- Email: [email protected]
The long and winding road from earthquakes to damage
Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
www.elsevier.com/locate/soildyn
The long and winding road from earthquakes to damage A. Ansal a,*, D. Slejko b a
b
Civil Engineering Faculty, Istanbul Technical University, Maslak, Istanbul 80626, Turkey Departimento Oceanogra®a e Geo®sica Ambientale, Istituto Nazionale di Oceanogra®a e di Geo®sica Sperimentale, Borgo Gigante, 42c, 34010 Sgonico, Trieste, Italy
Abstract The damage during earthquakes is caused by many factors depending on source, site and structural characteristics. The earthquake source mechanism and response of local site conditions are complex and may vary signi®cantly from one earthquake to the other and from one site to the other. These dif®culties and ambiguities in forecasting the seismic hazard in terms of expected earthquake characteristics affecting the man-made environment are discussed and possible alternative approaches are suggested. q 2001 Published by Elsevier Science Ltd. Keywords: Seismic risk; Seismic hazard; Hazard map; Hazard scenario
1. Introduction The poor correlation between earthquake magnitude and damage indicates that the process is rather complex and there are other factors that are equally important in the observed damage. Earthquake induced damage on manmade structures is important worldwide but especially in the Mediterranean region the damage was observed to be unwarranted also in terms of casualties and fatalities. This was mostly due to total collapse or heavy damage of residential and commercial buildings indicating the importance of structural features that are the most dominant factors controlling building vulnerability. The other important factor affecting building vulnerability is ground motion characteristics during an earthquake. However, earthquake ground motion is controlled mainly by earthquake source, path and site characteristics. Thus a comprehensive evaluation of regional seismicity, geologic and tectonic formations to determine possible fault locations and earthquake source characteristics is crucial. In conventional seismic hazard analysis, the source is accounted for using a single parameter (like the magnitude or the macroseismic intensity) and the path adopting an isotropic attenuation law. However, as pointed out by Ref. [1] earthquake source characteristics could vary signi®cantly depending on the fault orientation, stress drop, rupture pattern, directivity effects, fault roughness, and velocity of rupture propagation, especially in the near ®eld. Therefore, probabilistic approaches appear more suitable to account for * Corresponding author. E-mail address: [email protected] (A. Ansal). 0267-7261/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0267-726 1(01)00018-5
all these variabilities in the factors controlling the earthquake ground motion characteristics. The second group of factors affecting ground motion characteristics and thus the structural vulnerability are local geotechnical and geological site conditions. As observed in vertical strong motion arrays during recent strong earthquakes, soil strati®cation, as well as properties of soil layers could lead to signi®cant variations in earthquake ground motion. Large number of strong motion records obtained within relatively short distances have shown that ground motion characteristics can have signi®cant differences from one point to the other [9,14], most likely due to differences in the interaction among earthquake source, path and geotechnical site conditions. The aim of this paper is to discuss the principal factors that can transform a natural hazard into a human disaster and to propose possible approaches for risk reduction. The focus of the analysis will be on man-made structures and not on the additional adverse effects caused by earthquakes such as: decrease in the regional productivity, destruction of the social structure, etc., that depend on other factors outside the scope of Earth sciences. 2. Causes of damage The main factor in¯uencing structural damage and casualties observed in human settlements depends mainly on building vulnerability controlled by structural de®ciencies arising from inadequate structural design and construction techniques. Even though earthquakes have been effective in destroying cities and civilizations ever since
370
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
mankind started to establish settlements, the science of earthquake engineering is still very young. There have been signi®cant advances in our experimental and analytical capabilities in understanding the earthquake generated forces and response of structures under these forces. However, there are still ambiguities and unknowns especially in the near ®eld region. As a result, the major portion of the existing building stock that was designed and constructed based on the older building codes may not be considered very adequate in the light of our present knowledge on earthquake resistant design and construction. The earthquake engineering is an interdisciplinary science requiring input from different branches of Earth and engineering sciences. Unfortunately there are no simple prescriptions for prevention against structural vulnerability in the modern man-made environments. For a person without suf®cient engineering knowledge, it may be dif®cult to understand this complexity and thus respond accordingly. Therefore, one of the critical issues in the mitigation of the earthquake hazard is to increase the awareness of the public of®cials and general public and to convince them about the necessity to implement measures to decrease the structural vulnerability of the man-made environment. On the other hand, the effect of structural de®ciencies are ampli®ed by other factors related to earthquake ground motion characteristics controlled by earthquake source and site conditions. In this perspective each earthquake possesses unique characteristics due to the fault tectonics and fracture mechanism that are partly re¯ected in the obtained strong motion records and partly in the observed damage. Damage patterns and instrumental records in recent earthquakes indicate that ground motion characteristics such as direction, pulse or ¯ing effects and duration could have signi®cant in¯uence on forces generated during earthquakes and thus could play important roles in response of structures [7,13,17,21]. In addition, unexpected severe ground motions are often encountered and sometimes additional damages are caused by summation effects during strong aftershocks as in the 1997 Col®orito earthquakes in central Italy. Various factors contribute to the unexpected intensity of the ground shakings: (1) earthquakes occurring where they are not expected, (2) earthquake ground motion intensity being much larger than those forecasted by the national seismic zonation and (3) frequency content of the ground motion being different from those speci®ed in the national seismic codes. Identi®cation of seismic regions can be simple where the activity rate is high or where a long seismic history is documented but sources with long return periods can be missing in the seismic atlas. This is the reason why the present seismic hazard assessment in the USA [10] with very well documented present seismicity but with a poor historical documentation, is moving back from source dependent models to catalogue based models (from the seismotectonic probability to the historical probability according to Ref. [20]). Unexpected high ground shakings are rather common and
