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  1. 1. Seismic Risk: An Introduction
  2. 2. Introduction <ul><li>Earthquake engineering is a super-specialisation of structural engineering </li></ul><ul><li>Earthquake engineering deals with understanding earthquakes, their causes, their consequences, and designing structures to withstand earthquake forces </li></ul><ul><li>The field of study is highly multi-disciplinary with lead taken by structural engineers </li></ul>
  3. 3. Background <ul><li>Earthquakes are one of the most devastating forces in nature </li></ul><ul><li>Earthquakes disasters have been known since ancient times </li></ul><ul><li>Earthquakes have been instrumental in changing the course of history </li></ul><ul><li>Some of the most significant disasters in the last hundred years have been caused by earthquakes </li></ul>
  4. 4. <ul><li>The causes of earthquakes have been guessed by different civilisations since historical times </li></ul>Earthquake Background Mongolian Legend
  5. 5. <ul><li>The causes of earthquakes have been guessed by different civilisations since historical times </li></ul>Earthquake Background Japanese Legend
  6. 6. Earthquake Risk Risk Vulnerability Site Effects Hazard Probability of ground motion <ul><li>Amplification due to : </li></ul><ul><li>Soil </li></ul><ul><li>Topography </li></ul><ul><li>Effect on structures due to : </li></ul><ul><li>Building type and age </li></ul><ul><li>Population density </li></ul><ul><li>Land use </li></ul><ul><li>Month and time </li></ul>Probability of damage and losses
  7. 7. Seismic Hazard Assessment
  8. 8. <ul><li>Records of every major earthquake in China during the last 3000 years </li></ul><ul><li>Records of major earthquakes in India up to last 2500 years </li></ul><ul><li>Records of major earthquakes over 2000 years in Middle-East </li></ul><ul><li>Legends about earthquakes in India and several other ancient civilisations </li></ul>Historical Background
  9. 9. <ul><li>Modern study of seismology has been carried out over the last 45-50 years only </li></ul><ul><li>Most useful data has been collected using a world-wide network of seismological stations </li></ul><ul><li>Records show that earthquakes are not uniformally distributed but concentrated along well defined lines </li></ul>Modern Studies
  10. 10. Earthquake Sources
  11. 11. <ul><li>Most earthquakes are concentrated along boundaries of earth’s plates </li></ul><ul><li>Some earthquakes also occur away from plate boundaries </li></ul><ul><li>Earthquakes in many places are also associated with volcanic activities </li></ul><ul><li>In recent times, earthquakes may have been triggered by human structures and activities (dams, mining etc.) </li></ul>Earthquake Sources
  12. 12. <ul><li>Earth is not a rigid and motionless mass </li></ul><ul><li>Cross-section of the earth can be classified into four distinct concentric layers: </li></ul><ul><ul><li>Inner Core (Solid) </li></ul></ul><ul><ul><li>Outer Core (Liquid) </li></ul></ul><ul><ul><li>Mantle (Liquid) </li></ul></ul><ul><ul><li>Crust (Solid) </li></ul></ul>Structure of Earth
  13. 13. Structure of Earth
  14. 14. Structure of Earth
  15. 15. <ul><li>Motion of earth’s plates are explained using Plate Tectonics </li></ul><ul><li>According to Plate Tectonics: </li></ul><ul><ul><li>Earth’s land-mass were earlier joined together </li></ul></ul><ul><ul><li>The land-mass have broken up and have drifted apart </li></ul></ul><ul><ul><li>Relative motion is still continuing, relative motion at plate boundaries cause earthquakes </li></ul></ul>Plate Tectonics
  16. 16. Plate Tectonics
  17. 17. Earth’s Plates
  18. 18. Earth’s Plates
  19. 19. Drift of Indian Subcontinent
  20. 20. <ul><li>Considerable evidence now exist to support Plate Tectonics </li></ul><ul><li>Types of evidence: </li></ul><ul><ul><li>Geological and geomorphological - similar rock formations </li></ul></ul><ul><ul><li>Anthropological - similar vegetation and animal life </li></ul></ul><ul><ul><li>Geomagnetic - magnetic anomalies support drifting away of land mass from Atlantic ridge and other places </li></ul></ul>Plate Tectonics
  21. 21. Prehistoric Flora and Fauna
  22. 22. Elastic Rebound Theory
  23. 23. Example of Fault Rupture Chile Earthquake
  24. 24. Example of Fault Rupture Taiwan Earthquake
  25. 25. Example of Fault Rupture Kobe Earthquake
  26. 26. Example of Fault Rupture Kobe Earthquake
  27. 27. Example of Fault Rupture Kobe Earthquake
  28. 28. Earthquake Waves <ul><li>Elastic rebound produces waves from the point of rupture </li></ul><ul><li>The rupture may be localised at a point, along a slip line or a slip surface </li></ul><ul><li>Earthquake waves have clearly identifiable components </li></ul><ul><ul><li>Primary wave (refractory) </li></ul></ul><ul><ul><li>Secondary or shear wave (transverse) </li></ul></ul><ul><ul><li>Raleigh wave (refractory) </li></ul></ul><ul><ul><li>Love wave (transverse) </li></ul></ul>
  29. 29. Earthquake Waves
