SEISMIC SITE RESPONSE, ANALYSIS AND

CHARACTERIZATION OF MAJOR CITIES IN U.A.E.

A THESIS IN CIVIL ENGINEERING

Presented to the faculty of the American University of Sharjah College of Engineering in partial fulfillment of

the requirements for the degree

MASTER OF SCIENCE

by

MUHAMMAD IRFAN B.S. 2009

Sharjah, UAE June 2011

2011 Mohammad Irfan

ALL RIGHTS RESERVED

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We approve the thesis of Muhammad Irfan Date of Signature

Dr. Magdi El-Emam Assistant Professor Civil Engineering Graduate Committee

Dr. Zahid Khan Assistant Professor Civil Engineering Graduate Committee

Dr. Jamal Abdalla Head Civil Engineering Graduate Comittee

Dr. Aman Mwafy Assistant Professor Civil Engineering Graduate Committee

Dr. Hany El-Kadi Associate Dean College of Engineering

Dr. Yousef Al-Assaf Dean College of Engineering

Dr. Gautem Sen Vice Provost Research and Graduate Studies

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SEISMIC SITE RESPONSE, ANALYSIS AND

CHARACTERIZATION OF MAJOR CITIES IN U.A.E.

Muhammad Irfan, Candidate for the Masters of Science Degree

American University of Sharjah, 2011

ABSTRACT

United Arab Emirates (UAE) has experienced significant economic growth in recent years. The accelerated schedule driven projects are compelling designers to use values of seismic hazard (ground motion) that are either significantly conservative or unreliable. Moreover, not all estimates of a seismic hazard analysis such as mapped spectral accelerations, representative hazard spectra, and deaggregation covering all parts of UAE are available. Studies that have attempted to define the seismic hazard in UAE in the past are not in agreement and they either focused on few cities or did not provide all the necessary information. The variations in their results could be attributed to the use of questionable source zonation, activity parameters, and

superseded prediction equations. Consequently, designers in UAE have to rely on inaccurate estimate of seismic hazard for the region. Considering substantial

development in United Arab Emirates (UAE) and considerable ambiguity faced by the designers in choosing the seismic hazard from disagreeing studies, a new seismic hazard analysis is urgently required. This study is based on the use of homogenized catalogue of various degrees of

completeness for temporal distribution of events (Surface magnitudes, Ms), activity parameters based on doubly bounded magnitude-frequency relationships, modified

zonation of area sources, and new generation prediction equations. The study aims to provide a comprehensive seismic hazard assessment for all parts of UAE that will provide designers with Hazard curves, values of peak ground accelerations (PGA), mapped spectral accelerations at 0.2s and 1s (S0.2 and S1), Uniform Hazard Spectra (UHS), and deaggregation of seismic hazard.

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In addition to the estimation of seismic hazard, this study provides estimates of the site amplification for three major cities of UAE (Sharjah, Dubai, and Abu Dhabi). Effect of local site conditions in modifying the seismic waves is well documented in many studies. Site amplification factors as a guideline for typical UAE building sites are not available. As a result, designers in UAE have to rely on factors developed for other regions. These factors are typically obtained by performing

equivalent-linear or non-linear site response analysis of sites of known dynamic properties. Site response analyses were performed for different representative

subsurface soil models obtained from various sources. Sites were grouped as per the provisions of International Building Code (IBC 2009). The results of this part of the study will provide structural engineers with region specific amplification factors for the development of design spectra instead of relying on factors developed for other regions.

The results are generally provided for a return period of 2475 years (2 % probability of excedance in 50 years) in conformance to the provisions given in American codes. The results mapped seismic hazard presented in this study

corresponds to rock sites classified as Site Class B according to International Building Code (IBC 2009). The results indicate slightly larger values of seismic hazard compared to some recently published studies. The effect of west coast fault is significant especially at larger return periods and should be taken into account if

future studies confirm the presence of a fault along the west coast of UAE and prevalent building codes adopts lower probability of exeedance. The activity in

Arabian Craton (background seismicity) contributes mostly to the hazard in most southern part of UAE. The contribution of other sources such as Zargos (Iran) and Oman mountains increases as one move towards the North. The west of the country is dominated by seismicity from Zargos whereas the east by seismicity from Oman

mountains.

The results of site response analyses (site classes C and D) suggest more amplification in Sharjah than in Dubai and Abu Dhabi because of deep engineering bedrocks in Sharjah. The response spectra of Abu Dhabi and Dubai are scattered as compared to Sharjah because of the variance in soil column depths in Dubai and Abu Dhabi. The amplification factors for Sharjah are in the range of 4 to 6 and for Dubai it is estimated to be around 3 to 4; whereas, the amplification factors for Abu Dhabi ranged from 4 to 8.

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CONTENTS

ABSTRACT iii

LIST OF FIGURES vii

LIST OF TABLES x

ACKNOWLEDGEMENTS xi

Chapter 1 INTRODUCTION 1

General Introduction 1 Problem Definition 2 Objectives of Study 3 Available Data and Collection 4 Organization of Thesis 6

2 LITERATURE REVIEW 7

Background 7 Review of Regional Studies 19

3 STUDY AREA - GEOLOGY, TECTONICS AND SEISMICITY 30 Study Area 30 Geology 31 Regional Tectonic Setting 32

Regional Seismicity 34

4 METHODOLOGY 35

Seismic Hazard Analysis 35

Spectral Matching 40 Site Response Analysis 42

5 RESULTS AND DISCUSSION 49 Gridded Seismic Hazard Analysis 49 Spectral Matching 63 Site Response Analysis 70

6 CONCLUSIONS AND RECOMMENDATIONS 79

Conclusions 79

Recommendations 81

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REFERENCE LIST 82

Appendix

A SOIL COLUMNS 92

B SOFTWARE INTERFACE 128

C MANUAL INTEGRATION FOR PSHA 132

VITA 136

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FIGURES Figure Page

2.1 Typical Seismic Hazard Curve 10

2.2 Typical Uniform Hazard Spectrum (UHS) 10

2.3 Typical Seismic Hazard Map (NEHRP 2003) 11

2.4 Typical Deaggregation Plot 12

2.5 Typical plots to calculate site amplification factors 16

2.6 NEHRP Design Spectrum 17

2.7 Seismic source model of Al-Haddad et al. (1994) 19

2.8 Seismic source model of Abdalla and Al Homoud 2004 20

2.9 Cluster of Earthquake Records in the Iranian Region (Source: USGS NEIC) 21

2.10 Seismic Source Model of Peiris et al (2006) 22

2.11 Seismic Source Model of Musson et al. (2006) 23

2.12 Seismic source model of Aldama et al. (2009) 24

2.13 UHS from past studies for a return period of 2475 years 26

3.1 Location of U.A.E in the Arabian Gulf. (Source: Google Earth) 30

3.2 Spatial distribution of the Emirates of U.A.E. (Source: Wikipedia) 31

3.3 Tectonic Setting around U.A.E. 33

3.4 Seismicity Catalogue 34

4.1 Seismic source model for this study 36

4.2 Grid of nodes used in Gridded Seismic Hazard Analysis 39

4.3 Modulus reduction curves 43

4.4 Damping ratio curves 43

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4.5 Plot of shear wave velocity versus depth 45

4.6 Response spectra on surface and half space using LSM2270 47

5.1 Seismic curves of the eight cities of U.A.E. 50

5.2 Comparison of seismic curves for Abu Dhabi (PGA) 52

5.3 Comparison of seismic curves for Ras Al Khaimah (PGA) 53

5.4 Comparison of seismic curves for Dubai (PGA) 53

5.5 UHS for the eight cities of U.A.E. 54

5.6 Comparison of UHS for Dubai (return period - 2475 years) 54

5.7 Comparison of UHS for Dubai (return period - 475 years) 55

5.8 Contour map for 2475 year return period Peak Ground Acceleration 56

5.9 Contour map for 2475 year return period spectral acceleration at 0.2s. 56

5.10 Contour map for 2475 year return period spectral acceleration at 1s. 57

5.11 Proposed zonation of UAE based on equal increments of mapped hazard 58

5.12 UHS representing the proposed zonation of UAE 58

5.13 Deaggregation of hazard for Abu Dhabi 59

5.14 Deaggregation of hazard for Ras Al Khaimah 60

5.15 Effect of west coast fault on hazard curves 62

5.16 Matching ANG-090 response on Abu Dhabi Target Response Spectrum 64

5.17 Matching LSM2270 response on Abu Dhabi Target Response Spectrum 64

5.18 Matching GIL337 response on Dubai Target Response Spectrum 65

5.19 Matching TCU129-E response on Dubai Target Response Spectrum 65

5.20 Matching ANG000 response on Sharjah Target Response Spectrum 66

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5.21 Matching LSM2000 response on Sharjah Target Response Spectrum 66