may be explained in terms of source characteristics such as directivity effects and in terms of local site conditions such as soil ampli®cation. The Italian experience indicates that most of the recent earthquakes produced peak ground accelerations (PGA's) in agreement with probabilistic estimates with the exception of the 1972 Ancona earthquake where the recorded values were absolutely unexpected for the area (Fig. 1). The probabilistic estimates were obtained based on the catalogue updated until 1980 and, therefore, no major events were missing in the analysis with the exception of the 1997 Col®orito earthquake, where PGA's were in agreement with the predicted values. Unexpected frequency contents of the ground shakings are also common and the reasons are similar to those given for the high acceleration amplitudes. A good example is given in Fig. 2, where the spectra of the main shock (Ms 6.8) and the strongest aftershock (Ms 5.8) of the 1992 Erzincan earthquake in Turkey [6] are shown. The seismic sequence displays remarkable differences most likely due to the rupture mechanism, directivity effects and due to coupling between source and site conditions [5]. These are some of the aspects that make it dif®cult to correlate earthquake magnitude and observed damage but the capacity to forecast in advance the occurrence of high ground shakings and their characteristics is the key element for risk mitigation. 3. Seismic hazard and risk In assessing the earthquake recurrence for a region, tectonic formations that can generate earthquakes and the seismic history in the region can best be evaluated in a probabilistic manner [1]. However, for earthquake scenario studies deterministic approaches have been preferred by large number of researchers [19,30]. Seismic risk is de®ned [3] as the composition of seismic hazard, vulnerability, and exposed value. Seismic risk is the probability of observing a certain damage, or loss of functionality, in a ®xed time period. Seismic hazard is the probability of observing a certain ground shaking (PGA, macro-seismic intensity, etc.) in a ®xed time period. Vulnerability is the tendency of a structure (building, complex system, etc.) to suffer damage. Exposed value is an economic, but also social, etc., quanti®cation of the object exposed to earthquakes. The role of seismic risk is crucial for forecasting future damage to existing buildings and this information can be used to determine the priorities in retro®tting policies. On the other hand, the seismic hazard assessment becomes very important when planning new settlements and designing new structures. Risk scenarios are fundamental for preparedness against earthquakes. They can also be utilized for assessing the behavior of transportation systems, natural gas, electricity and telecommunication networks, water supply and sanitary facilities, and to forecast the expected
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
371
Fig. 1. PGA with a 475-year return period in Italy for an average soil [29] and characteristics of the recent earthquakes (year, place, Ms magnitude, maximum recorded PGA in g, number of deaths: strong motion data [2] and personal communications; macroseismic data [16]). The colour of the boxes indenti®es the soil type at the strong-motion recording station: dark grey rock; white stiff soil; pale grey soft soil [2].
damage. The quanti®cation of the exposed value is extremely complex and the general tendency is to express risk neglecting its complexity [34]. Furthermore, vulnerability is considered only for some objects, such as buildings, but its global estimate for all the objects affected by earthquakes is almost impossible. In this chain of simpli®cations, an hazard map in terms of macroseismic intensity can be seen also as a risk map because intensity describes earthquake characteristics based on damage. The main cause of structural damage, high vulnerability of buildings, can be minimized with a retro®tting policy over the whole national territory on the basis of risk maps
while risk scenarios can be used to reduce the impact of future earthquakes also in terms of number of casualties. The damage summation effect may also be forecasted by a detailed analysis of the past earthquakes because the behavior of the seismic sources often reproduces themselves. This peculiarity can be considered in the risk map computation. 4. Seismic hazard maps and scenarios The problem of the unexpected high ground shaking
372
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
Fig. 2. Response spectra of the main shock and the major aftershock of the 1992 Erzincan sequence.
concerns the hazard assessment and can be minimized by large-scale hazard maps and detailed scenarios. They refer to different goals: hazard maps are useful for de®ning the seismic zonation and, in general, for comparing the expected shakings in a wide territory; scenarios are useful for describing the impact on the territory for a speci®c earthquake. Different approaches can be followed in hazard map and scenario preparations, according to the different situations: when various seismic sources represent danger to the investigated region a probabilistic approach is suitable, if only one source conditions the hazard of the site a deterministic approach may be preferred [19]. Ref. [20] identi®es ®ve distinct generation of probabilistic hazard maps, which he refers as: historical determinism, historical probabilism, seismotectonic probabilism, nonPoissonian probabilism, and earthquake prediction. The historical determinism approach consists in mapping the observed information, and it can be obtained from the maximum shaking experienced in the past. This approach lacks any statistical meaning but gives a minimum value of reference hazard. The historical probabilistic approach treats the earthquake catalogue information statistically. Data processing can be based on a simple counting procedure as well as on more robust statistics (e.g.: the theory of extreme values; [12]). The seismotectonic probabilistic approach is based on the de®nition of a seismogenic zonation by joint analysis of the geological and seismological data on the study region. The most common application of this approach is the method detailed by Ref. [8], which is based on speci®c assumptions: the event magnitude is exponentially distributed, the recurrence times form a Poisson process, and the seismicity is uniformly distributed over the seismogenic zone. The non-Poissonian probabilistic approach assumes that major earthquakes do not occur randomly in time. The Markov approach or the more simple semi-Markov approach that considers only the last event can better predict some
future earthquakes than the Poisson seismicity model (obviously only in a statistical sense, [31,32]), but the two methods give very similar results when applied over a reasonably long period, as in the most common case in seismic zonation. Earthquake prediction can be considered as the ultimate stage of hazard assessment, when probabilities concerning timing, location, and size of the impending event are computed. The seismic codes and zonations of the various countries are based on seismic hazard estimates computed with the most suitable approach for the national seismotectonic knowledge available [18]. The ®rst three types of hazard map are very popular, while non-Poissonian probabilism, and its recent hybrid variation [33], remain mainly a research topic. The most popular of the probabilistic scenarios are the deterministic ones where different approaches can be considered as well: earthquake maps reference shaking description, and shaking scenario. The earthquake map is the simplest representation of the impact of an earthquake and is developed upon rough hypothesis on the source geometry and an attenuation relationship [26]. The reference shaking description requests a good knowledge on the geometry and mechanism of the source, and of the regional crustal structure. Ground motions are computed by synthetic seismogram constructions [22]. The shaking scenario is the most detailed forecast of the earthquake impact and requests excellent knowledge on the geometry and mechanism of the source, the regional and local geology, and a heavy computing power. The ground motion is calculated by ®nite difference or ®nite element modeling [27]. 5. Seismic hazard parameters Different hazard parameters can be considered in hazard map and scenario representations. Considering only physical parameters, it must be said that even if PGA is the most widely used in seismic hazard analysis for its easy and practical meaning, nevertheless, it is a rough shaking indicator because it is generally associated with a short impulse of very high frequency and, therefore, cannot be easily correlated with the observed damage. For this reason the majority of building codes adopt the design spectrum to de®ne the seismic actions and it is derived from the available uniform hazard response spectra. The uniform hazard response spectrum is computed simply considering spectral attenuation relations in the probabilistic computation, it gives a complete description of the expected shaking at the site and can account for different site conditions. A direct view of hazard at large-scale is dif®cult considering spectra, and, for this reason, spectral hazard maps, extracted from the spectra, offer a direct comparison at national scale,
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
373
6.1. Seismic hazard maps and microzoning
Fig. 3. De®nition of effective peak acceleration (EPA) and Housner spectrum intensity (SI) over an example of uniform hazard spectrum. EPA is the mean of spectral ordinates between 0.1 and 0.5 s, divided by 2.5 (the white horizontal line). SI, in the original formulation, is the integral of a pseudovelocity response spectrum in the range of 0.1±2.5 s; the area marked here is just an indication [25].
