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Module: Seismicity and seismic risk DECivil Mário Lopes ([email protected]) Departamento de Engenharia Civil, Arquitectura e Geo-Recursos do Instituto Superior Técnico, Lisboa Bolonha, 3-6 March 2014 3 – Characterization of seismic actions Prediction of earthquakes DECivil Seismicity 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 Prediction of earthquakes It is not possible to predict well in advance the date of occurrence of the next strong earthquake at a certain location. DECivil It is extremely unlikely that before an earthquake happens there is any sign that it is about to happen, allowing to evacuate towns (short term prediction). There is only one sucess case in all history of mankind, in Haicheng, China, in 1975. Even if the short term prediction was reliable, it would not prevent the destruction of towns and of the economy. It is possible to estimate the probability that earthquakes of given characteristics occur at a given location during a certain period of time (long term prediction). Seismicity studies for long term prediction DECivil It is used for the definition of the design seismic action It is based on information from three components: - Hystorical seismicity - Instrumental seismicity - Geological evidence Hystorical seismicity Information from past earthquakes, when there were no records, that allows the characterization of those earthquakes, as for instances: DECivil Date Zones more affected by the earthquake Level of damage Occurrence of tsunami Analysis of characteristics of earthquakes based on hystorical information - Epicentral location using isosseismals maps. DECivil Account for site effects and irregular exposition, that may distort the isoseismals map - Epicentral distance based on the tsunami DECivil The tsunami can be used to determine the epicentral distance using information on the difference of time between the earthquake and the arrival of tsunami, knowing the speeds of the seismic waves and the tsunami. - Magnitude based on damage Knowing the characteristics of the constructions at the location and time of the earthquake, damage on the constructions can be used to calculate the ground accelerations that would induce that damage. Knowing the attenuation laws of seismic waves it is possible to evaluate the characteristics of those waves at the epicenter and therefore the magnitude. - Maximum credible earthquake at the fault DECivil Knowing the epicenter, by means of geological and tectonic studies of the zone and and it is possible to estimate the maximum credible earthquake that can take place at the fault. Shortcomings of hystorical information DECivil - Reliability and precision – part of the information may not be accurate, given the circunstances in which it is obtained: possible panic and lack of knowledge of the witnesses, and errors in registering the information - It is available usually for a period of time much smaller than the period of the large tectonic movements and the return period between large earthquakes Instrumental seismicity DECivil It is the information of real earthquakes based on records of seismographs or accelerometers. It dates from the early 20th century, in a more systematic way only from the middle of the XX century onwards. Advantages - It is much more accurate than the hystorical information. DECivil It allows better determination of epicenters and magnitudes. - It allows the identification of occult faults, that do not reach the Earth´s surface. - It allows to register earthquakes of small magnitude, not felt by humans. Disadvantage It concerns an extremely short period, lower than the period associated with the hystorical seismicity, DECivil compared with the return period of large earthquakes Seismographs , Accelerographs DECivil Lai, 2013 Seismographic network (2008) DECivil Accelerometric network (2008) Magnitudes and epicenters of earthquakes recorded in Portugal DECivil Geological evidence - Location of faults DECivil - Dimension of faults and characteristic earthquakes of those faults - Activity in those faults - Past earthquakes in those faults For instances paleossismological studies on adequate trenches open on active faults, may allow the identification periods of ocurrence and magnitudes of earthquakes that took place thousands of years ago. DECivil Paleosseismological field works at the Vilariça Fault (north of Portugal). It allowed the detection of two earthquakes of magnitude above 7 during the last 18 000 years, much more than the largest hystorical earthquake known, with magnitude 5,8 and dated from 1858 Paleosseimological studies on the effects of tsunamis, like movements of large stones and sand deposits, DECivil may also be used to characterize tsunamis and the earthquakes that gave rise to them thousands of years ago. Earthquake catalogues Based on all sources of information, the main earthquakes and all the available information on their DECivil main characteristics are assembled in earthquake