5.22 Comparing ANG090 Original to Matched Time History 67

5.23 Comparing LSM 2270 Original to Matched Time History 67

5.24 Comparing GILL337 Original to Matched Time History 68

5.25 Comparing TCU129E Original to Matched Time History 68

5.26 Comparing ANG000 Original to Matched Time History 69

5.27 Comparing LSM2000 Original to Matched Time History 69

5.28 Response Spectra for Sharjah for Site Class C 71

5.29 Response spectra for Sharjah for Site Class D 71

5.3 Amplification factors for Sharjah for Site Class C 72

5.31 Amplification factors for Sharjah for Site Class D 73

5.32 Response Spectra for Dubai for Site Class C 73

5.33 Response Spectra for Dubai for Site Class D 74

5.34 Amplification factors for Dubai for Site Classes C and D with two input motions 75

5.35 Response Spectra for Abu Dhabi for Site Class C 75

5.36 Response Spectra for Abu Dhabi for Site Class D 76

5.37 Amplification factors for Abu Dhabi for Site Class C 77

5.38 Amplification factors for Abu Dhabi for Site Class D 77

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TABLES

Table Page

2.1 NEHRP Site Classification 18

2.2 Recurrence Parameters used by Abdallah and Al Homoud (2004) 20

2.3 Recurrence Parameters used by Musson et al. (2006) 23

2.4 Recurrence Parameters used by Aldama et al. (2009) 25

2.5 Comparison of PGAs 25

2.6 Results after using three attenuation equations on one source model 27

2.7 Results after using one attenuation equations on three source models 27

4.1 Verification Results 35

4.2 Activity parameters used in this study 37

4.3 Criteria for selecting time histories 41

4.4 Time histories selected for spectral matching 41

5.1 Spectral Accelerations at 2475 years for the eight cities of U.A.E. 50

5.2 Spectral Accelerations at 475 years for the eight cities of U.A.E. 51

5.3 Spectral Accelerations at 10000 years for the eight cities of U.A.E. 51

5.4 Comparing PGAs of this study with some of the previous hazard studies 52

5.5 Contribution of different sources to the hazard in selected cities 61

5.6 No. of boreholes for each city 70

5.7 Site amplification factors 78

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ACKNOWLEDGEMENTS

Foremost thanks and praises are to Almighty who blessed me with the

strength, capability, and knowledge to undertake and complete the research. First of all, I express my gratitude to the Department of Civil Engineering of

the American University of Sharjah for accepting me as a Research Assistant for this study.

The greatest credit of this work goes to my esteemed supervisors Dr. Magdi El-Emam and Dr. Zahid Khan who have given bulk of their precious time and

experience during this study to assist me in achieving the goals of this study. Their continuous supervision and valuable suggestions have been instrumental in

completing this research.

Special thanks to Dr. Jamal Abdalla and Dr. Mousa Attom for their occasional valuable suggestions on my research work.

I also appreciate the help of Dr. Tarig Ali for help with ArcGIS for plotting the contour maps in from results of the Gridded Probabilistic Seismic Hazard Analysis.

I am extremely thankful to the Geotechnical Department of Sharjah Municipality for their support in providing me the borehole logs of sites in Sharjah. Without their generous help, site response analysis phase of this study wouldnt have

been possible. For Dubai, I would like to appreciate the help of some private companies for

providing the borehole logs of sites in Dubai. I would also like to thank Dr. Ali Shaaban Ahmed Megahed from Abu Dhabi

Municipality for providing the borehole logs for various sites in Abu Dhabi. Lastly, I would like to appreciate the support of my family during this long

and sometimes difficult journey. By family, I mean my wife and my lovely children Ali and Amna. Special thanks to my parents for their love and support, and for instilling in me the value of learning and hard work, and providing me with the

opportunities to advance my life. My sisters have also been a great source of motivation for me during this study.

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DEDICATION

To my family:

My Parents, Wife, Sisters and two lovely Children Ali and Amna

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Chapter 1: INTRODUCTION

General Introduction

Earthquakes are one of the most devastating natural hazards faced by various

countries around the world. Recently, many governments have begun to realize the importance of managing the risk posed by the earthquakes. As part of the risk management strategies, developing countries such as U.A.E, Saudi Arabia, and Iran are beginning to develop building codes which will incorporate seismic loads consideration for the design of structures. The seismic design of structures is primarily based on Seismic Hazard Analysis and Site Response Analysis of the area.

Numerous studies have been performed to assess the seismic hazard for a particular area [1], [2]. Seismic Hazard Analyses are usually performed for rock conditions ignoring the effects of local site conditions. Hence, the results of Seismic Hazard Analysis only give a preliminary view of the seismic loads expected on the structure. Depending on what type of structure and where the structure is, the designers extract the relevant results. The analysis which would complete the seismic

design prerequisites is called Site Response Analysis. Site Response analysis is the process of quantifying the effects the local site conditions have on the seismic waves

which originate from bedrock. Site Response Analysis is one of the most critical steps in geotechnical earthquake engineering. The amplification of seismic waves due to the geological structure of a particular site has been found to be considerable by many researchers.

Some of the examples are the 1994 Northridge earthquake [3], the disastrous 1985 Mexico earthquake in which the amplification of seismic waves was five times the

ground motion from the rock [4], and the 2003 Bam earthquake [5]. The degree of the amplification caused by site conditions depends on the dynamic characteristics of the soil, the characteristics of the base rock motions, the impedance contrast between the soil profile and the base rock and the depth of semi-infinite half space [6]. Designing the structures according to the building codes applicable to the area where the structures are built is extremely important. Due to the lack of availability, some designers around the world are forced to design the structures using the procedures developed by developed countries such as U.S. and U.K. This can lead to extreme consequences because the design of structures using inapplicable studies

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would be unreliable. Moreover, the intensity of the effects of earthquakes largely depends on the types and strength of structures present in the area of shaking. The

recent earthquakes in Chile and Haiti suggest that although the earthquakes were of similar intensities, the casualties in Haiti were much greater than those in Chile. Many studies have attempted to perform Seismic Hazard Analysis for UAE [7, 8]. But significant variations exist amongst the results of those studies. Two of the studies in the past have attempted to perform site response analysis for Dubai and Sharjah [9, 10]. But the results of these cannot be reliable due to various shortcomings discussed in the literature review section. Considering the substantial development in cities such as Dubai and Abu Dhabi and the ambiguity faced by the designers in U.A.E. in choosing the seismic hazard, a comprehensive study of seismic hazard analysis is needed. Moreover, the lack of studies on site response analysis for U.A.E. also justifies a study on site response analysis. Considering the time driven nature of projects in U.A.E., not every project performs site specific response analysis. Hence this study aims at performing a comprehensive Probabilistic Seismic Hazard Analysis for U.A.E. to assess the hazard posed by the earthquake activity around U.A.E.

Moreover, numerous site response analyses would be performed for different parts of U.A.E. to provide the designers with guidelines to incorporate site effects without performing site response analysis for the project. The results and conclusions of this study would contribute significantly towards developing the regional building codes

for different cities of U.A.E.

Problem Definition

In the last 20 years, U.A.E. has undergone tremendous development in terms

of its infrastructure, including mega projects like the Palm Island, Dubai metro, and Burj Khalifa. Although historically U.A.E. has not been hit by a major earthquake, the frequent seismicity in the surrounding areas such as Iran and Oman can pose a significant threat to the infrastructure of U.A.E. Recent earthquakes of considerable magnitudes in U.A.E. and Oman have also enhanced the need for risk management plans for major cities of U.A.E. [11, 12]. The advancement in seismic networks of U.A.E. has enabled the recording of seismic activities which were previously unknown and underestimated.

Tall buildings have high natural periods. The seismic waves coming from long distances also vibrate at long periods. If the natural periods of the structures match the

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predominant periods of the long distance seismic waves (i.e. resonance), the results could be catastrophic. Therefore, even though the seismic activity in Iran is at a

considerable distance, the long period and high intensity waves are a concern for integrity of the sky scrapers in U.A.E.

Moreover, the seismic waves are subject to amplification due to the different types of soils underlying the surface. The amplification due to site effects causes the

waves to increase the ground motion. Several studies have attempted to evaluate the risk of U.A.E. in general and

major cities in particular. These studies presented significant variations in their results and emphasized on calculating general seismicity of the area or for particular cities only. The discrepancies in their findings are attributed to several shortcomings as discussed in the Literature Review chapter. Considering substantial development in the region especially in Dubai and Abu Dhabi and considerable ambiguity faced by the designers in choosing the seismic hazard, some municipalities in UAE are at

different stages of developing building codes. In light of the above challenges, a comprehensive seismic hazard analysis based on systematic approach is urgently

required.

Objectives of Study

To prepare a homogenized seismicity catalogue for U.A.E.

To develop a representative seismic source model for U.A.E.

To select the appropriate Ground Motion Prediction Equations, suitable for

regional geology. In case the needed equations are not available, use the widely accepted equations and the recently developed equations for world

wide applications.

To perform Gridded Probabilistic Seismic Hazard Analysis to develop Seismic

Curves and Uniform Hazard Spectra for different areas of U.A.E.

Develop contour maps for PGA and Spectral Accelerations at 0.2s and 1s for

return period of 2475 years (2% of exceedence in 50 years) To create a suite of spectrally matched ground motion time histories.

To develop site amplification factors using the site classifications provisions

of NEHRP (2003) specifically for use in U.A.E.