although focusing at speci®c frequencies. Consequently, an average parameter as effective peak acceleration (EPA) has been sometimes suggested and it describes hazard better than PGA [28]. Recently, the spectrum intensity (SI) has been used for characterizing hazard at national scale [11]. More precisely a modi®cation of the original de®nition of SI [15] has been proposed [25] for pointing out the most severe shaking, responsible of collapses, from the vibration causing damage only (Fig. 3). For doing this, different frequency ranges have been associated to different return periods (T ): SI in the range 0.1±0.5 s to the T of 100 years, for quantifying the frequent damaging earthquakes and SI in the range 0.2± 2.0 s to the T of 475 years, for quantifying the rare destructive events. This need is motivated by the fact that in several European countries (e.g.: Italy) the economic loss caused by moderate quakes is not much different than that caused by strong ones. The choice of the best suitable parameter is very important for hazard description according to the different purposes. 6. Tools for hazard mitigation Different ways can be followed for properly describing the regional/national hazard and for preparing the adequate tools for its mitigation. Although, it is worth recalling that the long tectonic processes are hardly described by the seismic history, which covers at best 10±20 centuries. Additional information may be obtained by paleoseismology but, in spite of this, seismic hazard remains a subject without, or with few, control points. In this framework, until further proof, hazard maps and scenarios are considered adequate tools, for national and regional hazard mitigation, respectively.
The different generations of hazard maps identi®ed by Ref. [20] relate to increasing knowledge, it means that a sophisticated approach can be dangerous where the information does not support it, and a simple one can give more robust results. This could be seen by a quanti®cation of the global uncertainty related to the results, but this is a rather dif®cult task. Referring to the causes of damage previously pointed out, some variability can be introduced in the hazard map preparation for avoiding possible de®ciencies. The identi®cation of seismic zones possibly missing in the elaborations can be done through paleoseismological investigations, explicitly focused on silent, or long term silent areas. In addition, historical investigation for picking up forgotten earthquakes can improve the seismogenic geography knowledge as well. When the level of knowledge for a speci®c area is considered low, cautious working hypotheses can be taken into account, for example considering all possible uncertainties on source, maximum magnitude, etc., and also reassessment of neotectonic structures for which no seismic evidence is available [23]. Investigations on historical seismicity, with detailed reconstruction of sequence of aftershocks, can help in understanding the time/space evolution of earthquake occurrence. The choice of the suitable return period (short for frequent damaging earthquakes, and long for rare destructive events) can avoid unexpected large shakings and unexpected frequencies. Spectral hazard maps can better describe shakings at a ®xed frequency and thus can limit unexpected frequencies. Microzonation may take into account geological and topographic site effects. As shown in Fig. 4, their relevance may be important due to the variability introduced by the local site conditions [4]. It is a complementary aspect of regional seismic hazard maps for reducing unusual shaking intensities and unexpected frequency contents. In absence of a microzonation study, soil effects can be introduced as a ®rst approximation into an hazard map by de®ning typical soil conditions for the municipalities of the study region and calculating hazard for those speci®c soil conditions [24]. Such a hazard map at the free ®eld would better describe the expected shaking intensities than the customary maps referring to speci®c, or to average soil conditions. It is clear that this is nothing more than a crude substitute for a more comprehensive modeling of soil effects. The limits of the hazard maps are remarkable and depend on the scale of the investigation. In fact, hazard maps have national or at least regional relevance, and therefore, cannot be too detailed. It means that average shaking intensities are forecasted, generally referring to bedrock, which can be easily exceeded during an earthquake. A precise idea about the shaking intensity which is likely to occur during several events can be obtained only by coupling hazard maps with microzonation studies.
374
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
Fig. 4. Results of the microzonation study for Izmir city: (a) equivalent shear wave velocity and (b) ampli®cation.
