Seismic Risk: An Introduction

1. Seismic Risk: An Introduction Prof. Ravi Sinha Special Lecture: Introduction to Civil Engineering (CE-152) February 1, 2006

2. Introduction • Earthquake engineering is a super-specialisation of structural engineering • Earthquake engineering deals with understanding earthquakes, their causes, their consequences, and designing structures to withstand earthquake forces • The field of study is highly multi-disciplinary with lead taken by structural engineers

3. Background • Earthquakes are one of the most devastating forces in nature • Earthquakes disasters have been known since ancient times • Earthquakes have been instrumental in changing the course of history • Some of the most significant disasters in the last hundred years have been caused by earthquakes

4. Earthquake Background • The causes of earthquakes have been guessed by different civilisations since historical times Mongolian Legend

5. Earthquake Background • The causes of earthquakes have been guessed by different civilisations since historical times Japanese Legend

6. Earthquake Risk Hazard Probability of ground motion • Amplification due to: • Soil • Topography Site Effects • Effect on structures due to: • Building type and age • Population density • Land use • Month and time Vulnerability Risk Probability of damage and losses

7. Seismic Hazard Assessment

8. Historical Background • Records of every major earthquake in China during the last 3000 years • Records of major earthquakes in India up to last 2500 years • Records of major earthquakes over 2000 years in Middle-East • Legends about earthquakes in India and several other ancient civilisations

9. Modern Studies • Modern study of seismology has been carried out over the last 45-50 years only • Most useful data has been collected using a world-wide network of seismological stations • Records show that earthquakes are not uniformally distributed but concentrated along well defined lines

10. Earthquake Sources

11. Earthquake Sources • Most earthquakes are concentrated along boundaries of earth’s plates • Some earthquakes also occur away from plate boundaries • Earthquakes in many places are also associated with volcanic activities • In recent times, earthquakes may have been triggered by human structures and activities (dams, mining etc.)

12. Structure of Earth • Earth is not a rigid and motionless mass • Cross-section of the earth can be classified into four distinct concentric layers: • Inner Core (Solid) • Outer Core (Liquid) • Mantle (Liquid) • Crust (Solid)

13. Structure of Earth

14. Structure of Earth

15. Plate Tectonics • Motion of earth’s plates are explained using Plate Tectonics • According to Plate Tectonics: • Earth’s land-mass were earlier joined together • The land-mass have broken up and have drifted apart • Relative motion is still continuing, relative motion at plate boundaries cause earthquakes

16. Plate Tectonics

17. Earth’s Plates

18. Earth’s Plates

19. Drift of Indian Subcontinent

20. Plate Tectonics • Considerable evidence now exist to support Plate Tectonics • Types of evidence: • Geological and geomorphological - similar rock formations • Anthropological - similar vegetation and animal life • Geomagnetic - magnetic anomalies support drifting away of land mass from Atlantic ridge and other places

21. Prehistoric Flora and Fauna

22. Elastic Rebound Theory

23. Example of Fault Rupture Chile Earthquake

24. Example of Fault Rupture Taiwan Earthquake

25. Example of Fault Rupture Kobe Earthquake

26. Example of Fault Rupture Kobe Earthquake

27. Example of Fault Rupture Kobe Earthquake

28. Earthquake Waves • Elastic rebound produces waves from the point of rupture • The rupture may be localised at a point, along a slip line or a slip surface • Earthquake waves have clearly identifiable components • Primary wave (refractory) • Secondary or shear wave (transverse) • Raleigh wave (refractory) • Love wave (transverse)

29. Earthquake Waves

30. Earthquake Magnitude • Earthquake magnitude is most commonly defined in Richter magnitude • It is logarithm of the maximum displacement (in µm) recorded on a particular type of seismograph 100 km from the epicentre • Richter magnitude is open-ended and has no maximum value • Scientifically more useful measure is based on seismic moment and measures the total energy that is released • Both magnitudes give similar value for moderate earthquakes (M 5.0 - M7.5)

31. Earthquake Intensity • Earthquake intensity is a measure of its consequence • Most popular intensity scales are primarily based on structure damage • MMI (Defines 12 intensities) based only on performance of buildings • MSK (Defines 12 intensities) based on building performance, geotechnical effects as well as human perception • Most countries (including India) use MSK intensity scale or its modifications to suit local conditions

32. Indian Seismicity

33. Seismic Hazard

34. Assessment of Site Effects

35. Influence of Local Conditions Maximum ground motion also depends on local soil/rock properties Maximum ground displacement in Northridge earthquake (1994) 10 km

36. Mumbai Description

37. Influence of Local Conditions Anjar Town-Plan and Local Soil Conditions

38. Seismic Hazard • Challenges in Hazard Assessment • Location of seismogenic features • Fault size, movement rate, return period • Inadequate historical earthquake data • Information on local soil conditions • These require significant additional scientific studies to provide reliable information

39. Seismic Vulnerability Assessment

40. Seismic Vulnerability • Depends on type of structures (structure category) and their age • Depends on land use in city (space betweenadjacent buildings, height of buildings etc.) • Depends on month and time (buildings maybe weaker during the rainy season, and residential buildings more fully occupiedduring nights) • Depends on population density (impact of damage of a building to number of people)

41. Urban Construction Practice Engineered Constructions • Reinforced concrete buildings • Brick masonry buildings with RCC roof Non-Engineered Constructions • Informal brick masonry buildings • Other non-engineered buildings using light weight materials

42. 100% Non-engineered % DAMAGE Masonry Steel 50% RCC 0% V VI VII VIII IX X XI EARTHQUAKE INTENSITY (MSK) Seismic Vulnerability • Seismic vulnerability can be expressed in terms of vulnerability curves

43. Seismic Vulnerability • Must consider the consequences of very poor building stock – example: Mumbai Number of building collapses in Mumbai – without any earthquake

44. Seismic Risk

45. Earthquake Risk • Risk has the following components • Hazard • Site Effects • Vulnerability • Inadequate understanding of earthquake hazard in large parts of India • Vulnerability of different structure types are poorly assessed

46. Time Time MSK VI MSK VI MSK VII MSK VII MSK VIII MSK VIII Midnight Midnight 11,200 31,400 118,400 42,600 277,600 100,100 6 A.M. 6 A.M. 25,000 9,000 34,000 94,600 80,000 222,100 12 Noon 12 Noon 6,700 18,800 25,500 71,000 60,100 166,500 Example :: Mumbai Research indicates following scenario for an earthquake disaster in Mumbai (2001) Estimated number of fatalities and injuries due to building collapse

47. Summary • Significant knowledge about earthquakes and its consequences exist in scientific community • Interest in earthquakes continue since prehistoric times • Earthquakes can cause severe damage due to strong ground motions and/or deformations • Seismic risk depends on hazard, site conditions and structure vulnerability

48. Summary • India is divided into 4 seismic zones (low to very high seismic hazard) • Most large cities have moderate to high seismic hazard • The damage at a locality is influenced by the local soil conditions • Building performance depends on ground motions as well as structural characteristics

49. Summary • Vulnerability is expected to be very high due to poor building stock • Information on hazard, site effect and vulnerability can be combined to assess seismic risk • Seismic risk is high due to uncertainty about hazard and high vulnerability

50. Thank You