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Available Data and Collection

Seismic Hazard Analysis

The first set of data required to perform seismic hazard analysis was the seismicity in and around the study area. The seismicity records were retrieved from various databases and catalogues available online or in the literature [13, 14, 15, 16, 17, 18]. Some of these resources contained historic seismicity along with the instrumental seismicity. In this study, only the instrumental seismicity was used. The final collection of seismicity records was cleaned up to avoid repetition of any seismic

events. Along with seismicity, plate tectonics and geology studies were also consulted [19, 20, 21]. These were required to develop the recurrence parameters for the Gutternberg-Richter relationship and for developing the seismic source model to be used in seismic hazard analysis. Ground Motion Prediction Equations (GMPEs) were needed to be assigned to the seismic sources. These were acquired from the Pacific Earthquake Engineering Research (PEER) center studies along with other studies such as [22, 23]. The commercially available software, EZFRISK was purchased from Risk Engineering Inc. for performing the Gridded Seismic Hazard Analysis. Another

computer program called ArcGIS was obtained to plot the seismic hazard contour maps of UAE

Spectral Matching

Several strong ground motion time histories were obtained from Pacific Earthquake Engineering Research (PEER) database. PEER database has the option of using criteria such as magnitude, distance or PGA to select the time histories. So, the time histories were selected based on the deaggregation results from seismic hazard analysis of Dubai, Sharjah and Abu Dhabi. A computer program of SeismoSignal, available for free from SeismoSoft Ltd. for research purposes on the web, was used to

obtain various strong motion parameters of time histories for comparing the original and matched time histories. Moreover, RSP Match EDT, commercially available

software, was procured and used to perform spectral matching on ground motion time histories.

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Site Response Analysis

To correlate the dynamic properties of soils at different site selected in UAE major cities, reports of soil investigation conducted at these sites are required. Numerous boreholes reports are available with the municipalities of the emirates of U.A.E. However, due to complications in formal procedures and lack of cooperation

from some municipalities, it was not easy to acquire many boreholes from Emirates such as Umm Al Quwain, Fujairah and Ras Al Khaimah. Around 100 boreholes from Dubai, Sharjah, Abu Dhabi and Ajman were collected. Most of the boreholes from Dubai and Abu Dhabi varied from 15 to 30 m depth. However, some boreholes from Sharjah are extended to 50m depth. Several studies of correlations between SPT-N values and shear wave velocities are available in literatures [24, 25, 26, 27]. These correlations were used to correlate the data from the boreholes to the soil dynamic properties required for site response analysis phase. To estimate the shear wave

velocities for the bedrock, three studies were used to correlate the Unconfined Compression Strength (UCS- MPa) [28, 29, 30]. The computer program SHAKE 2000 is used extensively for performing 1-D site response analysis in this study.

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Organization of Thesis

This thesis is prepared for two major phases of this study. First phase was the Gridded Seismic Hazard Analysis and the second was Site Response Analyses.

Chapter 2 presents the general background and review of some of the subjects of this thesis such as Source Zonation, Ground Motion Prediction Equation and Recurrence Parameters. Chapter 2 also reviews the regional seismic hazard and site response studies performed. Moreover, the results of previous studies are compared and

reasons for discrepancies in the results are discussed. Chapter 3 describes the tectonic setting, geology and seismicity of the study area along with the geographic setting of UAE.

Chapter 4 presents the methodology used for Gridded Seismic Hazard Analysis and Site Response Analyses. The computer programs used for the two

phases are also described. Format of results from Gridded Seismic Hazard Analysis and Site Response Analyses have been presented. Chapter 5 presents the results of Gridded Seismic Hazard Analysis in the form of Uniform Hazard Spectra, seismic hazard curves and contour maps. Deaggregation

graphs for cities of Abu Dhabi and Ras Al Khaimah have been plotted. Comparison of results between this study and past studies has been made. Matched time histories

along with their response spectra have been plotted to compare the results before and after matching. Response spectra and amplification factors for Sharjah, Dubai and Abu Dhabi have been plotted to show the results of site response analyses performed for 100 boreholes. Chapter 6 summarized most important conclusions made in this research as well as suggestions for further research.

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CHAPTER 2: LITERATURE REVIEW

Background

Probabilistic Seismic Hazard Analysis

The time, size and location of occurrence of earthquakes in future cannot be predicted with certainty. The concept of probability is incorporated in Seismic Hazard

Analysis to analyze factors of time, size and location with the uncertainty involved. Cornell 1968 [31] developed Probabilistic Seismic Hazard Analysis for the estimation of ground motion. The probabilistic approach considers all possible magnitude earthquakes, at all possible distances from all possible source zones with consideration given to likelihood of each combination. The ground motion obtained from this approach has a specified probability of exceedence. Uncertainty in the

events of earthquake occurrence and the associated hazards of damaging ground motion is inherent. The reliability of results from this approach depends on factors

described in the following. These factors are required for performing the Probabilistic seismic hazard analysis [31, 32, 33, 34, 35, 36]. They include identification of sources, establishment of recurrence relationships, magnitude distribution and average rate of occurrence for each source, selecting attenuation relationship and computing

the Uniform Hazard Spectrum and site hazard curve.

Identification of seismic sources

The identification of seismic source zones is based on the interpretation of

tectonic, geological and seismological data. Describing the whole process of developing the seismic model is a broad topic; therefore, the identification process is

briefly described in this section.

Seismicity around a region of interest is grouped into many seismic sources. These sources are identified based on the spatial distribution of earthquakes. Seismic sources can be faults, areas and points. Area sources are widely preferred where the accurate knowledge of line and point sources is not conclusive. Once the sources close to the site of interest are identified, uniform probability distributions within the

sources are assigned to each source i.e. earthquakes can occur at any point within the source zone.

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Recurrence parameters for seismic sources

Determining recurrence parameters is a major difference between the deterministic and probabilistic approaches of Seismic Hazard Analysis. The uncertainty in size and time of occurrence of future earthquakes is characterized through a recurrence relationship assigned to each source. A recurrence relationship specifies the average rate at which an earthquake of some size will be

exceeded. A linear relationship was observed by Gutenberg and Richter (1944) when the logarithm of annual rate of exceedence was plotted against earthquake magnitude

(Equation 2.1).

Log N = a bM [2.1]

Where M is the earthquake magnitude and N is the number of earthquakes having magnitude greater than or equal to M. a and b are constants where a indicates the number of earthquakes greater than magnitude zero, and it depends on

the number of events, the size of source region and the number of years of seismic date. b is the relative number of small magnitude to large magnitude earthquakes [7].

Selection of Ground Motion Prediction Equations (GMPEs)

Ground Motion Prediction Equations (GMPEs or attenuation equations) are used to predict the ground motion produced by an earthquake at a certain distance

from epicenter or hypocenter. The GMPEs are constrained by many factors such as the distance from epicenter, distance from hypocenter, type of fault rupture mechanism, damping of transmitting media, and characteristic of the soil of the site if included [38, 39]. These ground motion prediction equations are developed from the regression of accelerations recorded at different distances. The uncertainty in the regression is quantified by the standard deviation of the peak ground acceleration. Majority of the attenuation relations relate the peak ground acceleration to the magnitude of an earthquake (M), and the distance (R) from epicenter/hypocenter. Some attenuation relations also include other parameters which are used to

characterize the earthquake source, wave propagation and local site conditions. Typical form of the GMPE relationships is given by

ln Y = C +CM+CM +C lnR + C expCM + CR + fsource + fsite [2.2]

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The values of coefficients (C1, C2, C3 etc) vary depending on which ground motion parameter (Y) is being predicted. These coefficients are computed by performing the regression analysis on the particular ground motion parameter. Typically, these coefficients represent the relationship between the ground motion parameter, spectral period, and the variable (magnitude or distance). The relationship between the variable and ground motion parameter is also determined using the

regression analysis. These relationships could be linear, parabolic or exponential. GMPEs are then assigned to different seismic sources. More than one equation can be

assigned to a seismic source.

Results of Probabilistic Seismic Hazard Analysis

Main result of PSHA is seismic hazard curve that relates the annual rate of exceedence (or return period) to any spectral acceleration (such as PGA). Figure 2.1 presents a typical seismic curve for Peak Ground Acceleration which is the spectral

acceleration at spectral period of zero (0) second. In addition to seismic curves, a plot which shows different spectral accelerations for different spectral periods at a common rate of exceedence is called Uniform Hazard Spectrum (Figure 2.2). Results of PSHA are also plotted as ground motion hazard maps such as the one produced by

the USGS for the National Earthquake Hazards Reduction Program (Figure 2.3). Typically, PGA and Spectral acceleration for 0.2s and 1s are plotted on these maps to

facilitate designers in choosing ground motion amplitudes for a particular return period i.e. a particular probability of exceedence.

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Figure 2.1 Typical Seismic Hazard Curve

Figure 2.2 Typical Uniform Hazard Spectrum (UHS)

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Figure 2.3 Typical Seismic Hazard Map (NEHRP 2003)

PSHA deaggregation

The dynamic analysis of a structures, engineering models and computer codes require an earthquake acceleration time history representative of local conditions from

the results of PSHA. A procedure called Deaggregation is used to determine the dominant distance and magnitude from the results of PSHA. Many studies have

described the process of Deaggregation [34, 39, 40, 41, 42, 43]. Figure 2.4 shows a typical deaggregation plot. For different spectral accelerations at different spectral periods, the peaks of histogram will change. The magnitude and distance range that represents the peak in histogram is used to select the earthquake time history for

structure specific dynamic analysis. In deterministic hazard analysis, selecting a representative earthquake for

dynamic analysis could be difficult because deterministic approach considers the effect of a single scenario earthquake at a site. On the other hand, the probabilistic approach considers all possible combinations of earthquake magnitudes and distances in order to determine which one contributes the greatest to a particular hazard level.

