire rubber in terms of shear modulus (G_O), and damping ratio in shear (D_min). Torsional resonant column tests are performed on dry, dense specimens of soil-rubber mixtures in a range of soil to rubber particles size 5:1–1:10 and rubber content from 0 to 35% by mixture weight. The experimental results indicate that the response of the mixtures is significantly affected by the content of rubber and the relative size of rubber to soil particles. Concering the small-strain shear modulus, an equivalent void ratio is introduced that considers the volume of rubber particles as part of the total volume of voids. Based on a comprehensive set of test results a series of equations were developed that can be used to evaluate the shear modulus and damping ratio at small shear strain levels if the confining pressure, the content of rubber by mixture weight, the grain size of soil and rubber particles, and the dynamic and physical properties of the intact soil are known.     

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.