6.2. Seismic hazard scenarios Hazard scenarios are much more detailed than hazard maps and can well represent the expected shaking intensities caused by a particular earthquake. A good knowledge of the source location and geometry, the crustal strata crossed by the seismic waves, and the local geology of the study site are necessary for performing a robust earthquake scenario. Deterministic approaches are generally used for scenario de®nition [19] and the expected shaking intensity can be realistically computed when suf®cient information is available and the analysis is detailed (very speci®c results can be given by FEM modeling, while only a general view can be
obtained by using attenuation relations). A comprehensive modeling could accurately forecast amplitude and frequency of the expected ground shaking. The limit of the scenario consists in its great sensitivity on the input data, which is hardly constrained as the information about the deep Earth structures and source mechanisms is at hypothesis level without possibility of experimental veri®cation. 7. Conclusions The purpose in this introductory paper was to point out
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
the dif®culties in forecasting the damage caused by earthquakes and to suggest possible alternatives for avoiding underestimation in the expected shaking intensities. In general, the con®dence in risk and hazard estimates is low because of the limited knowledge of seismogenic activity. Probabilistic hazard maps may give average values. However, they need to be integrated with microzonation studies to yield a more reliable estimate about the variation of design earthquake characteristics. Deterministic scenarios may represent more accurately expected earthquake intensity distribution during a speci®c earthquake and can be useful for preparedness activities. In the light of the observations that earthquake damage is mainly due to high structural vulnerability and unexpected earthquake forces; thus: ² cautious hazard maps taking into account the lack of seismotectonic knowledge ² more suitable hazard parameters representing induced ground motion and building vulnerability ² microzonation studies to determine site ampli®cations and site effects are the essential stages in the earthquake hazard mapping and scenario studies. However, there are still dif®culties remaining in modeling near ®eld effects and in the special installations requiring speci®c design parameters. References [1] Allen CR. Earthquake hazard assessment: has our approach been modi®ed in the light of recent earthquakes. Earthquake Spectra 1995;11(3):357±66. [2] Ambraseys NN, Simpson KA, Bommer JJ. Prediction of horizontal response spectra in Europe. Earth Eng Struct Dyn 1996;25:371±400. [3] Ambraseys NN. Evaluation of seismic risk. In: Ritsema AR, Gurpinar A, editors. Seismicity and seismic risk in the offshore North Sea area, Dordrecht: Reidel Publishing Company, 1983. p. 317±45. È zkan M. A preliminary microzonation study [4] Ansal AM, Iyisan R, O for the town of Dinar. In: SaÃco e Pinto P, editor. Seismic Behaviour of Ground and Geotechnical Structures, Proceedings 14th ICSMFE, Hamburg, Rotterdam: Balkema, 1997. p. 3±9. [5] Ansal AM, Siyahi BG. Effects of coupling between source and site characteristics during earthquakes. In: Elnashai AI, editor. European Seismic Design Practice, Proceedings of the SECED Conference, Balkema: Rotterdam, 1995. p. 83±89. [6] Ansal AM, Lav A. Geotechnical factors in 1992 Erzincan earthquake. Proc 5ICSZ 1995(1):667±74. [7] Bolt BA. Discussions of `enduring lessons and opportunities lost from the San Fernando earthquake of February 6, 1971' by Paul C. Jennings. Earthquake Spectra 1997;13(3):545±7. [8] Cornell CA. Engineering seismic risk analysis. Bull Seism Soc Am 1968;58:1583±606. [9] Field EH, Hough SE. The variability of PSV response spectra across a
[10] [11] [12] [13] [14] [15]
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
375
dense array deployed during the Northridge aftershock sequence. Earthquake Spectra 1997;13(2):243±57. Frankel A. Mapping seismic hazard in the Central and Eastern United States. Seismological Research Letters 1995;66(4):8±21. di Lavoro G. Proposta di riclassi®cazione sismica del territorio nazionale. Ingegneria Sismica 1999;14(1):5±14. Gumbel EJ. Statistics of extremes. New York: Columbia University Press, 1958. p. 375. Hall JF, Heaton TH, Halling MW, Wald DJ. Near-source ground motion and its effects on ¯exible buildings. Earthquake Spectra 1995;11(1):569±605. Hartzell S, Cranswick E, Frankel A, Carver D, Meremonte M. Variability of site response in the Los Angeles urban area. Bull Seism Soc Am 1997;87(6):1377±400. Housner GW. Spectrum intensities of strong-motion earthquakes. In: Duke CM, Feigen M, editors. Proc Symp on Earthquake and Blast Effects Structures, Los Angeles: University of California, 1952. p. 21±36. ISC. Bulletin of the International Seismological Centre. Newbury Thatcham: ISC, 1972±1997. Jennings PC. Enduring lessons and opportunities lost from the San Fernando earthquake of February 6, 1971. Earthquake Spectra 1997;13(1):25±53. McGuire RK. The practice of earthquake hazard assessment, Denver: IASPEI ESC, 1993. p. 284. McGuire RK. Deterministic vs probabilistic earthquake hazards and risks, Soil Dynamics and Earthquake Engineering, 21, 377±84. Muir-Wood R. From global seismotectonics to global seismic hazard. Annali di Geo®sica 1993;36:153±68. Naeim F. On seismic design implications of the 1994 Northridge earthquake records. Earthquake Spectra 1995;11(1):91±109. Panza GF, Vaccari F, Fah D. Seismic input modelling for zoning and microzoning. Earthquake Spectra 1996;12(3):529±66. Papoulia J, Slejko D. Cautious neotectonic hypotheses for assessing the seismic hazard in Northeastern Italy. Natural Hazards 1992;5:249±68. Peruzza L, Rebez A, Slejko D. Seismic hazard mapping for administrative purposes. Natural Hazards 2001;23:431±42. Peruzza L, Rebez A, Slejko D, Bragato PL. The Umbria±Marche case: some suggestions for the Italian seismic zonation. Soil Dynamics and Earthquake Engineering 2000;20:361±71. Pessina V. Empirical prediction of the ground shaking scenario for the Catania area. J Seismology 1999;3(3):265±77. Priolo E. 2-D spectral element simulations of destructive ground shaking in Catania (Italy). J Seismology 1999;3(3):289±309. Rebez A, Peruzza L, Slejko D. Spectral probabilistic seismic hazard assessment for Italy. Boll Geof Teor Appl 1999;40:31±51. Slejko D, Peruzza L, Rebez A. Seismic hazard maps of Italy. Annali di Geo®sica 1998;41:183±214. Whitman RV, Anagnos T, Kircher CA, Lagorio HJ, Lawson RS, Schneider P. Development of a national earthquake loss estimation methodology. Earthquake Spectra 1997;13(4):643±61. Working Group on California Earthquake Probabilities. Probabilities of large earthquakes in the San Francisco bay region, California. U.S.G.S. circular 1053, U.S.G.S., Denver, 51 pp, 1990. Working Group on California Earthquake Probabilities. Seismic hazards in southern California: probable earthquakes, 1994±2024. Bull Seism Soc Am 1995;85:379±439. Wu SC, Cornell CA, Winterstein SR. A hybrid recurrence model and its implication on seismic hazard results. Bull Seism Soc Am 1995;85:1±16. Yang WS, Slejko D, Viezzoli D, Gasparo F. Seismic risk in Friuli± Venezia Giulia: an approach. Soil Dynamics and Earthquake Engineering 1989;8:96±105.