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Figure 2.4 Typical Deaggregation Plot

Description of EZFRISK

EZFRISK is commercially available software by Risk Engineering Ltd which

implements the Cornell-McGuire approach. Seismic Hazard calculations of EZFRISK represent an application of the total probability theorem. The process of entering the input data is extremely user friendly. Constructing a seismic zone model and assigning the recurrence parameters on the seismic zones are relatively simple steps.

The program has a big database of predefined Ground Motion Prediction Equations which is frequently updated. EZFRISK is capable of delivering various results such as

seismic curves for different spectral periods, uniform hazard spectra for numerous return periods and deaggregation for several combinations of magnitude and distance. Time consumed for a single site seismic hazard analysis performed by EZFRISK was small which enabled the Gridded Seismic Hazard Analysis to be performed within reasonable amount of time.

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Time Histories for Site Response Analysis

Designing of strong motion time histories is an essential part of soil structure

interaction done by geotechnical earthquake engineers, and nonlinear dynamic

analysis of critical structures done by structural engineers. The earthquake time histories are selected and adjusted to match the target response spectrum of a particular site. The target response spectrum is part of the results produced by Probabilistic Seismic Hazard Analysis (PSHA). There are two methods for designing the strong ground motion time histories: scaling ground motion and spectral matching [44]. Both methods involve the use of past natural or synthetic time histories. Scaling ground motions is conducted by multiplying the natural or synthetic acceleration values at all-time intervals by certain factor. Though the natural phasing

of the recorded ground motion and peaks and troughs in the spectral shape are maintained, getting the average response spectrum shape to match the target response

spectrum would be a major challenge using this method. In the second method (i.e. spectral matching method) the frequency content of an earthquake time history (natural or synthetic) could be modified to match the response spectrum of that target time history (i.e. target response spectrum). Various methods of spectral matching have been described by Preumont (1984) [45]. Generally, there are two approaches of spectral matching: frequency domain and time

domain. The first approach involves replacing the Fourier amplitude spectrum of the initial time history with a Fourier spectrum which is consistent with the target

spectrum based on random vibration theory. However, the later involves adding wavelets to the initial time history. Time domain approach is a better option because of good convergence properties and preserving the non-stationary character of the original time histories. Several popular computer programs such as RSP Match EDT and SeismoMatch use the time domain approach for computing the modified time

histories. Spectral matching is a preferred option over the scaling of ground motions because lesser hassle is involved in achieving a satisfactory comparison of the

response spectra of both original and target time histories.

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RSP Match EDT

RSP match EDT is a Windows based program that uses a time domain approach to modify the original time histories to make them compatible with a target

response spectrum. This program was developed by Abrahamson (1992) [46] and applies the methodology of Lilhanand and Tseng (1987, 1988) [47, 48]. Different modification models are used to perform the modification of time histories. This helps in preserving the non-stationary phasing of the original time history as mentioned above. Different sources are used to get the recorded strong ground motion time histories for spectral matching. This program has its own specific format in which the

original acceleration-time history could be input into it. Hence, the ground motion time histories from any database are converted to a RSP Match EDT compatible

format before being used scaled. The formatting process is done within the program option that is available in RSP Match EDT.

Site Response Analysis

Site response analysis is the process of analyzing the seismic hazard at a micro level. Soil conditions are considered to quantify the alterations caused by the soil on

seismic waves propagating from the bed rock. The results of site response analysis primarily depend upon the type of soils, and the soil profile configuration. Hence, differences would be encountered in the results of site response analysis from one site to another. Therefore, a site specific response analysis is highly recommended for

every structure that intended to be built. However, site response analysis is expensive to perform making it impractical to be done for each individual structure site. To help

the designers save the cost of performing site specific response analysis, building codes include site amplification factors which are used to quantify the amplification or deamplification of the seismic waves due to the soil conditions. For example, the National Earthquake Hazard Reduction Program [49] has site amplification factors estimated for the North American region. The local site affects the important characteristics of the surface ground

motion such as amplitude and frequency content. The intensity depends on the properties of the subsurface materials, site geometry, and distance of earthquake [50, 51] and on the characteristics of the bedrock ground motion itself [39]. Site Specific Response Analysis is generally divided into three main tasks [52].

15

Characterization of soil properties in the site is the first, major, and most expensive task. Geophysical or geotechnical investigation is used to determine the

dynamic properties of the soil by laboratory or field methods such as Resonant Column Test, Cyclic Triaxial test, Seismic Refraction or Spectral Analysis of Surface Waves (SASW). The other two tasks are: the selection of bedrock acceleration-time Histories, and conducting the ground Response Analysis.

The ground response analysis (usually one dimensional) is performed for the specified site using the bedrock time histories selected in the second task to compute

the time histories propagated to the ground surface. The ratio of response spectra of the time histories measured at the ground surface to the input motion response spectra is used to quantify the local site effects (Figure 2.5). The use of one dimensional ground response analysis is most suitable for modern seismic analysis for many reasons. Software packages for conducting one dimensional site response analysis are available in abundance in personal computers

and have been tried tested and verified. They are believed to produce conservative results, because majority of the design projects in the past which were designed using this methodology have survived strong earthquakes. The two major assumptions in one dimensional analysis are: (1) soil layers are horizontal and extend to infinity, and (2) the ground surface is level and the shear waves propagate vertically upwards. These assumptions can be justified for various reasons such as the horizontal ground motions are more important than vertical ground motions, soil properties generally vary more in the vertical direction than in the horizontal directions and many more

reasons which make the use of one dimensional analysis viable for use in the site response analysis [52]. One dimensional site response analysis is typically performed as either equivalent linear or non linear analysis.

16

Figure 2.5 - Typical plots to calculate site amplification factors

Estimating Site Amplification Factors

Performing a Site Specific Response Analysis for every structure is not practical. Therefore, typical buildings and other structures often employ the site amplification factors to develop a site specific design response spectrum (Figure 2.6). These factors are provided in modern building codes as short and long period acceleration amplification factors.

The development of site amplification factors involves Site Response

Analyses on a large scale. For a particular region, the site amplification factors are

determined by using the soil profiles of various sites in that region. The use of the

average shear wave velocity in the top 30 m of the soil profile (Vs30) is commonly used to classify the soil profiles [53, 54, 55, 56]. This type of soil classification is used for its simplicity and making the soil classification uniform (Table 2.1).

17

Figure 2.6 NEHRP Design Spectrum

18

Table 2.1 NEHRP Site Classifications

Site Class Description A Hard rock with measured shear wave velocity, vS > 5000 ft/sec (1500 m/s) B Rock with 2,500 ft/sec < vs 5000 ft/sec (760 m/s < vs 1500m/s C Very dense soil and soft rock with 1,200 ft/sec < vs 2,500 ft/sec (360 m/s

< vs 760 m/s) or with either N > 50 or su > 2,000 psf (100 kPa) D Stiff soil with 600 ft/sec vs 1,200 ft/sec (180 m/s vs 360 m/s) or

with either 15 N 50 or 1,000 psf su 2,000 psf (50 kPa su 100 kPa)

E A soil profile with vs < 600 ft/sec (180 m/s) or with either N < 15, su < 1,000 psf, or any profile with more than 10 ft (3 m) of soft clay defined as soil with

PI > 20, w 40 percent, and su < 500 psf (25 kPa) F Soils requiring site-specific evaluations:

1. Soils vulnerable to potential failure or collapse under seismic

loading such as liquefiable soils, quick and highly sensitive clays, collapsible weakly cemented soils. Exception: For structures

having fundamental periods of vibration less than or equal to 0.5 second, site-specific evaluations are not required to determine spectral accelerations for liquefiable soils. Rather, the Site Class may be determined in accordance with Sec. 3.5.2, assuming liquefaction does not occur, and the corresponding values of Fa and Fv determined from Tables 3.3-1 and 3.3-2.

2. Peat and/or highly organic clays (H > 10 ft [3 m] of peat and/or highly organic clay, where H= thickness of soil)

3. Very high plasticity clays (H > 25 ft [8 m] with PI > 75) 4. Very thick, soft/medium stiff clays (H > 120 ft [36 m]) with su <

1,000 psf (50 kPa)

19

Review of regional studies

Seismic Hazard Analysis

Many studies have attempted to estimate the seismic hazard for the Arabian Peninsula region in the past. These studies have several shortcomings and generalizations which will be discussed in this section. Due to the generalizations, the results of these studies have significant variations, and all of them draw different conclusions on the regional seismic hazard.

