PART I: EARTHQUAKE HAZARD AND STRONG MOTION 67 C. S. Oliveira, A. Roca and X. Goula (eds.). Assessing and Managing Earthquake Risk, 67 - 89.  2005 Springer. Printed in the Netherlands. CHAPTER 4 LOCAL SITE EFFECTS AND MICROZONATION A. Roca 1 , C. S. Oliveira 2 , A. Ansal 3 and S. Figueras 1 1. Institut Cartogràfic de Catalunya, Barcelona, Spain 2. Instituto Superior Técnico, Lisbon, Portugal 3. Boğaziçi University, Istanbul, Turkey 4.1. Introduction Seismic waves generated at the earthquake source propagate through different geological formations until they reach the surface at a specific site. The travel paths of these seismic waves in the uppermost geological layers strongly affect their characteristics, producing different effects on the earthquake motion at the ground surface. In general, thicker layers of soft, unconsolidated deposits tend to amplify selectively different wave frequencies. These complex physical phenomena are known as soil effects. On the other hand, the local topography can also modify the characteristics of the incoming waves, leading to the so called topographic effects. Soil and topographic effects are considered under the general denomination of local site effects. Beyond these effects and under certain circumstances, induced effects may occur for large amplitude incoming waves, among which are slope instabilities (landslides) and liquefaction. Within a more generalized scope, active faulting should also be considered as, in case of fault ruptures. In addition permanent differential displacements and near fault effects are other important issues to be recognized. In many past and recent earthquakes it has been observed that the local site conditions - soil and topographic effects, as well as induced effects - have a great influence on the damage distribution. It is therefore very important to take into account and predict these possible local site effects when assessing the earthquake hazard at regional and local scale. Seismic microzonation is the generic name for subdividing a region into individual areas having different potentials for hazardous earthquake effects, defining their specific seismic behaviour for engineering design and land-use planning. Seismic microzoning, including approaches for assessing local ground response, slope instability and liquefaction, has become a useful tool for cost effective earthquake risk mitigation. There is a demand from international, national, regional and municipal administrations for microzoning urban areas generating maps to be taken into account in urban planning, in seismic codes and in civil protection preparedness procedures. Various approaches are currently applied for microzonation studies. Experimental techniques, together with theoretical approaches involving ground motion modelling under different hypotheses, are used to classify urban areas in various zones of different earthquake response characteristics. Several review and synthesis papers on the different methods used for geological and geotechnical site characterisation and microzoning have been published (e.g. Bard, 1994; Kudo, 1995; Pitilakis and Anastasiadis, 1998; Bard, 1999; Mulas, 2002; Kawase,