catalogues. These catalogues usually refer to the last centuries, sometimes more in some zones. The degree of knowledge and reliability of the information varies depending on how long ago the earthquake took place and the sources of information. For the events to be time independent, remove foreshocks and aftershocks Probabilistic definition of the seismic action A given zone on a territory may be subjected to DECivil earthquakes of different seismic sources. Therefore it is usual to divide the territory in seismogenic zones of similar seismicity, either spread in that zone or concentrated in certain faults previously identified. The seismicity of each zone is characterized based on the 3 previously referred sources of information (earthquake catalogue and known faults). Example of seismogenic zones for seismicity studies in Portugal DECivil Main seismic sources (earthquakes M>5), that affect the Italian territory DECivil INGV, 2010 Example of seismogenic zones for seismicity studies in Italy DECivil The seismic action potentially felt at a given location results from 3 main processes. DECivil 1. the occurrence of earthquakes 2. the propagation of seismic waves from the source to the location being studied 3. site effects Occurrence of earthquakes The occurrence of earthquakes is characterized by their DECivil distribution along time, the location of the ruptures of the faults and the magnitude. Given an earthquake catalogue, the frequency of occurrence of earthquakes of a given magnitude is usually assumed to follow the Gutemberg-Richter law (1944), as follows: 𝑙𝑜𝑔 𝑀 = 𝑎 − 𝑏. 𝑀 M is the annual probability of occurrence of earthquakes of magnitude M DECivil a, b – constants that depend on the seismicity of the seismogenic region being studied and are derived from the earthquake catalogue Basic assumption: the process is memory less, this is, the ocurrence of earthquakes is independent of time. It does not accounts for how much time passed since the last event of similar characteristics If we consider the earthquakes of all the XX century in the whole Earth we obtain the following relationship, with b= 0,898 and a=8. Note that in average there is 1 to 2 earthquakes M>8 DECivil Incertainties The above is a process with very large uncertainties due to: the short period range of earthquake catalogues, specially for DECivil large magnitude earthquakes whose return period may be much larger than the period spanned by the catalogue; in fact the rate of occurrence of large earthquakes results from an extrapolation of the rate of occurrence of smaller earthquakes The incompleteness of earthquake catalogues, as some hystorical events may not have been registered The fact that some active faults may not have been identified, specially faults on the oceanic crust (fault trace under the sea) or faults that don´t reach the Earth surface Propagation of seismic waves Seismic waves propagate from the focus of the DECivil earthquake in a way similar to waves on a lake if we throw a stone on the water: the amplitude of the waves atenuates as the distance to the source increases. There are two main sources of attenuation: Geometric attenuation Damping at the earth crust Geometric attenuation – as the distance DECivil to the epicenter increases, the energy of the waves is distributed by a greater surface, therefore the amplitude decreases. Damping – as the waves travel by the Earth crust, the friction between solid particles that displace relatively DECivil to each other, reduces the total energy of the seismic waves, decreasing their amplitude. This attenuation effect is stronger on the high frequencies. Therefore at larger distances from the epicenter the frequency contents of the seismic waves changes (becoming richer in low frequencies) as compared with the frequency contents near the epicenter (richer in high frequencies). Attenuation relationships DECivil Relationship between a ground motion descriptor, such as PGA (Peak Ground Acceleration), Spectral acceleration at a given period, Intensity, Arias Intensity, and the distance to the fault or epicenter DECivil DECivil DECivil DECivil Teves Costa et al, 2002 DECivil Sabetta F, Pugliese A. Estimation of response spectra and simulation of nonstationary earthquake ground motions. Bull Seismol Soc Am. 1996;86(2):337-52. DECivil INCERTAINTIES DECivil Attenuation relationships have very large uncertainties of two types: - Due to the inherent scatter of ground motion data - Due to the simplified and empirical nature of the models Site effects It refers to the effects of the most superficial soil layers on the amplitudes and frequency contents of the seismic DECivil waves that propagate from the bed rock to the surface. The soil behaves like a multidegree of freedom oscillator, that increases or decreases the amplitudes of some frequencies Transform a function in a sum of synusoidal functions. Fourier series DECivil Dynamic amplification. Ressonance DECivil DECivil Changes in seismic waves due to site effects, recorded in Ashigara