The earliest study was performed by Al-Haddad et al. (1994) [57]. Although the studys focus was on Saudi Arabia, the results were mapped over the whole Arabian Peninsula. The study used a ground motion prediction equation which was

derived for Western North America [58] but the coefficients for that equation were taken from Thenhaus et al. (1986) [59]. The seismic source model of this study is presented in Figure 2.7. The figure shows partial seismic model relevant for the area covered in this study. The large source which combines the Zagros region with Makran region is not justified because two different regions have been combined into one seismic source. The results of this study indicated that the PGA values

corresponding to a return period of 475 years for the cities of Abu Dhabi and Dubai are less than 0.05g. Hazard for U.A.E. was estimated by mapping the hazard of Saudi Arabia which could produce unreliable results for U.A.E.

Figure 2.7 Seismic source model of Al-Haddad et al. (1994)

20

A Global Seismic Hazard Assessment Project was completed in 1999 for generating the PGA maps (return period of 475 years) for Europe, Africa and Middle East [60] The results of this study suggested over conservative values of PGA of 0.32g and 0.24g for Dubai and Abu Dhabi respectively. The results were deduced

from the calculated hazard at Dead Sea and Zargos area without performing actual seismic hazard analysis for sites in UAE. Abdallah and Al Homoud (2004) [7] performed the pioneering seismic hazard assessment specifically for United Arab Emirates and its surroundings. The seismic zones considered in this study are shown in Figure 2.8 and the recurrence parameters are given in Table 2.2. This study used one attenuation equation for all the seismic

sources adopted from Zare (2002) [61]. The estimated PGA from this study for Dubai and Abu Dhabi for a return periods of 475 years are 0.15g and 0.10g respectively.

Figure 2.8 Seismic source model of Abdalla and Al Homoud 2004 [7]

Table 2.2 - Recurrence Parameters used by Abdallah and Al Homoud (2004) [7]

Seismic Source Fault

Mechanism Mmin Mmax at Mmin - beta Main Zagros Thrust Region Area 4 7 194984 2.81 North East Arabian Gulf Region Area 4 6 1698 2.16 Northern Emirates Region Area 4 6 104.71 1.842 Lut Region Area 4 6.8 37154 2.56 Central Iran Region Area 4 7.2 6026 2.05 Makran Region Area 4 6.7 0.347 1.842 South East Arabian Gulf Region Area 4 7.5 47.86 1.842

Central Iran Region

Lut Region

Makran Region

South East Arabian Gulf Region

Northern Emirates Region

North East Arabian Gulf Region

Main Zagros Thrust Region

21

This study indicated larger seismic hazard in comparison to most recent studies. The difference in the results are attributed to a source zone (region III Northern Emirates Region) with very high activity parameter of (12.02 at Mmin = 4). In addition to that, this seismic source seems to inflate the seismicity in U.A.E. because this seismic source combines the Southern Zagros region with the northern region of U.A.E. As a result, the probability of a high magnitude earthquake occurring

in the northern emirates region is similar to that of Southern Zagros region. This is contrary to the cluster of earthquakes records shown in Figure 2.9 where it clearly shows that barely any major earthquake has occurred close to the northern emirates region. In addition, the high standard deviation of the attenuation equation used in this study also contributes to larger seismic hazard [62].

Figure 2.9 Cluster of Earthquake Records in the Iranian Region (Source: USGS NEIC)

Sigbjornsson and elnashai 2006 [74] performed the seismic hazard for Dubai only. They adopted the seismic source zonation of Tavakoli and Ghafory (1999) [1] in addition to the inclusion of Dibba and West Coast Faults. They used attenuation equations by Ambraseys et al. 1996 [63] and Simpson 1996 [64] for all the sources in

22

the seismic source model. The results were presented in the form of hazard curves for PGA and Uniform Hazard Spectra for return periods of 975 and 2475 years for Dubai. The PGA values of this study for Dubai were 0.16g and 0.22g for return periods of 475 and 2475 years respectively. In comparison, the PGA at 475 years is slightly higher than that of Abdalla and Al-Homoud 2004 [7] and significantly higher than some of the other studies. The larger values of hazard are possibly because of the

inclusion of west coast fault as a very active source.

Peiris et al 2006 [8] performed the seismic hazard study for Dubai and Abu Dhabi beside other Arabian cities by using five different ground motion prediction equations. Equations by Atkinson and Boore 1997 [65] and Dahle et al. 1990 [66] were used for the Arabian Stable Craton whereas equations by Ambraseys et al. 1996 [63] and Sadigh et al. 1997 [67] were used for Zagros and Makran regions. The seismic source zonation of this study is similar to that of Al Haddad et al. 1994 [57] (Figure 2.10) in addition to regional faults like Dibba and West coast. The results in this study were presented in the form of seismic curves for some cities and UHS for two return periods for Dubai only. The PGA values estimated for Dubai and Abu

Dhabi for a return period of 475 years were 0.06g and 0.05g respectively.

Figure 2.10 Seismic Source Model of Peiris et al 2006 [8]

23

The study by Musson et al. 2006 [68] presented the results of seismic hazard assessment of UAE that was performed by British Geological Survey on behalf of the

Government of Dubai. Although significantly different tectonic nature of different source zones were appreciated, only two attenuation equations were used for all the seismic sources in their model (Figure 2.11). Table 2.3 presents the recurrent parameters used in that study. Ambraseys et al 1996 [63] was used for the computation of spectral accelerations, whereas Ambraseys 1995 [69] was used for predicting Peak Ground Accelerations (PGA). The results were presented in the form of PGA maps and Uniform Hazard Spectra for the seven emirates for return periods of 475, 1000 and 10000 years. The results indicated a PGA of 0.05g for Dubai for a return period of 475 years. These results are similar to those of Peiris et al. 2006 [8] and Al Haddad et al. 1994 [57].

Figure 2.11 - Seismic Source Model of Musson et al. 2006 [68]

Table 2.3 - Seismicity Parameters used by Musson et al. 2006 [68]

Seismic Source Fault Mechanism Mmin Mmax at Mmin - beta DIBB Strike slip 4 5 0.0139 1.428 EHOS Strike slip 4 5.1 0.0832 1.7731 FORE Reverse 4 5.8 0.525 2.464 MUSP Strike slip 4 4.9 0.007 2.602 OMOB Strike slip 4 5.5 0.0139 1.428 QESH Reverse 4 6.4 0.851 1.704 ZEMI Strike slip 4 5.8 0.0794 1.658 ZMFF Reverse 4 6.5 1.023 1.59

24

Husein Malkawi et al. 2007 [70] presented seismic hazard assessment for major cities of UAE. The seismic source model of this study consists of a single source which includes the Makran Region, Zagros Region and parts of the Arabian Craton. A single ground motion prediction equation of Atkinson and Boore 1997 [65] was used. The results of this study are considered highly unreliable considering the uncertain zone model and superseded ground motion prediction equation.

The latest study for U.A.E. was presented by Aldama et al. 2009 [71]. The study focused on three cities: Dubai, Abu Dhabi and Ras al Khaimah. A total of 20

seismic source zones were considered (Figure 2.12), and seven attenuation equations including a New Generation Attenuation (NGA) equation were used for different seismic source zones. The recurrence parameters used for various source zones are given in Table 2.4. The results were presented in the form of uniform hazard spectra and hazard curves for the three cities for different return periods. The results are in agreement with the findings of Peiris et al 2006 [8] and Musson et al. 2006 [68]. This study did not provide seismic hazard assessment for other parts of UAE. Shama 2011 [72] presented a seismic hazard assessment for a site in Dubai. This study used many attenuation models for different seismic sources. Many local faults such as West coast and Dibba were considerd as very active and hence included in this study. The study presented significantly higher values of hazard in Dubai with PGA values of 0.17g and 0.33g for a return period of 475 and 2475 years respectively. The seismic catalogue used in the study was based on the database of IRIS [73] which includes many events that are dislocated and are not present in the original database

cross referenced by IRIS 2008 [73].

Figure 2.12 Seismic source model of Aldama et al. 2009 [71]

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Table 2.4 Seismicity parameters used by Aldama et al. 2009 [71]

Seismic Source Fault Mechanism Mmin Mmax at Mmin beta High Zagros Reverse 4 7.3 9.56 1.91 South Zagros Reverse 4 6.9 2.65 1.59

Oman Mountains Strike Slip 5 6.8 0.1478 2.5158 Makran Top Intraslab 4 6.8 1.07 1.63

Makran Bottom Right Interface 4 8.5 2 1.796 Makran Bottom Interface 4 8.5 2 1.796

Zagros Makran Transition Strike slip 5 7 0.1892 2.4946

The review of all the studies presented in the preceding section indicates that

their results have significant variations. Table 2.5 shows the comparison of PGA for a return period of 475 years of some of the above mentioned studies for Dubai. Figure 2.13 shows the Uniform Hazard Spectra for a return period of 2475 years from three of the previous studies. These two sets of data clearly show variations in the results

presented by previous studies. The reasons for these contradictions can be attributed to the use of different seismic source zones, different activity parameters assigned to

those source zones and the use of different attenuation equations. In the following section, a parametric study is performed to elaborate the reasons behind variations in the previous studies.