  30. 30. Earthquake Magnitude <ul><li>Earthquake magnitude is most commonly defined in Richter magnitude </li></ul><ul><ul><li>It is logarithm of the maximum displacement (in µm ) recorded on a particular type of seismograph 100 km from the epicentre </li></ul></ul><ul><ul><li>Richter magnitude is open-ended and has no maximum value </li></ul></ul><ul><li>Scientifically more useful measure is based on seismic moment and measures the total energy that is released </li></ul><ul><ul><li>Both magnitudes give similar value for moderate earthquakes (M 5.0 - M7.5) </li></ul></ul>
  31. 31. Earthquake Intensity <ul><li>Earthquake intensity is a measure of its consequence </li></ul><ul><li>Most popular intensity scales are primarily based on structure damage </li></ul><ul><ul><li>MMI (Defines 12 intensities) based only on performance of buildings </li></ul></ul><ul><ul><li>MSK (Defines 12 intensities) based on building performance, geotechnical effects as well as human perception </li></ul></ul><ul><li>Most countries (including India) use MSK intensity scale or its modifications to suit local conditions </li></ul>
  32. 32. Indian Seismicity
  33. 33. Seismic Hazard
  34. 34. Assessment of Site Effects
  35. 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. 36. Mumbai Description
  37. 37. Influence of Local Conditions Anjar Town-Plan and Local Soil Conditions
  38. 38. Seismic Hazard <ul><li>Challenges in Hazard Assessment </li></ul><ul><ul><li>Location of seismogenic features </li></ul></ul><ul><ul><li>Fault size, movement rate, return period </li></ul></ul><ul><ul><li>Inadequate historical earthquake data </li></ul></ul><ul><ul><li>Information on local soil conditions </li></ul></ul><ul><li>These require significant additional scientific studies to provide reliable information </li></ul>
  39. 39. Seismic Vulnerability Assessment
  40. 40. Seismic Vulnerability <ul><li>Depends on type of structures (structure category) and their age </li></ul><ul><li>Depends on land use in city (space between adjacent buildings, height of buildings etc.) </li></ul><ul><li>Depends on month and time (buildings may be weaker during the rainy season, and residential buildings more fully occupied during nights) </li></ul><ul><li>Depends on population density (impact of damage of a building to number of people) </li></ul>
  41. 41. Urban Construction Practice Engineered Constructions <ul><li>Reinforced concrete buildings </li></ul><ul><li>Brick masonry buildings with RCC roof </li></ul>Non-Engineered Constructions <ul><li>Informal brick masonry buildings </li></ul><ul><li>Other non-engineered buildings using light weight materials </li></ul>
  42. 42. Seismic Vulnerability <ul><li>Seismic vulnerability can be expressed in terms of vulnerability curves </li></ul>V VI VII VIII IX X XI 50% 100% 0% % DAMAGE EARTHQUAKE INTENSITY (MSK) Non-engineered Masonry RCC Steel
  43. 43. Seismic Vulnerability Number of building collapses in Mumbai – without any earthquake <ul><li>Must consider the consequences of very poor building stock – example: Mumbai </li></ul>Year Collapses 1993-94 236 1994-95 253 1995-96 224 1996-97 272 1997-98 259 1998-99 305 1999-00 154 2000-01 260 2001-02 273
  44. 44. Seismic Risk
  45. 45. <ul><li>Risk has the following components </li></ul><ul><ul><li>Hazard </li></ul></ul><ul><ul><li>Site Effects </li></ul></ul><ul><ul><li>Vulnerability </li></ul></ul><ul><li>Inadequate understanding of earthquake hazard in large parts of India </li></ul><ul><li>Vulnerability of different structure types are poorly assessed </li></ul>Earthquake Risk
  46. 46. Example :: Mumbai Research indicates following scenario for an earthquake disaster in Mumbai (2001) Estimated number of fatalities and injuries due to building collapse Time MSK VI MSK VII MSK VIII Midnight 11,200 42,600 100,100 6 A.M. 9,000 34,000 80,000 12 Noon 6,700 25,500 60,100 Time MSK VI MSK VII MSK VIII Midnight 31,400 118,400 277,600 6 A.M. 25,000 94,600 222,100 12 Noon 18,800 71,000 166,500
  47. 47. Summary <ul><li>Significant knowledge about earthquakes and its consequences exist in scientific community </li></ul><ul><li>Interest in earthquakes continue since prehistoric times </li></ul><ul><li>Earthquakes can cause severe damage due to strong ground motions and/or deformations </li></ul><ul><li>Seismic risk depends on hazard , site conditions and structure vulnerability </li></ul>
  48. 48. <ul><li>India is divided into 4 seismic zones (low to very high seismic hazard) </li></ul><ul><li>Most large cities have moderate to high seismic hazard </li></ul><ul><li>The damage at a locality is influenced by the local soil conditions </li></ul><ul><li>Building performance depends on ground motions as well as structural characteristics </li></ul>Summary
  49. 49. <ul><li>Vulnerability is expected to be very high due to poor building stock </li></ul><ul><li>Information on hazard, site effect and vulnerability can be combined to assess seismic risk </li></ul><ul><li>Seismic risk is high due to uncertainty about hazard and high vulnerability </li></ul>Summary
  50. 50. Thank You