www.elsevier.com/locate/soildyn
The long and winding road from earthquakes to damage A. Ansal a,*, D. Slejko b a
b
Civil Engineering Faculty, Istanbul Technical University, Maslak, Istanbul 80626, Turkey Departimento Oceanogra®a e Geo®sica Ambientale, Istituto Nazionale di Oceanogra®a e di Geo®sica Sperimentale, Borgo Gigante, 42c, 34010 Sgonico, Trieste, Italy
Abstract The damage during earthquakes is caused by many factors depending on source, site and structural characteristics. The earthquake source mechanism and response of local site conditions are complex and may vary signi®cantly from one earthquake to the other and from one site to the other. These dif®culties and ambiguities in forecasting the seismic hazard in terms of expected earthquake characteristics affecting the man-made environment are discussed and possible alternative approaches are suggested. q 2001 Published by Elsevier Science Ltd. Keywords: Seismic risk; Seismic hazard; Hazard map; Hazard scenario
1. Introduction The poor correlation between earthquake magnitude and damage indicates that the process is rather complex and there are other factors that are equally important in the observed damage. Earthquake induced damage on manmade structures is important worldwide but especially in the Mediterranean region the damage was observed to be unwarranted also in terms of casualties and fatalities. This was mostly due to total collapse or heavy damage of residential and commercial buildings indicating the importance of structural features that are the most dominant factors controlling building vulnerability. The other important factor affecting building vulnerability is ground motion characteristics during an earthquake. However, earthquake ground motion is controlled mainly by earthquake source, path and site characteristics. Thus a comprehensive evaluation of regional seismicity, geologic and tectonic formations to determine possible fault locations and earthquake source characteristics is crucial. In conventional seismic hazard analysis, the source is accounted for using a single parameter (like the magnitude or the macroseismic intensity) and the path adopting an isotropic attenuation law. However, as pointed out by Ref. [1] earthquake source characteristics could vary signi®cantly depending on the fault orientation, stress drop, rupture pattern, directivity effects, fault roughness, and velocity of rupture propagation, especially in the near ®eld. Therefore, probabilistic approaches appear more suitable to account for * Corresponding author. E-mail address: [email protected] (A. Ansal). 0267-7261/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0267-726 1(01)00018-5
all these variabilities in the factors controlling the earthquake ground motion characteristics. The second group of factors affecting ground motion characteristics and thus the structural vulnerability are local geotechnical and geological site conditions. As observed in vertical strong motion arrays during recent strong earthquakes, soil strati®cation, as well as properties of soil layers could lead to signi®cant variations in earthquake ground motion. Large number of strong motion records obtained within relatively short distances have shown that ground motion characteristics can have signi®cant differences from one point to the other [9,14], most likely due to differences in the interaction among earthquake source, path and geotechnical site conditions. The aim of this paper is to discuss the principal factors that can transform a natural hazard into a human disaster and to propose possible approaches for risk reduction. The focus of the analysis will be on man-made structures and not on the additional adverse effects caused by earthquakes such as: decrease in the regional productivity, destruction of the social structure, etc., that depend on other factors outside the scope of Earth sciences. 2. Causes of damage The main factor in¯uencing structural damage and casualties observed in human settlements depends mainly on building vulnerability controlled by structural de®ciencies arising from inadequate structural design and construction techniques. Even though earthquakes have been effective in destroying cities and civilizations ever since
370
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
mankind started to establish settlements, the science of earthquake engineering is still very young. There have been signi®cant advances in our experimental and analytical capabilities in understanding the earthquake generated forces and response of structures under these forces. However, there are still ambiguities and unknowns especially in the near ®eld region. As a result, the major portion of the existing building stock that was designed and constructed based on the older building codes may not be considered very adequate in the light of our present knowledge on earthquake resistant design and construction. The earthquake engineering is an interdisciplinary science requiring input from different branches of Earth and engineering sciences. Unfortunately there are no simple prescriptions for prevention against structural vulnerability in the modern man-made environments. For a person without suf®cient engineering knowledge, it may be dif®cult to understand this complexity and thus respond accordingly. Therefore, one of the critical issues in the mitigation of the earthquake hazard is to increase the awareness of the public of®cials and general public and to convince them about the necessity to implement measures to decrease the structural vulnerability of the man-made environment. On the other hand, the effect of structural de®ciencies are ampli®ed by other factors related to earthquake ground motion characteristics controlled by earthquake source and site conditions. In this perspective each earthquake possesses unique characteristics due to the fault tectonics and fracture mechanism that are partly re¯ected in the obtained strong motion records and partly in the observed damage. Damage patterns and instrumental records in recent earthquakes indicate that ground motion characteristics such as direction, pulse or ¯ing effects and duration could have signi®cant in¯uence on forces generated during earthquakes and thus could play important roles in response of structures [7,13,17,21]. In addition, unexpected severe ground motions are often encountered and sometimes additional damages are caused by summation effects during strong aftershocks as in the 1997 Col®orito earthquakes in central Italy. Various factors contribute to the unexpected intensity of the ground shakings: (1) earthquakes occurring where they are not expected, (2) earthquake ground motion intensity being much larger than those forecasted by the national seismic zonation and (3) frequency content of the ground motion being different from those speci®ed in the national seismic codes. Identi®cation of seismic regions can be simple where the activity rate is high or where a long seismic history is documented but sources with long return periods can be missing in the seismic atlas. This is the reason why the present seismic hazard assessment in the USA [10] with very well documented present seismicity but with a poor historical documentation, is moving back from source dependent models to catalogue based models (from the seismotectonic probability to the historical probability according to Ref. [20]). Unexpected high ground shakings are rather common and
