The investigation of the design parameters of the Iranian earthquake code of practice based on hazard analysis

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1 The investigation of the design parameters of the Iranian earthquake code of practice based on hazard analysis G. Ghodrati Arniri & H. Rabet Es-haghi Department of Civil Engineering, Iran University of Science & Technology, Tehran, Iran Abstract The correct and satisfactory design of the structures in each country depends on having an appropriate earthquake code of practice based on the regional and seismic conditions of that country, In this regard, the Iranian earthquake code of practice has undergone some rapid revisions in the recent versions. The reason is that comprehensive researches based on the analysis of seismic data haven t been performed. In this research, the seismic design parameters have been investigated and analyzed based on the hazard analysis studies and the documented data ftom the past earthquakes in Tehran. In order to process the data in this research, a user-friendly software named STIUSHA was developed. Based on the obtained results, some changes in the design parameters of the 2800 Iranian earthquake code of practice can be made parallel to the advanced earthquake codes of practice of the world. 1 Introduction Earthquake is an unexpected event, during which a considerable energy would be released and this energy causes ground vibration and sometimes destruction over ground surface and death of thousands of humans. Iran plateau is situated over one of the active seismic belts of the world (Alpide- Himalaya) and many active faults of Iran are located near important centers and populated areas and once in a while, a shallow earthquake occurs in these regions, which sometimes causes a great catastrophe. The study of the

2 648 Risk Analysis III historical Earthquakes, recent earthquakes and the investigation of the seismograph network data show a direct relationship between these events with geologic and geomorphic conditions of the Iran plateau, Based on the probabilistic analyses and past earthquakes, geomorphic and seismotectonic characteristic of the Iran plateau, different regions were adopted. According to one suggestion, Iran seismotectonic plateau was divided into 4 regions which include: Zagros seismic belt, Eastern and central areas of Iran, Azarbayejan and mountain ranges of Alborz. In later Researches, more details were considered and the Iran plateau was divided into 23 seismotectonic provinces, The western and southwestern areas of Iran are located in Zagros region. This region which extends from sea of Oman to West of Iran is tightly folded and is a highly active seismic region. Alborz Region starts from Ghafghaz and includes parts of western, central and eastern Alborz and some parts of Khorasan province, Abyek fault in south of this region caused the catastrophic earthquake of Booin-Zahra (1 t of September 1964) which killed people. This Region is also a very highly active seismic one. Another part of the Iran plateau is eastern areas of Iran which has a high potential of producing severe earthquakes, The famous Tabbas earthquake with 7.7 magnitude occurred in this region in the year The central part of Iran has low seismic activities. (Bargi [1], Berberian [2]) Considering the aforementioned remarks, it is obvious that Iran has a very high potential of seismic activity, Therefore buildings must be designed against earthquakes based on the 2800 Iranian earthquake code of practice and this standard must be updated regularly taking into account the new advances in earthquake engineering, In this research, probabilistic seismic hazard analysis of Capital of Iran (Tehran) is performed using a software named STIUSHA which is developed in Iran University of Science and Technology (IUST) by the authors of this paper and the results are compared with the 2800 Iranian Earthquake code of practice. 2 Theoretical aspects For designing structures against earthquake, the engineer needs to know the design acceleration of the site to calculate the earthquake loading. The value of the design acceleration can be normally obtained using the earthquake codes of practice for normal structures but for critical and major structures like dams, nuclear power plants and likewise, a more exact and elaborate method must be employed. Hazard analysis is one of the methods that can be used for calculation of the design acceleration and construction of the site specific response spectra. There are two ways for describing ground motion in hazard analysis. One is PGA (Peak Ground Acceleration) and the other is spectral ordinates (S, or S.). So in this paper, whenever the term of ground motion is used, it can be either PGA or spectral ordinates. (Kramer [3], Naeirn [4], Us-Army Corps of Engineers [5,6]) There are two methods for hazard analysis:

3 2.1 Probabilistic Seismic Hazard Analysis (PSHA) Risk Analysis III 649 In PSHA method, the site ground motions are estimated for selected values of annual probability of exceedance (APE) or return periods for ground motion exceedance, or probability of ground motion exceedance in a certain exposure time (or design time period), The probability of exceeding a certain level of ground motion at a site is a function of the locations of seismic sources and the uncertainty of Mure earthquake locations on the sources, the tiequency of occurrence of earthquakes of different magnitudes on the various sources and the source to site ground motion attenuation including its uncertainty, The step by step procedure of this method is described below. 2.1,1 Identification and modeling of seismic sources The frost step of PSHA method is identification of sources and modeling them. The type and accuracy of the selected model depends on geologic, geotectonic, geomorphic and historic data and the engineering judgment of the analyzer, The purpose of this step is collecting necessary information for delineating faults and the areas in which the seismic activity can be assumed to be uniform. Based on these data, the sources are modeled as either line sources or area sources or point sources Evaluation of source seismicity The second step in seismic hazard analysis is the evaluation of the seismic parameters of each modeled source. This evaluation involves the following components: Collection of data and formulation of the recurrence relationship The database for seismic events on a given source is often incomplete, nonhomogenous in time, and lacking in refinement. The appropriate processing of this occurrence information is very important because the reliability of the results of the hazard analysis is strongly dependent on the consistency and the completeness of the input data base. The magnitude-fi-equency or recurrence relationship is formulated from the number of the earthquakes that a source has generated and their respective magnitudes. The most common method of determining this relationship is fkom historic data, Statistical regression is commonly used to obtain the best line fit with the least squared error. Expert subjective opinion can also be incorporated in order to supplement the historic database. The most commonly used recurrence relationship is the one suggested by Gutenberg and Richter [7]. This relationship is given by the following equation: LnN(m) = a + b*m (1) Where N(m) = average number of events greater than or equal to the magnitude m a, b = constants Other forms of recurrence relationships have been used by researchers. Dalal [8] has used Gaussian and log-normal probability distribution models, Mortgat et

4 (55(1 Risk Analysis III al. [9] have used a bilinear relationship and Cornel and Merz [10] have used a quadratic form for their recurrence relationships, Determination of the maximum earthquake The estimate of the size of the maximum earthquake for a given source is based on the following factors: 1, Geologic evaluation of the regional tectonic fkamework 2, Historical seismicity of the source and the surrounding region 3, Geologic history of the displacement (horn trenching investigations) 4. Relationship between earthquake magnitude and fault rupture length. 5. Relationship between earthquake magnitude and amount of fault displacement Selection of attenuation relationship When a rupture along a fault plane occurs, vibratory ground motions are generated, These motions travel out from the source as body and surface waves. As these waves travel farther out from the source, they are attenuated. The type and amount of attenuation depends on many factors, the most important of which are listed below: 1. Size or source severity of the event on the source 2. Type of fault mechanism 3. Transmission path of the seismic waves from source to the site 4, Vibration or wave frequency of interest of the seismic ground motion 5. Distance from the source to the site 6. Local site soil response effect Attenuation relationships describe the variation of the amplitude of a ground motion parameter as a fimction of earthquake magnitude and source to site distance, A number of attenuation relationships have been developed for PGA and also for response spectral accelerations or velocities for different structural periods of vibration (Douglas [11]), These relationships have been developed for different environments and broad categories of subsurface conditions, particularly for the categories of rock and firm soils, In some cases, attenuation relationships have distinguished the effects of different types of faulting (e.g., strike slip vs. reverse faulting). It is important to select a set of attenuation relationships that are most applicable to the site under consideration. The general form of attenuation relationships is: Log y = f(c)+f(m,r)+ & (2) Where y is the ground motion parameter (PGA or S, or S,), M is earthquake magnitude, R is the distance between the seismic source and site, and s is the random error with mean value equal to zero 2.1,4 Selection of probabilistic forecasting model Step 4 of PSHA analysis is to forecast source severity of future earthquakes on each of the identified sources, once the seismic sources have been identified and the seismicity of the identified sources has been determined, These forecasting models are not based on extrapolation of past data, but are based on stochastic models, The most widely used model is called the homogeneouspoisson Model.

5 Risk Analysis III 651 In the Poisson model, earthquake occurrence in time is assumed to be random and memory less, The probability of an earthquake in a given time period is thus determined by the average tiequency of earthquakes, and is independent of when the last earthquake occurred. This model has been shown to be consistent with earthquake occurrence on a regional basis; however it does not conform to the process believed to result in earthquakes on an individual fault, More realistic real time earthquake recurrence models have been developed that predict the probability of an earthquake in the next time period, rather than any time period, taking into account the past history of large earthquakes on a fault. Usually there are insufficient geologic and seismic data on the timing of past earthquakes to justi the use of these models, Conducting Probabilistic Seismic Hazard Analyses (PHSA) The seismic source characterization and ground motion attenuation characterization are combined in the selected probabilistic model to develop relationships between amplitude of a ground motion parameter and the probability or the tlequency of its exceedance, These relationships are termed hazard curves, There are two alternatives approaches for obtaining response spectra based on PSHA, Approach one is to anchor a normalized design response spectrum shape to the PGAs determined from PSHA (The ground motion parameter is PGA in the analysis) and approach two is developing equal hazard spectra directly from PHSA (The ground motion parameter is spectral ordinate) PSHA and computer program Since in PSHA method, there are too much and cumbersome calculations, it is usually not possible to conduct a PSHA analysis without computer. So a computer program is needed for PHSA analysis, Therefore, a completely userfiiendly software was developed based on PSHA in Iran university of science and technology by the authors of this paper and its name is STIUSHA which stands for: Science and Technology Iran University Seismic Hazard Analysis. STIUSHA results were compared and tested with EQRisk [12] and SeisRisk 111. This software is used in this paper. 2.2 Deterministic Seismic Hazard Analysis (DSHA) rhe deterministic method is used exclusively for those important structures where the consequences of failure are catastrophic such as nuclear power plants, liquefied natural gas facilities, and dams, This method tends to compound conservatism (certainty of occurrence, largest magnitude and closest distance from epicenter to the site) and will generally result in extremely large design requirements, For most structures, these highly conservative design values cannot be justified economically for use. This disadvantage of extreme conservatism has actually resulted in the adoption of probabilistic procedures even for some critical facilities. Deterministic method therefore, won t be used in this paper.