Table 2.5 - Comparison of PGAs

Study PGA-return period of 475 years for Dubai Al-Haddad et al. 1994 [57] < 0.05g

Abdallah and Al Homoud 2004 [7] 0.15g Sigbjornsson and elnashai 2006 [74] 0.16g

Peiris et al 2006 [8] 0.06g Musson et al. 2006 [68] 0.05g Aldama et al. 2009 [71] < 0.05g

Shama 2011[73] 0.17g

26

Figure 2.13 UHS from past studies for a return period of 2475 years

Reasons for contradictions in past studies

The contradictions in the results of the previous studies can be attributed to the three main steps of Probabilistic Seismic Hazard Analysis i.e. seismic source model, activity parameters assigned to the source model and attenuation equations. In this

section, the results of a parametric study are presented to illustrate the effect of using different zones, activity rates and attenuation equations. EZFRISK by Risk

Engineering is used to perform the seismic hazard calculations for two scenarios. In the first scenario, the seismic source zones and activity rates are kept constant and three different equations are used. In the second scenario, single attenuation equation will be used for three different seismic source models and activity parameters

Same Seismic model, but different attenuation equations

The seismic source model from Aldama et al. 2009 [71] presented in Figure 2.12 was used with three different equations. Not all the seismic sources were

extracted from the study because these have been found to be most critical for the hazard contribution. The hazard analysis was performed for Dubai.

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3 3.5 4

Spec

tra

l Acc

eler

atio

n (g)

Period (s)

Aldama et al. 2009

Sigbjornsson and elnashai

(2006)

Peiris et al (2006)

27

The Attenuation equations that were used for this analysis are as follows: Abrahamson and Silva 1997 [22] Ambraseys et al. 1996 [63] Sadigh et al. 1997 [67]

The results of this analysis are given in Table 2.6 for the Peak Ground Acceleration for the return period of 2475 years.

Table 2.6 Results after using three attenuation equations on one source model

Attenuation Equation PGA Ambraseyes et al. 1996 [63] 0.08272g

Abrahamson and Silva 1997 [22] 0.1091g Sadigh et al.1997 [67] 0.0715g

Although the functional forms of these equations are similar, the table

indicates that using different attenuation equations can produce different results. These results could vary significantly if other equations are also considered without considering their applicability and constraints

Same attenuation equation but different seismic models and activity parameters

Three different seismic source models of Abdallah and Al Homoud 2004 [7], Musson et al. 2006 [68] and Aldama et al. 2009 [71] are used in this step. These seismic source models are presented in Figures 2.8, 2.11 and 2.12 respectively. Their seismicity parameters are given in Tables 2.2, 2.3, and 2.4 respectively. The

attenuation equation used for this iteration was Abramson and Silva 1997 [22] for rock sites and the results with different source models are presented in Table 2.7. It is

evident from Table 2.7 that variations in the results of past studies are due to the source models and activity rates

Table 2.7 - Results after using one attenuation equations on three source models

Attenuation Equation Seismic Source Model PGA Abrahamson and Silva 1997 [22] Aldama et al. 2009 [71] 0.114g Abrahamson and Silva 1997 [22] Musson et al. 2009 [68] 0.062g Abrahamson and Silva 1997 [22] Abdallah and Al Homoud 2004 [7] 0.2g

28

Other emirates of U.A.E. are developing at a fast pace and many major infrastructures are being built in other cities such as Abu Dhabi, Ajman and Sharjah and even in small towns. The designers in these emirates do not have proper guidance on calculating seismic loads. Therefore, there is a need for a new comprehensive Gridded Seismic Hazard Analysis for U.A.E.

Spectral Matching

The spectral matching is the process of matching historical ground motion

time histories to the Uniform Hazard Spectra of a particular area resulting from Seismic Hazard Analysis for that area. Sigbjornsson and Elnashai 2006 [74] have presented synthetic time histories in their study for dynamic analysis for Dubai only. Moreover, no spectral matching was performed. This study will aim to create a suite of spectrally matched time histories for the major cities of U.A.E. such as Dubai, Sharjah and Abu Dhabi by performing spectral matching on UHS of Dubai, Sharjah and Abu Dhabi

Site Response Analysis

Two studies have been performed on the consideration of local site effects for

U.A.E. None of these studies present the site amplification factors, similar to that of NEHRP provisions, which can be used as a general guideline for the development of design spectrum (Figure 2.6). Balwan 2008 [10] performed site response analyses for various sites of Sharjah using a total of 140 boreholes logs selected at various sites in Sharjah. The study of Al Bodour 2005 [75] was used to obtain the PGA map for United Arab Emirates. Single acceleration time history was used for all the sites. The amplification potential of Sharjah was given in the form of zonation maps for PGA. Spectral acceleration at different periods was not considered in assessing the amplification. The time history was selected because the PGA of this recording was

within the range of PGA given by Al Bodour 2005 [75] for Sharjah. No Spectral matching was performed for any Uniform Hazard Spectrum.

In another study, Ansal et al. 2008 [9] developed microzonation maps for site conditions of Dubai. This study presents amplification factors of different areas of Dubai after performing site response analyses using different borehole logs. The input ground motions were based on the results of the seismic hazard assessment for return

periods of 475 and 2475 years. A total of 1094 borings from the city of Dubai were

29

used to determine the variation of shear wave velocities. Correlations between shear wave velocity and the number of blows from SPT tests were adopted. The scaling of

time histories was simply based on the Peak Ground Acceleration of the time histories, and not on the spectral matching. Consequently, the time histories did not exactly represented the hazard spectra for the sites. Moreover, this study used unreliable damping ratio and shear modulus reduction curves which can produce

significant offset in the results [76]. This study presents larger degradation in dynamic properties for rock than for clays which is in disagreement with the findings of [6, 39, 77]. Despite the influence of soil conditions being very critical in earthquake design, not much effort has been made on site characterization of the major cities of U.A.E to account for the seismic wave amplification. None of the two studies described above can be relied on due to their shortcomings. Therefore, there is scope for a new site response analysis study for major cities of U.A.E. This study aims at characterizing the major cities of U.A.E. according to the amplification intensity of the soils in respective cities by performing site response analysis on numerous

boreholes. The results in the format of site amplification factors for major cities of U.A.E. would be easier to apply by the practical designers in U.A.E.

30

CHAPTER 3: STUDY AREA: GEOLOGY, TECTONICS AND SEISMICITY OF U.A.E.

Study Area

U.A.E. is a small country located in the southeast of Arabian Peninsula in Southwest Asia on the Persian Gulf covering an area of approximately 83,600 km2 (Figure 3.1). The country comprises of seven emirates with Abu Dhabi being the capital. The spatial distribution of the seven emirates is shown in Figure 3.2. Although Abu Dhabi has a large area, majority of the infrastructure is located in the northern region of Abu Dhabi. Even in other major cities such as Sharjah, Dubai and Ajman, the developed area is relatively small, and covers the western side of these emirates bordering the Persian Gulf. The Arabian Peninsula is not considered as active seismically. However,

recent shakings of the neighboring areas such as Oman and areas such as Dibba have raised the awareness of a potential hazard to UAE [11, 12]

Figure 3.1 Location of U.A.E in the Arabian Gulf (Source: Google Earth)

31

Figure 3.2 Spatial distribution of the Emirates of U.A.E. (Source: Wikipedia)

Geology

The geology of the United Arab Emirates, and the Arabian Gulf area, has been substantially influenced by the deposition of marine sediments associated with

numerous sea level changes during relatively recent geological time. With the exception of mountainous regions shared with Oman in the north-east, the country is

relatively low lying; with near-surface geology dominated by Quaternary to late Pleistocene age, mobile Aeolian dune sands, and sabkha/evaporate deposits. Conditions in Dubai area essentially consist of a linear coastline dissected by channels or creeks. Superficial deposits consist of beach dune sands together with marine sands and silts. In addition, wind erosion, capillary action and evaporation has led to extensive sabkha deposits in certain areas, notably around the creeks. These superficial deposits are underlain by alternating beds of calcarenite, carbonate

sandstone, sands and cemented sands.

32

Regional Tectonic Setting

U.A.E. is located on the Arabian plate which is regarded as stable seismically [19, 20]. The tectonic setting on regional scale is depicted in Figure 3.3. Significant crustal deformations and recorded seismic events are rare within the Arabian Peninsula [78]. Although the Arabian plate is bounded by many active tectonic boundaries, major contribution to the seismic hazard in UAE is from Zagros and the Makran region. The separation of the Arabian plate from the African plate creates a subduction zone with the Eurasian plate. The Arabian plate is moving north at a rate

of approximately 21 mm/year [79] and slight rotational movement also creates subduction zone at the boundary of Makran [80]. Movement of Arabian plate is also associated with the formation of Zagros fold and thrust belt in Iran that extends to the edge of the Persian Gulf [81]. In addition to Zagros and Makran regions, the active tectonic structures present in the Oman Mountains (Dibba fault) can also contribute significantly to the seismic hazard in UAE especially in the north and east of the

country [82]. The possibility of existence of fault on the west coast of UAE is supported by little and unclear information [82, 83]. A comprehensive assessment of this feature including geomorphic and paleoseismological studies is required. Since some

instrumentally recorded earthquakes can be associated with the west coast fault (Figure 3.4), any seismic hazard assessment of the region shall include optional hazard values with west coast fault included. Most of the earthquakes in Zagros region are shallow earthquakes at an

average depth of 15 km associated with blind thrust faults in the Precambrian metamorphic rocks [21, 84]. The region has the potential to generate earthquakes with magnitude (Ms) larger than 7. The depths of earthquake foci tend to get deeper (40 km) towards the transition between zagros and makran regions. This transition creates complex faulting systems known as Zindan-Minab zone [85]. The Makran region itself is subducting at an estimated rate of approximately 25 mm/yr [79].