may be explained in terms of source characteristics such as directivity effects and in terms of local site conditions such as soil ampli®cation. The Italian experience indicates that most of the recent earthquakes produced peak ground accelerations (PGA's) in agreement with probabilistic estimates with the exception of the 1972 Ancona earthquake where the recorded values were absolutely unexpected for the area (Fig. 1). The probabilistic estimates were obtained based on the catalogue updated until 1980 and, therefore, no major events were missing in the analysis with the exception of the 1997 Col®orito earthquake, where PGA's were in agreement with the predicted values. Unexpected frequency contents of the ground shakings are also common and the reasons are similar to those given for the high acceleration amplitudes. A good example is given in Fig. 2, where the spectra of the main shock (Ms 6.8) and the strongest aftershock (Ms 5.8) of the 1992 Erzincan earthquake in Turkey [6] are shown. The seismic sequence displays remarkable differences most likely due to the rupture mechanism, directivity effects and due to coupling between source and site conditions [5]. These are some of the aspects that make it dif®cult to correlate earthquake magnitude and observed damage but the capacity to forecast in advance the occurrence of high ground shakings and their characteristics is the key element for risk mitigation. 3. Seismic hazard and risk In assessing the earthquake recurrence for a region, tectonic formations that can generate earthquakes and the seismic history in the region can best be evaluated in a probabilistic manner [1]. However, for earthquake scenario studies deterministic approaches have been preferred by large number of researchers [19,30]. Seismic risk is de®ned [3] as the composition of seismic hazard, vulnerability, and exposed value. Seismic risk is the probability of observing a certain damage, or loss of functionality, in a ®xed time period. Seismic hazard is the probability of observing a certain ground shaking (PGA, macro-seismic intensity, etc.) in a ®xed time period. Vulnerability is the tendency of a structure (building, complex system, etc.) to suffer damage. Exposed value is an economic, but also social, etc., quanti®cation of the object exposed to earthquakes. The role of seismic risk is crucial for forecasting future damage to existing buildings and this information can be used to determine the priorities in retro®tting policies. On the other hand, the seismic hazard assessment becomes very important when planning new settlements and designing new structures. Risk scenarios are fundamental for preparedness against earthquakes. They can also be utilized for assessing the behavior of transportation systems, natural gas, electricity and telecommunication networks, water supply and sanitary facilities, and to forecast the expected
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
371
Fig. 1. PGA with a 475-year return period in Italy for an average soil [29] and characteristics of the recent earthquakes (year, place, Ms magnitude, maximum recorded PGA in g, number of deaths: strong motion data [2] and personal communications; macroseismic data [16]). The colour of the boxes indenti®es the soil type at the strong-motion recording station: dark grey rock; white stiff soil; pale grey soft soil [2].
damage. The quanti®cation of the exposed value is extremely complex and the general tendency is to express risk neglecting its complexity [34]. Furthermore, vulnerability is considered only for some objects, such as buildings, but its global estimate for all the objects affected by earthquakes is almost impossible. In this chain of simpli®cations, an hazard map in terms of macroseismic intensity can be seen also as a risk map because intensity describes earthquake characteristics based on damage. The main cause of structural damage, high vulnerability of buildings, can be minimized with a retro®tting policy over the whole national territory on the basis of risk maps
while risk scenarios can be used to reduce the impact of future earthquakes also in terms of number of casualties. The damage summation effect may also be forecasted by a detailed analysis of the past earthquakes because the behavior of the seismic sources often reproduces themselves. This peculiarity can be considered in the risk map computation. 4. Seismic hazard maps and scenarios The problem of the unexpected high ground shaking
372
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
Fig. 2. Response spectra of the main shock and the major aftershock of the 1992 Erzincan sequence.
concerns the hazard assessment and can be minimized by large-scale hazard maps and detailed scenarios. They refer to different goals: hazard maps are useful for de®ning the seismic zonation and, in general, for comparing the expected shakings in a wide territory; scenarios are useful for describing the impact on the territory for a speci®c earthquake. Different approaches can be followed in hazard map and scenario preparations, according to the different situations: when various seismic sources represent danger to the investigated region a probabilistic approach is suitable, if only one source conditions the hazard of the site a deterministic approach may be preferred [19]. Ref. [20] identi®es ®ve distinct generation of probabilistic hazard maps, which he refers as: historical determinism, historical probabilism, seismotectonic probabilism, nonPoissonian probabilism, and earthquake prediction. The historical determinism approach consists in mapping the observed information, and it can be obtained from the maximum shaking experienced in the past. This approach lacks any statistical meaning but gives a minimum value of reference hazard. The historical probabilistic approach treats the earthquake catalogue information statistically. Data processing can be based on a simple counting procedure as well as on more robust statistics (e.g.: the theory of extreme values; [12]). The seismotectonic probabilistic approach is based on the de®nition of a seismogenic zonation by joint analysis of the geological and seismological data on the study region. The most common application of this approach is the method detailed by Ref. [8], which is based on speci®c assumptions: the event magnitude is exponentially distributed, the recurrence times form a Poisson process, and the seismicity is uniformly distributed over the seismogenic zone. The non-Poissonian probabilistic approach assumes that major earthquakes do not occur randomly in time. The Markov approach or the more simple semi-Markov approach that considers only the last event can better predict some
future earthquakes than the Poisson seismicity model (obviously only in a statistical sense, [31,32]), but the two methods give very similar results when applied over a reasonably long period, as in the most common case in seismic zonation. Earthquake prediction can be considered as the ultimate stage of hazard assessment, when probabilities concerning timing, location, and size of the impending event are computed. The seismic codes and zonations of the various countries are based on seismic hazard estimates computed with the most suitable approach for the national seismotectonic knowledge available [18]. The ®rst three types of hazard map are very popular, while non-Poissonian probabilism, and its recent hybrid variation [33], remain mainly a research topic. The most popular of the probabilistic scenarios are the deterministic ones where different approaches can be considered as well: earthquake maps reference shaking description, and shaking scenario. The earthquake map is the simplest representation of the impact of an earthquake and is developed upon rough hypothesis on the source geometry and an attenuation relationship [26]. The reference shaking description requests a good knowledge on the geometry and mechanism of the source, and of the regional crustal structure. Ground motions are computed by synthetic seismogram constructions [22]. The shaking scenario is the most detailed forecast of the earthquake impact and requests excellent knowledge on the geometry and mechanism of the source, the regional and local geology, and a heavy computing power. The ground motion is calculated by ®nite difference or ®nite element modeling [27]. 5. Seismic hazard parameters Different hazard parameters can be considered in hazard map and scenario representations. Considering only physical parameters, it must be said that even if PGA is the most widely used in seismic hazard analysis for its easy and practical meaning, nevertheless, it is a rough shaking indicator because it is generally associated with a short impulse of very high frequency and, therefore, cannot be easily correlated with the observed damage. For this reason the majority of building codes adopt the design spectrum to de®ne the seismic actions and it is derived from the available uniform hazard response spectra. The uniform hazard response spectrum is computed simply considering spectral attenuation relations in the probabilistic computation, it gives a complete description of the expected shaking at the site and can account for different site conditions. A direct view of hazard at large-scale is dif®cult considering spectra, and, for this reason, spectral hazard maps, extracted from the spectra, offer a direct comparison at national scale,
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
373
6.1. Seismic hazard maps and microzoning
Fig. 3. De®nition of effective peak acceleration (EPA) and Housner spectrum intensity (SI) over an example of uniform hazard spectrum. EPA is the mean of spectral ordinates between 0.1 and 0.5 s, divided by 2.5 (the white horizontal line). SI, in the original formulation, is the integral of a pseudovelocity response spectrum in the range of 0.1±2.5 s; the area marked here is just an indication [25].