6 (552 Risk Analysis III 3 Methodology In this paper, seismic hazard of a typical site in Tehran is performed using STIUSI-LA The assumed site coordinates are: longitude= 51.4 degrees east and latitude = 35,8 degrees north. The evaluation of seismic hazard is performed in a region with the assumed site at the center and a radius of 200 km, In this region, all the major active faults are identified and considered in the analysis, As it was mentioned in theoretical aspects, the following steps are required in PHSA. 3.1 Identification and modeling of seismic sources in the considered region The active faults in the considered region are identified and listed in Table 1. All of these seismic sources are modeled as line sources in STIUSHA. Since in Iran, most earthquakes are shallow earthquakes, the considered focal depth is 5 KM in the program, Of all the mentioned faults in Table 1, the most critical and significant faults are Mosha and North Tehran, 3.2 Evaluation of source seismicity Collection of data and formulation of the recurrence relationship One of the most important steps in seismic hazard analysis is obtaining the Gutenberg-Richter parameters (a, b). There are many different methods for calculating these parameters, One of the best method specially for regions with incomplete and insufficient seismic database is Kijko method [14,15]. Using this method and considering both historic and instrumentally recorded earthquakes in the considered region, the following information was obtained: R = 200 km (m, = 4.5) = 0,38 p=l.51 Considering the Gutenberg-Richter relationship in the form of Ln N(m)= a-bin We have: b= =1.51 = N(m,=4.5) Ln(O.38) = a *4.5 a = 5.83 It must be noted that these parameters are calculated for the considered region. STIUSHA itself calculates these parameters for each seismic source automatically, Determination of the minimum and maximum earthquake magnitude The minimum earthquake magnitude is considered to be M, = 4 based on engineering judgment. There are a couple of methods for the determination of maximum earthquake magnitude. In this paper, the fault rupture length method and Nowroozi relationship [16] are used. This relationship is: M,= Log L (3) Where M, is earthquake surface magnitude and L is rupture length in Meter.

7 Table 1: Seismic sources in the considered region Risk Analysis III 653 Number Fault Name Longitude Latitude 53,7 36 I I Max Ms 1 2 Mosha , , ,6 North Tehran I I I 51.1 I Niavaran North Rey 6.5 1%%# 5 South Rey 6.57 I I =--w%= Garrnsar Pishva I 51,91 ] I 52 ] Parchin 6,52 I I Taleghan 6,9 H Ipak 7 50,

8 654 Risk Analysis III 3.3 Selection of attenuation relationship One of the most important and difficult steps in hazard analysis is the selection of the appropriate attenuation relationship, Since STIUSHA accepts nearly all the attenuation relationships, in this research, different attenuation relationships based on the local conditions of Tehran, were used which include Campbell 90, Crouse 91, Ambraseys & Bommer 91, Campbell 93. (Douglas [11]) 3.4 Performing Probabilistic Seismic Hazard Analysis (PSHA) After the above steps, PSHA analysis shall be performed with STIUSHA, 4 Results and discussion The data, which were collected in the above steps, were given to STIUSHA and PSHA analysis was performed by this software and the results were obtained for each attenuation relationship, As an example, the seismic hazard curve for one of the attenuation relationships (Campbell 93) is shown in Figure 1, It is customary to design structures for Maximum Design Earthquake (annual probability of exceedance of ) and control their serviceability for Design Basis Earthquake (annual probability of exceedance of ). In Table 2, the results for MDE and DBE for each attenuation relationship are given, According to this table, MDE for Tehran varies from 0,35g to 0.44g and DBE varies from 0, 18g to o,23g. The name of the Iranian earthquake code is 2800 standard, Two of seismic design parameters in this standard are B, A. B is the normalized design response spectrum and A is PGA which is used to scale B for obtaining the design spectrum of the site under study. In this standard, Iran is divided into 4 regions which are regions with low seismic activity, medium seismic activity, high seismic activity and very high seismic activity. For each of these regions, the value of A for MDE is given as 0.2, 0,25, 0,3, and 0.35 respectively. According to this standard, Tehran is a region with a very high seismic activity. Therefore the value of A for Tehran, according to the 2800 standard, is 0,35g. But in our analysis, the maximum calculated design maximum PGA for MDE is 0.44g, It clearly shows that for important and critical structures, the Iranian earthquake code of practice is not enough and seismic hazard analysis is required for constructing design response spectrum. 5 Conclusions In this research, seismic hazard analysis using PSHA method was performed for the Capital of Iran (Tehran) using a software named STIUSHA which was developed in Iran University of Science and Technology (IUST) and the results were compared with the 2800 Iranian earthquake code of practice,, Based on the obtained results, some changes in the design parameters of the 2800 Iranian earthquake code of practice can be made.