Oman Mountains towards the northeast of UAE exhibit active seismicity. Kusky et al 2005 [86] also reports historical seismicity associated with this Cretaceous Ophiolite Obsduction. Instrumented earthquake with magnitude greater than 5 has been recorded with association to this faulting mechanism. Recent studies associate this fault system (Dibba fault, Wadi Shimal, and Wadi Ham fault) as an

33

extension of Zindab-Minab line. Since the seismic activity is not well documented for this source, rates of uplift and deformation rates shall be used to characterize the

source.

Figure 3.3 Tectonic Setting around U.A.E.

Plate Movement Thrust fault Transform fault

Strike slip fault Plate boundary

34

Regional Seismicity

Different databases from sources such as United States Geological Survey (USGS) and National Geosceinces of Iran were used to develop a seismic catalogue for the sources around UAE. The earthquake database from National Geoscience uses various references such as National Earthquake Information Center [13], International Seismological Center [14], Ambraseys and Melville 1982 [15], Nowroozi 1987 [16], Nabavi 1978 [17], National Oceanic and Atmospheric Administration [18] among many others. Events with magnitude greater than four and between 1900 and 2010 were selected as the basis of catalogue to identify the sources. The catalogue was cleaned using standard protocols of removing duplicated events and aftershocks and for completeness using methods suggested by Reasenberg 1985 [87] and Knopoff 2000 [88]. Historical records of earthquakes in the region were especially considered for Arabian Craton, Oman Mountains, and Makran region. Sources like Zargos and Zindam Minab were characterized by instrumentally recorded data since 1910. The abundance of instrumented events was considered sufficient for defining the slope of Gutenberg Richter relationship which has significant effect on the outcome of Hazard. Historical events were given due consideration in selecting the upper bound magnitudes. Figure 3.4 presents the homogenized (Ms) seismicity catalogue of instrumentally recorded events from National Geoseisnces of Iran.

Figure 3.4 Seismicity Catalogue

18

20

22

24

26

28

30

32

45 50 55 60 65 70

Lati

tud

e

Longitude

35

CHAPTER 4: METHODOLOGY

Seismic Hazard Analysis

In this study, a computer program of EZFRISK was used to perform the

seismic hazard analysis. EZFRISK is an implementation of the Cornell 1968 [31] PSHA framework. The accuracy of this software was evaluated by performing a

sensitivity analysis. A simple verification example of PSHA was performed for a site in UAE using three different seismic zone models in CRISIS [89], EZFRISK and using manual calculations. Manual calculations were done by following the procedure described in Kramer 1996 [39]. To make the manual calculations short and simple, only one seismic source was used along with one attenuation relationship assigned to the seismic source. The PGA values for a return period of 2475 years were computed. The results of this analysis are given in Table 4.1

Table 4.1 Verification Results

Source Attenuation Relation Manual Calculations EZFRISK CRISIS

Oman Mountains Abramson-Silva 1997 [22] 0.046g 0.0413 0.0530 South Zagros Fold Belt Spudich et al. 1999 [110] 0.048g 0.0546g 0.0515g

Oman Peninsula Spudich et al. 1999 [110] 0.090g 0.1035g 0.1075g

For South Zagros fold belt and Oman Peninsula, the variation in the results of

CRISIS and EZFRISK is very small. Whereas, the variation for South Zagros fold belt and Oman Peninsula is 0.003 and 0.004 respectively. The difference between the

results of CRISIS and EZFRISK increases to 0.0117 with Oman Mountains. This increase in difference might be attributed to the use of a different attenuation equation. However, the difference in the results between EZFRISK and manual calculations is around 10% to 12% for all the three analyses. The increase in variation for manual calculations might be due to manual integration. The overall results indicate good agreement between the results of EZFRISK and CRISIS

36

Seismic Zones

The development of seismic source model is primarily based on the work of Berberian 1995 [21], Engdahl et al. 2006 [90] and Aldama et al. 2009 [71]. The seismic source model adopted for this study is shown in Figure 4.1. The seismic source model comprises of seven distinct seismic sources. The southern boundary of South Zargros has been extended into the Persian Gulf instead of being along the Iranian coast due to uncertainty associated with constraining of the boundary. Moving the boundary of South Zargros northward can increase the seismicity of stable Arabian Craton with potentially higher hazard levels in the southern and central cities such as Abu Dhabi and Dubai.

The proposed boundary of South Zargros although may slightly increase the level of hazard in northern cities but is not expected to cause significant increase in

hazard in other distant cities. Dividing the South Zargos into another small zone in the south based on the presence of Zargos foredeep [21] will push the seismicity associated with Zargos region northwards and will result in under estimation of seismic hazard. Although further subdivision of South Zargos can be justified by geological evidence, it is not in agreement with the spatial or temporal distribution of seismic events; therefore, a single zone of South Zargos was adopted.

Figure 4.1 Seismic source model for this study

18

20

22

24

26

28

30

32

45 50 55 60 65 70

Latit

ude

Longitude

Makran

Makran Bottom

Arabian Craton

Oman Mountains

Transition

37

Recurrence parameters

The parameters for all the source zones were calculated using the doubly

bounded exponential distribution [91]. The activity parameters ( at Mmin and ) for Oman mountains (includes all faults), west coast fault (when included) and Makran bottom (Inerplate fault) were computed by using the method proposed by Youngs and Coppersmith 1985 [92]. The slip rates and shape of the fault was used to estimate the seismic moments and then the magnitude-recurrence relationship to determine the activity parameters.

For Arabian Craton, the parameter was obtained from seismicity of the source. Previous studies [19, 20] indicate a larger value of this parameter. The value of 1.16 was selected because subsequent analysis of hazard for the region indicated insignificant effect on the total hazard due to major contribution of other dominant sources.

The upper bound magnitudes (Mmax) were selected as the maximum of historical seismicity, instrumented seismicity, and computation using relationships by Wells and Coppersmith 1994 [93] for known geometry of faults. The parameters for doubly bounded Gutenberg-Richter relationships for all source zones are presented in Table 4.2.

Table 4.2 Activity parameters used in this study.

Seismic Source Fault Mechanism Mmin Mmax at Mmin - beta High Zagros Reverse 4 7.1 16.27 2.2529 South Zagros Reverse 4 7.1 2.056 1.96

Oman Mountains Strike Slip 4 7.0 0.625 2.5 Makran Top Intraslab 4 6.8 1.07 1.63

Makran Bottom Interface 4 8.0 2 1.796 Zagros Makran Transition Strike slip 4 7 5.045 1.998

Arabian Craton Reverse 4 6.5 0.116 1.1555

Ground Motion Prediction Equations (GMPE)

Ground Motion Prediction Equations (GMPE) are used to estimate the ground motion parameter at certain location from a magnitude-distance scenario. The equations derived from the statistical analysis of recorded ground motion data for the

area of interest are preferred. There were no established seismograph networks in

38

UAE until recently established by the governments of Dubai and Abu Dhabi. Consequently ground motion prediction equations (GMPEs) specific to UAE are not available. All seismic hazard analysis performed for the region use GMPEs developed for other geographical areas. The choice of these equations often is based on guidelines proposed by Cotton et al 2006 [94]. Alternatively equations (New Generation Equations) that were developed after the analysis of worldwide seismicity are increasingly being used. A total of seven different GMPEs were used in this study including new

generation equations. Different seismic sources were assigned at least two GMPEs except for the Arabian Craton along with conversion to geometric mean wherever applicable. Three New Generation Equations of Boore and Atkinson 2008 [95], Abrahamson and Silva 2008 [96], Campbell and Borzognia 2008 [97] along with Abrahamson and Silva 1997 [22] were assigned to sources of Zagros and the Oman Mountains. For the Makran region, Atkinson and Boore 2003 [98] and Youngs et al. 1997 [23] were used due to their suitability for earthquakes generated in subduction zones. The equation by Atkinson and Boore 2006 [99] was assigned to the Arabian Craton.

Gridded Seismic Hazard Analysis

The computer application used in this study facilitates the option of performing single site and multi-site seismic hazard analysis. Hence, the shape of U.A.E. was defined in EZFRISK and a grid of nodes was plotted on the U.A.E. map.

Latitudes and Longitudes of all the nodes were recorded. EZFRISK already has a predefined seismic source model for the Middle East. But for this study, a separate seismic source model was defined in EZFRISK along with recurrence parameters for

each source. The attenuation equations obtained from EZFRISKs database were assigned to the seismic source zones. The input data was validated and gridded

seismic hazard analysis was performed. Figure 4.2 shows the gridded map of U.A.E. developed in EZFRISK.

39

Figure 4.2 Grid of nodes used in Gridded Seismic Hazard Analysis.