although focusing at speci®c frequencies. Consequently, an average parameter as effective peak acceleration (EPA) has been sometimes suggested and it describes hazard better than PGA [28]. Recently, the spectrum intensity (SI) has been used for characterizing hazard at national scale [11]. More precisely a modi®cation of the original de®nition of SI [15] has been proposed [25] for pointing out the most severe shaking, responsible of collapses, from the vibration causing damage only (Fig. 3). For doing this, different frequency ranges have been associated to different return periods (T ): SI in the range 0.1±0.5 s to the T of 100 years, for quantifying the frequent damaging earthquakes and SI in the range 0.2± 2.0 s to the T of 475 years, for quantifying the rare destructive events. This need is motivated by the fact that in several European countries (e.g.: Italy) the economic loss caused by moderate quakes is not much different than that caused by strong ones. The choice of the best suitable parameter is very important for hazard description according to the different purposes. 6. Tools for hazard mitigation Different ways can be followed for properly describing the regional/national hazard and for preparing the adequate tools for its mitigation. Although, it is worth recalling that the long tectonic processes are hardly described by the seismic history, which covers at best 10±20 centuries. Additional information may be obtained by paleoseismology but, in spite of this, seismic hazard remains a subject without, or with few, control points. In this framework, until further proof, hazard maps and scenarios are considered adequate tools, for national and regional hazard mitigation, respectively.
The different generations of hazard maps identi®ed by Ref. [20] relate to increasing knowledge, it means that a sophisticated approach can be dangerous where the information does not support it, and a simple one can give more robust results. This could be seen by a quanti®cation of the global uncertainty related to the results, but this is a rather dif®cult task. Referring to the causes of damage previously pointed out, some variability can be introduced in the hazard map preparation for avoiding possible de®ciencies. The identi®cation of seismic zones possibly missing in the elaborations can be done through paleoseismological investigations, explicitly focused on silent, or long term silent areas. In addition, historical investigation for picking up forgotten earthquakes can improve the seismogenic geography knowledge as well. When the level of knowledge for a speci®c area is considered low, cautious working hypotheses can be taken into account, for example considering all possible uncertainties on source, maximum magnitude, etc., and also reassessment of neotectonic structures for which no seismic evidence is available [23]. Investigations on historical seismicity, with detailed reconstruction of sequence of aftershocks, can help in understanding the time/space evolution of earthquake occurrence. The choice of the suitable return period (short for frequent damaging earthquakes, and long for rare destructive events) can avoid unexpected large shakings and unexpected frequencies. Spectral hazard maps can better describe shakings at a ®xed frequency and thus can limit unexpected frequencies. Microzonation may take into account geological and topographic site effects. As shown in Fig. 4, their relevance may be important due to the variability introduced by the local site conditions [4]. It is a complementary aspect of regional seismic hazard maps for reducing unusual shaking intensities and unexpected frequency contents. In absence of a microzonation study, soil effects can be introduced as a ®rst approximation into an hazard map by de®ning typical soil conditions for the municipalities of the study region and calculating hazard for those speci®c soil conditions [24]. Such a hazard map at the free ®eld would better describe the expected shaking intensities than the customary maps referring to speci®c, or to average soil conditions. It is clear that this is nothing more than a crude substitute for a more comprehensive modeling of soil effects. The limits of the hazard maps are remarkable and depend on the scale of the investigation. In fact, hazard maps have national or at least regional relevance, and therefore, cannot be too detailed. It means that average shaking intensities are forecasted, generally referring to bedrock, which can be easily exceeded during an earthquake. A precise idea about the shaking intensity which is likely to occur during several events can be obtained only by coupling hazard maps with microzonation studies.
374
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
Fig. 4. Results of the microzonation study for Izmir city: (a) equivalent shear wave velocity and (b) ampli®cation.
