Preface to the Pageoph Topical Volume “Seismic ground motion in large urban areas”

This volume summarizes the main results achieved in the framework of the UNESCO-IUGS-IGCP project 414 ‘‘Realistic Modelling of Seismic Input for Megacities and Large Urban Areas’’. It addresses the problems of pre-disaster orientation like hazard prediction, risk assessment, and hazard mapping, in connection with seismic activity. The unequal distribution of damage and casualties from earthquakes, which often affects urban areas, gives rise to problems that are still a challenge that has fostered numerous studies on the estimation of the seismic ground motion before the occurrence of a damaging earthquake. This task requires the detailed knowledge of both the subsurface structure within the city and of the probable location and characteristics of seismic sources around it. On the other hand one must use theoretical methods and related computer codes that allow us the realistic and reliable simulation of the expected seismic ground motion. These numerical simulations play an important role, especially in areas of complex geology, and they can provide realistic synthetic waveforms at places where no recordings are available. Synthetics are compared with observations wherever instrumental data are available. During recent years, several methods have been proposed for the theoretical estimation of the seismic response at a specific site. This volume contains results obtained using tools which enable us to estimate realistic amplification effects in complex structures, exploiting the available knowledge relative to geotechnical, lithological and geophysical parameters, topography of the medium, tectonic, historical data, and seismotectonic models. The ground motion modeling technique applied in most of the papers proves that it is possible to investigate the local effects even at large epicentral distances, too often neglected event, taking into account both the seismic source and the propagation path effects. This collection gathers original studies which offer quantitative information required for the design, construction and retrofitting of the built environment. In particular, these studies have a long-lasting impact on the reduction of the environmental hazard associated with the seismically active regions, and contribute to the definition of the source and response spectra to be used in the mosaic of the studied world seismic regions. The accurate knowledge of subsoil structures and probable complex source mechanisms is used to study the local site effects in large urban areas and Megacities like: Algiers, Alexandria, Beijing, Bucharest, Cairo, Delhi, Napoli, Santiago de Cuba, Sofia, Thessaloniki and Zagreb. Alongside, a comparison with traditional methods for seismic microzoning is made for selected regions and cities.