Presentation of results for Seismic Hazard Analysis

For each of the nodes in the grid in Figure 4.2, EZFRISK produced a seismic

curve and a uniform hazard spectrum (UHS). Seismic curves corresponding to spectral acceleration of 0.2s, 1s and 3s are also produced. Preferences for Uniform

Hazard Spectra (UHS) can also be predefined in EZFRISK depending on the need. UHS can be plotted for any return period. Probabilistic Seismic Hazard Analysis combines all the seismic source zones to determine the hazard at a particular site. However, designers and researchers

usually are also interested in the contribution of the sources to the hazard. The process of determining the contribution from the seismic sources to the hazard at site is called

Deaggregation. EZFRISK has the option of performing deaggregtation for any spectral acceleration. The results are presented in the form of 2D and 3D graphs showing the contribution of each combination magnitude and distance has to the hazard. Since our aim is to find the combination of magnitude and distance which

contributed greatest to the Peak Ground Acceleration (PGA) on the site of interest, the PGA should be known before performing deaggregation. Therefore, the seismic

hazard analysis was performed first without the deaggregation option. Once the PGA was known, the seismic hazard analysis was repeated with the deaggregation option

activated. Using the distance and magnitude combination that contributed greatest to

22

22.5

23

23.5

24

24.5

25

25.5

26

26.5

51 52 53 54 55 56 57

Latit

ude

Longitude

40

the hazard, ground motion time histories were selected for spectral matching and site response analysis for Sharjah, Dubai and Abu Dhabi. Deaggregation was performed for all the seven emirates of U.A.E.

Spectral Matching

Constructing an accurate representative time history for a target spectrum is integral in the outcome of any site response analysis. This will rely on the results of

deaggregation from gridded seismic hazard analysis. In this study, a commercial computer software called RSP Match EDT was used to match time histories results to the target spectra.

This application required two major inputs for matching: Target response spectrum is a result of Seismic Hazard Analysis. For Dubai,

Sharjah and Abu Dhabi, these were obtained from the results of Gridded Probabilistic Seismic Hazard Analysis (GPSHA) of U.A.E. This is called Target because the response spectrum of a time history is customized to be matched to

this response spectrum.

Time histories to be matched two time histories each for Dubai, Sharjah and Abu Dhabi were chosen according to the criteria described by Bommer and Avecedo 2004 [100] for selection of time histories. Bommer and Avecedo 2004 [100] mention some conditions for selecting the ground motion time histories such as the spectral shape and similarity in magnitude and distance. Therefore, each response spectrum of the chosen time history was compared to the target spectrum to choose the time history which gives the closest response spectrum in terms of

the shape along with the closeness in deaggregation results (Table 4.3). An alternative to obtain the input ground motion was to create an artificial time

histories to match regional mechanisms for the Arabian Peninsula region. However, selecting the time histories based on parameters such as magnitude, source to site distance and Peak Ground Acceleration is more important than based on the local mechanism [100].

Other input values such as the maximum waves, maximum wavelets and interpolation values were required by RSP Match EDT. The values used for those

inputs are given in Figure 21 which shows the screen shot of the main menu of RSP Match EDT. Defaults values for some of the parameters were used because, according

41

to the manual of RSP Match EDT, they were not known to affect the matching process significantly.

The ground motion time histories selected were in PEER (Pacific Earthquake Engineering Research) format. Therefore, the time histories had been converted to the compatible format before matching was done. Once the suit of time histories was ready, target response spectrum was defined and RSP Match EDT was run. The

details of time histories used for matching are given in Table 4.4.

Table 4.3 Criteria for selecting time histories

Cities PGA Range (g) Magnitude Range Distance Range (km) Dubai 0.10-0.12 5.5-6.5 20-40

Abu Dhabi 0.07-0.1 5.5-6 35-45 Sharjah 0.12-0.13 5.5-6 25-35

Table 4.4 Time histories selected for spectral matching

City Earthquake Station Component PGA (g) Distance

(km) Magnitude

Dubai Chi-Chi,

Taiwan-02 1757, 09/19/79

CWB 9999936 TCU129

TCU-129-E 0.1173 27 5.9

Dubai Morgan Hill 1984-04-24

21:15

CDMG 47006 Gilroy - Gavilan

Coll. GIL 337 0.1014 25 6.19

Abu Dhabi

Whittier Narrows-01 1987-10-01

14:42

USC 90062 Mill Creek, Angeles

Nat For A-ANG090 0.071 38 5.99

Abu Dhabi

Little Skull Mtn,NV 1992-

06-29

USGS 99999 Station #2-NTS

Control Pt. 1 LSM-2270 0.091 30 5.9

Sharjah Whittier

Narrows-01 1987-10-01

14:42

USC 90062 Mill Creek, Angeles

Nat For A-ANG000 0.089 38 5.99

Sharjah Little Skull

Mtn,NV 1992-06-29

USGS 99999 Station #2-NTS

Control Pt. 1 LSM-2000 0.119 30 5.19

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Site Response Analysis

Amplification of seismic waves has been witnessed in the past in earthquakes such as the Mexico City in 1985, Los Angeles in 1995 and San Francisco in 1989 [101, 102]. The soil amplification is sometimes known to be the sole reason behind the disastrous consequences of an earthquake. Although the Mexico City earthquake

originated from a distance of 400kms, the seismic waves in Mexico were amplified by five times the original intensity. Hence, the intensity of site amplification on seismic

waves is an important factor in designing structures to mitigate earthquake damage. To predict site amplification, the knowledge of variation of shear wave velocities laterally and in depth for different points in a region is essential. Other required information is the unit weights and both damping ratio and shear modulus curves for different soil in the site profile. While the static properties of soil profiles

can be retrieved from the geotechnical investigations done for majority of private and government projects, few projects attempt to perform geophysical investigations to determine the dynamic properties. Therefore, the geophysical data available for site response analysis is limited. In this study, 1D equivalent linear site response analysis was performed for around 100 boreholes from different parts of U.A.E. Borehole logs were selected based on the spatial distribution for cities of Dubai, Sharjah and Abu Dhabi. These borehole logs represented typical sand and rock composition in U.A.E. The

commercial program SHAKE 2000 was used to perform site response analyses on these 100 boreholes.

SHAKE 2000 is a FORTRAN program used for performing one dimensional, equivalent nonlinear site response analysis. It is one of the oldest geotechnical earthquake engineering programs developed for mainframe environments in 1970s by Schnabel et al. 1972 [77]. Since then it has gone through many modifications to make it more user friendly and compatible for todays advanced computer features.

Inputs for SHAKE 2000

Thickness and material type for Layers - will depend on the geology and composition of underground soils. The borehole logs were used to define the

material type and thickness values to be input to SHAKE 2000.

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Shear modulus and Damping ratio curves - depending on the type of soil in the

borhole profile, shear modulus and damping curves were assigned to those layers. Several damping and modulus curves have been proposed in the past such as Schnabel 1973 [103], Seed et al. 1986 [104], and Sun et al. 1988 [105]. These studies have been derived for specific soil types such as sand, clay and gravel. In UAE, majority of the top composition of soils are sandy. Hence, two widely accepted shear modulus and damping curves (Seed and Idriss 1970 [106] for sandy soils, and Schnabel 1973 [103] for rocks) were used in this study. Figures 4.3 and 4.4 show the plots of modulus reduction

and damping ratio curves used for both sandy soil and bedrock.

Figure 4.3 Modulus reduction curves

Figure 4.4 Damping ratio curves

0

0.2

0.4

0.6

0.8

1

1.2

0.0001 0.001 0.01 0.1 1

Mo

dulu

s R

edu

ctio

n (G

/Gm

ax

)

Strain (%)

Seed and Idriss (1970)Schnabel (1973)

0

5

10

15

20

25

30

0.0001 0.001 0.01 0.1 1

Dam

pin

g ra

tio (%

)

Strain (%)

Seed and Idriss (1970) Schnabel (1973)

44

Shear Wave Velocity - is the dynamic property that used to characterize the

strength of soil. Stiff soils are known to have greater shear wave velocities than soft soils. Various geophysical methods such as seismic refraction surveys, seismic crosshole and downhole tests and seismic cone penetration test (SCPT) have been developed over the years to measure shear wave velocity of soils. However, since the geophysical tests are usually expensive to

perform, many researchers have developed correlations which can be used to

predict shear wave velocity using in site tests such as Standard Penetration Test Number (SPT-N). In this study, the correlations proposed by Hasancebi and Ulusay 2006 [24], Shibata (1970) [25], Seed and Idriss (1981) [26] and Athanasopoulos (1995) [27] were used to estimate average shear wave velocities of different soil layers from the soils SPT-N values (Equations 4.1, 4.2, 4.3 and 4.4 respectively).

V# = 90.82 N+., [4.1] V# = 31.7 N+.0 [4.2] V# = 61.4 N+. [4.3] V# = 107.6 N+. [4.4]

Where Vs shear wave velocity in m/s

N Standard Penetration Test Number (SPT-N)

The variation of the shear velocity predicted using the four abovementioned equations for one borehole loge is shown in Figure 4.5. Despite the similarity in the trend of shear wave velocity predicted with the four equations with depth, some discrepancies are clear from the figure. For example; the shear wave velocity values

predicted by Shibata (1970) [4.2] and Hasancebi and Ulusay (2006) [4.1] are closer to each other and located in the lower side of the shear wave velocity axis. However, values predicted by Seed and Idriss (1981)