6.2. Seismic hazard scenarios Hazard scenarios are much more detailed than hazard maps and can well represent the expected shaking intensities caused by a particular earthquake. A good knowledge of the source location and geometry, the crustal strata crossed by the seismic waves, and the local geology of the study site are necessary for performing a robust earthquake scenario. Deterministic approaches are generally used for scenario de®nition [19] and the expected shaking intensity can be realistically computed when suf®cient information is available and the analysis is detailed (very speci®c results can be given by FEM modeling, while only a general view can be
obtained by using attenuation relations). A comprehensive modeling could accurately forecast amplitude and frequency of the expected ground shaking. The limit of the scenario consists in its great sensitivity on the input data, which is hardly constrained as the information about the deep Earth structures and source mechanisms is at hypothesis level without possibility of experimental veri®cation. 7. Conclusions The purpose in this introductory paper was to point out
A. Ansal, D. Slejko / Soil Dynamics and Earthquake Engineering 21 (2001) 369±375
the dif®culties in forecasting the damage caused by earthquakes and to suggest possible alternatives for avoiding underestimation in the expected shaking intensities. In general, the con®dence in risk and hazard estimates is low because of the limited knowledge of seismogenic activity. Probabilistic hazard maps may give average values. However, they need to be integrated with microzonation studies to yield a more reliable estimate about the variation of design earthquake characteristics. Deterministic scenarios may represent more accurately expected earthquake intensity distribution during a speci®c earthquake and can be useful for preparedness activities. In the light of the observations that earthquake damage is mainly due to high structural vulnerability and unexpected earthquake forces; thus: ² cautious hazard maps taking into account the lack of seismotectonic knowledge ² more suitable hazard parameters representing induced ground motion and building vulnerability ² microzonation studies to determine site ampli®cations and site effects are the essential stages in the earthquake hazard mapping and scenario studies. However, there are still dif®culties remaining in modeling near ®eld effects and in the special installations requiring speci®c design parameters. References [1] Allen CR. Earthquake hazard assessment: has our approach been modi®ed in the light of recent earthquakes. Earthquake Spectra 1995;11(3):357±66. [2] Ambraseys NN, Simpson KA, Bommer JJ. Prediction of horizontal response spectra in Europe. Earth Eng Struct Dyn 1996;25:371±400. [3] Ambraseys NN. Evaluation of seismic risk. In: Ritsema AR, Gurpinar A, editors. Seismicity and seismic risk in the offshore North Sea area, Dordrecht: Reidel Publishing Company, 1983. p. 317±45. È zkan M. A preliminary microzonation study [4] Ansal AM, Iyisan R, O for the town of Dinar. In: SaÃco e Pinto P, editor. Seismic Behaviour of Ground and Geotechnical Structures, Proceedings 14th ICSMFE, Hamburg, Rotterdam: Balkema, 1997. p. 3±9. [5] Ansal AM, Siyahi BG. Effects of coupling between source and site characteristics during earthquakes. In: Elnashai AI, editor. European Seismic Design Practice, Proceedings of the SECED Conference, Balkema: Rotterdam, 1995. p. 83±89. [6] Ansal AM, Lav A. Geotechnical factors in 1992 Erzincan earthquake. Proc 5ICSZ 1995(1):667±74. [7] Bolt BA. Discussions of `enduring lessons and opportunities lost from the San Fernando earthquake of February 6, 1971' by Paul C. Jennings. Earthquake Spectra 1997;13(3):545±7. [8] Cornell CA. Engineering seismic risk analysis. Bull Seism Soc Am 1968;58:1583±606. [9] Field EH, Hough SE. The variability of PSV response spectra across a
[10] [11] [12] [13] [14] [15]
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
375
dense array deployed during the Northridge aftershock sequence. Earthquake Spectra 1997;13(2):243±57. Frankel A. Mapping seismic hazard in the Central and Eastern United States. Seismological Research Letters 1995;66(4):8±21. di Lavoro G. Proposta di riclassi®cazione sismica del territorio nazionale. Ingegneria Sismica 1999;14(1):5±14. Gumbel EJ. Statistics of extremes. New York: Columbia University Press, 1958. p. 375. Hall JF, Heaton TH, Halling MW, Wald DJ. Near-source ground motion and its effects on ¯exible buildings. Earthquake Spectra 1995;11(1):569±605. Hartzell S, Cranswick E, Frankel A, Carver D, Meremonte M. Variability of site response in the Los Angeles urban area. Bull Seism Soc Am 1997;87(6):1377±400. Housner GW. Spectrum intensities of strong-motion earthquakes. In: Duke CM, Feigen M, editors. Proc Symp on Earthquake and Blast Effects Structures, Los Angeles: University of California, 1952. p. 21±36. ISC. Bulletin of the International Seismological Centre. Newbury Thatcham: ISC, 1972±1997. Jennings PC. Enduring lessons and opportunities lost from the San Fernando earthquake of February 6, 1971. Earthquake Spectra 1997;13(1):25±53. McGuire RK. The practice of earthquake hazard assessment, Denver: IASPEI ESC, 1993. p. 284. McGuire RK. Deterministic vs probabilistic earthquake hazards and risks, Soil Dynamics and Earthquake Engineering, 21, 377±84. Muir-Wood R. From global seismotectonics to global seismic hazard. Annali di Geo®sica 1993;36:153±68. Naeim F. On seismic design implications of the 1994 Northridge earthquake records. Earthquake Spectra 1995;11(1):91±109. Panza GF, Vaccari F, Fah D. Seismic input modelling for zoning and microzoning. Earthquake Spectra 1996;12(3):529±66. Papoulia J, Slejko D. Cautious neotectonic hypotheses for assessing the seismic hazard in Northeastern Italy. Natural Hazards 1992;5:249±68. Peruzza L, Rebez A, Slejko D. Seismic hazard mapping for administrative purposes. Natural Hazards 2001;23:431±42. Peruzza L, Rebez A, Slejko D, Bragato PL. The Umbria±Marche case: some suggestions for the Italian seismic zonation. Soil Dynamics and Earthquake Engineering 2000;20:361±71. Pessina V. Empirical prediction of the ground shaking scenario for the Catania area. J Seismology 1999;3(3):265±77. Priolo E. 2-D spectral element simulations of destructive ground shaking in Catania (Italy). J Seismology 1999;3(3):289±309. Rebez A, Peruzza L, Slejko D. Spectral probabilistic seismic hazard assessment for Italy. Boll Geof Teor Appl 1999;40:31±51. Slejko D, Peruzza L, Rebez A. Seismic hazard maps of Italy. Annali di Geo®sica 1998;41:183±214. Whitman RV, Anagnos T, Kircher CA, Lagorio HJ, Lawson RS, Schneider P. Development of a national earthquake loss estimation methodology. Earthquake Spectra 1997;13(4):643±61. Working Group on California Earthquake Probabilities. Probabilities of large earthquakes in the San Francisco bay region, California. U.S.G.S. circular 1053, U.S.G.S., Denver, 51 pp, 1990. Working Group on California Earthquake Probabilities. Seismic hazards in southern California: probable earthquakes, 1994±2024. Bull Seism Soc Am 1995;85:379±439. Wu SC, Cornell CA, Winterstein SR. A hybrid recurrence model and its implication on seismic hazard results. Bull Seism Soc Am 1995;85:1±16. Yang WS, Slejko D, Viezzoli D, Gasparo F. Seismic risk in Friuli± Venezia Giulia: an approach. Soil Dynamics and Earthquake Engineering 1989;8:96±105.