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Module: Seismicity and seismic risk DECivil (for civil engineers) Mário Lopes ([email protected]) Departamento de Engenharia Civil, Arquitectura e Geo-Recursos do Instituto Superior Técnico, Lisboa Bolonha, 3-6 March 2014 INDEX of CONTENTS 1 – The origin of earthquakes DECivil Plate tectonics The Earth inner structure Focus and epicenter Seismic waves Scales of Magnitude and Intensity Seismicity 2 – Natural effects, consequences and mitigation DECivil 2.1 - Seismic effects on the Earth surface Fault movements Soil vibrations Soil effects – landslides, liquefaction, subsidence Tsunamis Fires and Contaminations 2.2 – Global impact of earthquakes Types of earthquake effects: direct and indirect effects DECivil Earthquakes of the past Comparison with impact of other natural and technological catastrophes The future – earthquake simulators. Risk = hazard x exposition (people and economic assets exposed to seismic damage) x vulnerability 2.3 – Mitigation actions Fault movements DECivil Soil vibrations Soil effects – landslides, liquefaction, subsidence Tsunamis Fires and Contaminations Civil Protection 3 – Characterization of seismic actions Seismicity DECivil Probabilistic definition of the seismic action Definition of the seismic action for structural applications Annual probability of ocurrence. Return period Quantifications of other seismic effects DECivil Map of epicenters Earth´s plates and the epicenters of some of the 30 000 earthquakes recorded over a period of six years DECivil Press and Siever, 1994 Continental drift theory DECivil DECivil DECivil Latha, 2001 Origin of earthquakes In the lithosphere (10 km to 100 km), the crust and external part of the mantle DECivil Internal Earth structure DECivil DECivil (Press and Siever, 1994) DECivil Latha, 2001 DECivil Latha, 2001 DECivil Latha, 2001 DECivil DECivil Main faults in Italy (Buratti, 2013) DECivil Focus and epicenter DECivil (Lai, 2013) DECivil DECivil Rays of light reflecting when crossing borders between air and water. Seismic waves behave in a similar manner when crossing boundaries between different materials Press and Siever,1994 Seismic waves radiate from the focus in many directions DECivil P waves – vibration of the soil in the direction of propagation S waves – vibration of the soil perpendicular to the direction of propagation Dziewonski and Anderson, Phys. Earth Planet Intern., Vol 25, 1981 Velocity of P and S waves Surface waves: Rayleigh and Love DECivil Surface waves are confined to the vicinity of the surface, therefore their energy spreads less than body waves (P and S). Therefore they tend to produce the highest accelerations at larger distances from the epicenter. Surface waves: Rayleigh and Love DECivil (Lai, 2013) DECivil Determination of the epicenter DECivil DECivil Seismic scales Magnitude – measure of the energy released at the origin of the earthquake DECivil - It is determined from instrumental recordings - Depends only on characteristics of the earthquake (size of the rupture area and displacement between sides of the fault) Intensity – measures the way an earthquake is felt at a given location - Determined by the damage produced and human perception - It depends on characteristics of the earthquake (energy released at the origin, epicentral distance, geology and site effects) and of the built environment (vulnerability, type and amount of buildings) Scales of Magnitude Richter magnitude: DECivil it is a logaritmic scale. Each degree corresponds to a variation of 1/32 in the energy released at the origin of the earthquake. It is associated to the displacement amplitude on a Wood-Anderson seismograph at a certain distance of the epicenter. It tends to be less reliable at the higher magnitudes. Moment magnitude: it is calculated from the seismic moment 𝑀0 as 𝑀𝑤 = 2 3 𝑙𝑜𝑔10 𝑀0 -6,07. The seismic moment 𝑀0 is calculated as 𝑀0 = 𝜇𝐴 𝐷, in which is the shear stiffness of the crust, 𝐴 is the rupture area and 𝐷 is the relative displacement between faces of the fault. Scales of Intensity DECivil Modified Mercalli Scale: qualitative description of damage and human perception. Degrees I to XII Mercalli Intensity DECivil Description I Felt by very few people; barely noticeable. II Felt by a few people, especially on upper floors. III Noticeable indoors, especially on upperfloors, but may not be recognized as an earthquake. IV Felt by many indoors, few outdoors. May feel like heavy truck passing by. V Felt by almost everyone, some people awakened. Small objects moved. trees and poles may shake. VI Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster falls. Chimneys may be slightly damaged. VII Slight to moderate damage in well built, ordinary structures. Considerable damage to poorly built structures. Some walls may fall. VIII Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. Some walls collapse. IX Considerable damage to specially built structures, buildings shifted off foundations. Ground cracked noticeably. Wholesale destruction. Landslides X Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. Wholesale destruction. XI Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground. XII Total damage. Waves seen on ground. Objects thrown up into air. European Macroseismic Scale DECivil There are 12 degrees. Up to degree IX the intensity is defined as a function of: a) How the earthquake is felt by people b) Effects on soil and objects c) Damage on constructions Degrees X, XI and XII are characterized only as a function of damage on the constructions. In order to charaterized damage on buildings, these are divided in vulnerability classes, identified by the letters A to F, as a function of the material and characteristics of seismic resistance. For masonry and reinforced concrete buildings damage is defined as a function of the degree of destruction, according to a graphic definition and description. DECivil DECivil DECivil DECivil DECivil DECivil DECivil Isoseismals maps DECivil DECivil Benavente, Portugal, 1909 DECivil Faial, Azores, 1998 DECivil (Lai, 2013) DECivil DECivil DECivil Duration DECivil The duration of an earthquake is the time during which the energy is released. It influences the duration of seismic motions. The ones of large duration have larger number of cycles, what may influence the damage caused by the earthquake. The repetition of cycles may affect the accumulation of interstitial pressure in saturated loose sand deposits (liquefaction) and the degradation of the stiffness of structural elements. This effect is variable, being larger on the elements under large shear forces than in elements whose nonlinear behaviour is predominantly due to flexural deformations. Response spectrum DECivil