Valley (Japan) during an M=5.1 earthquake DECivil Reiter, 1991 Map of seismic hazard in Italy DECivil Maximum horizontal acceleration in soil type A, with a probability of exceedence of 10% in 50 years DECivil http://www.seismo.ethz.ch/GSHAP/ Definition of the seismic action for structural applications DECivil Probability of exceedance in 50 years for current buildings: 10% (EC 8) Most common formats: response spectra or set of accelerograms Se(T)= ag . S.2,5. Sd(T)= ag . S.2,5/q DECivil 𝑎𝑔 = 𝑎𝑔𝑅 × γ𝐼 𝑎𝑔𝑅 - peak horizontal acceleration on the bed rock (soil type A) γ𝐼 - Importance factor 𝑆 - soil factor - Factor that accounts for the fact that the damping coefficient is different of 5% q - behaviour factor (q factor in EC 8 terminology) Soil types in EC 8 DECivil The soil factor S depends on the stiffness of the soil. The strength of the soil and topographic effects may also be accounted for Vs is preferable to NSPT Response spectra for Lisbon, for far and near field events, and two types of soil, A and D DECivil The choice of the probability of collapse of a structure, that can not be zero, is a political choice, not a technical one. It depends on how much risk society is prepared to accept and how much is it willing to pay to reduce it. It depends on DECivil cultural, hystorical and economic factors. It is generally accepted that some structures are more important than average (hospitals, scools, government buildings, lifeline facilities) and others are less important (for instances agricultural facilities whose collapse is unlikely to cause human casualties). Therefore for these structures different probabilities of collapse are required/accepted. Alternatively, different lifetimes for these structures can be considered. The above is reflected in EC 8, in the Importance factors I, that multiply the reference seismic action, applicable to common housing and office buildings DECivil Portuguese National Annex of EC 8, Part 1, 2009 Seismicity descriptor, studies for often express an instances the peak earthquake horizontal acceleration on stiff soil, associated to a given type of earthquake, as a function of the DECivil return period, (average period of time between earthquakes). TR, DECivil It is therefore usefull to relate the return period with the probability of exceedance of earthquakes with certain characteristics, for instances PGA>a, during given periods of time (usually the lifetime, n years) TR=1/P1 P1=1/TR P1 – anual probability of occurrence of earthquakes of certain characteristics (for instances PGA > a) DECivil 1-P1=(1-1/TR) – anual probability of not occurring earthquakes with certain characteristics (1-1/TR)n – probability of not occuring earthquakes with certain characteristics in n years Pn=1- (1-1/TR)n – probability of occuring earthquakes with certain characteristics in n years Probabilidade anual de excedência (%) 10.00% 1.00% 0.10% DECivil 0.01% 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 PGA (g) 0.8 Therefore, structures of higher Class of Importance can be assigned lower probabilities of collapse (Pn) or higher lifetimes (n years), to which will correspond higher values of PGA on stiff soil, agR. The ratio of this value by the value of agR of the standard Class of Importance (II) is the respective Importance factor. Quantification of other seismic effects DECivil Displacements between fault faces at or near the Earth surface (i) the identification of faults traces can be done by identification of epicenters of previous earthquakes and/or direct observation (ii) the potencial displacement between fault faces can be done using geological evidence and information on the effects of previous earthquakes Landslides – stability of slopes can be assessed by analysing models with information on the topography, soil characteristics and water contents. At a large scale it is DECivil also possible to estimate the potencial for landslides from information of past occurrences, photogrametric techniques namely using Liquefaction DECivil certain – it may occur in saturated soil with percentages of sand. The potencial for liquefaction can be analysed by means of dynamic tests of the soils, involving cyclic changes in the loading pattern at certain rates, leading to the evaluation of stiffness and strength degradation Testing equipment for dynamic soil testing, and results DECivil Tsunamis – detection of tsunamis is done essencially by means of instrumentation on solid ground, that detects others consequences (vibrations) DECivil of the same event that triggers the tsunami (earthquake, the more frequent cause of tsunami, or underwater landslide). Equipment on the sea is more expensive, has high maintenance costs and very little durability. The amplitude of a movement at the sea bed necessary to trigger a tsunami can be evaluated based on the location of the epicenter, local geology, magnitude of the earthquake and models of tsunami propagation. Model of tsunami propagation for the repetition of the 1755 Lisbon earthquake DECivil DECivil