9 References Risk Analysis III 655 [1] Bargi, K., Fundamentals of Earthquake Engineering, Jahad Daneshgahi Publications (MAJED): Iran, Tehran, 1996, (in Persian) [2] Berberian, M., Active Faulting and Tectonics of Iran in Zagros-Hindu- Kush-Himalaya: Geodynamics Evolution, F,M. Delany and H.K.Gupta, Editors, Geodynamics Series, Am, Geophys, Union: pp , [3] Kramer, S.L., Translated into Persian By: Mir Hoseini, M & Arefpoor, B,, Geotechnical Earthquake Engineering, Iranian Earthquake Engineering Research Center Publications: Iran, Tehran, 2000, (in Persian) [4] Naeim, F.,The Seismic Design Handbook, Van Nostrand: 1989 [5] US-Army Corps of Engineers, Seismic Design Guidelines for Essential Buildings, TM , Department of Army-The Navy- and The Air Force: 1986, [6] US-Army Corps of Engineers, Seismic Design for Buildings, TI , Department of Army-The Navy- and The Air Force: 1999, [7] Gutenberg, B, & Richter, C, F,, Earthquake Magnitude, Intensity, Energy, and Acceleration. Bulletin of the Seismological Society oj America, Vol. 46, [8] Dalal, J.S,, Probabilistic Seismic Exposure and Structural Risk Evaluation, Technical Report No. 169, Department of Civil Engineering, Stanford University: [9] Mortgat, C.P., Zsutty, T.C., Shah, H,C. & Lubetkin, A study of Seismic Hazard For Costa Rica, Technical Report No 25, The John A, Blume Earthquake Center, Stanford University: 1978, [10] Mertz, H,A, & Cornell, C.A,, Seismic Risk Analysis Based on a Quadratic Magnitude-Frequency Law. Bulletin of the Seismological Society of America, Vol. 63, No. 6, pp , [11 Douglas, J. A Comprehensive Worldwide Summary of Strong- Motion Attenuation Relationships for Peak Ground Acceleration and Spectral Ordinates, Engineering Seismology and Earthquake Eng. EsEEE Report No, 01-1, Imperial College of Science, Technology and Medicine, Civil Engineering Department: January 2001, [12] McGuire, R.K., EQRisk: A Fortran Computer Program For Seismic Risk Analysis, US Geological Survey Open File Report 76-67: [13] Bender, B, & Perkins, D.M,, SeisRisk III: A Computer Program for Seismic Hazard Estimation, US Geological Survey (USGS) Bulletin 1772, [14] Kijko, A. & Slevolle, M,A., Estimation of Earthquake Hazard Parameters for Incomplete and Uncertain Data Files. Natural Hazard 3, pp. 1-13, 1990, Paper Presented at the 21st General Assembly of the European Seismological Commission, Sofia, 1990.

10 656 Risk Analysis III [15] Kijko, A. & Slevolle, M.A., Estimation of Earthquake Hazard Parameters for Incomplete Data Files. Part II. Incorporation of Magnitude Heterogeneity, Bulletin of the Seismological SocieQ of America, Vol. 82, No. 1, pp , [16] Nowroozi, A.A., Empirical Relations between Magnitudes and Fault Parameters for Earthquakes in Iran. VO1.75,No.5, pp , 1985., 0,1 0.!5 0,2 o% 0.3 0, k Not for Comamiai w, <- Figure 1: Seismic Hazard Curve using Campbell 93 [11] Table 2: Results of Hazard Analysis of Tehran I Attenuation Relationship I DBE (g) I MDE (g) I I Campbell 90 I 0.18 I 0,35 I I Ambrasevs & Bommer 92 I 0.19 I 0.41 I I Crouse 91 I 0.19 I 0.4 I Campbell