September 19, 1985

1. September 19, 1985 7:19 am The complete seismic event consisted of four quakes. A pre-event quake of magnitude 5.2 occurred on 28 May 1985. The main and most powerful shock occurred 19 September, followed by two aftershocks: one on 20 September 1985 of magnitude 7.5 and the fourth occurring seven months later on 30 April 1986 of magnitude 7.0.

2. An 8.1 magnitude earthquake hits Mexico City, Mexico.

3. The earthquake shook the buildings in Mexico City for a total of three horrifying minutes.

4. Thirty six hours later a 7.5 magnitude earthquake hits the city again.

5. The epicenter was later located 50 km (approximately 31 miles) off the coast of Mexico.

6. The Seismic Event

7. Historic seismic activity in this region causes Mexico to experience about five times as many major earthquakes as California, with greater frequency of recurrence.

8. Because of the long duration of shaking, deep soil deposits were excited resulting in amplified ground movement. • Mexico City is built partially on alluvial lake deposits that average from 100 – 150 feet thick. Original Water Level Building Structures 100-150’ Weak soil deposits

9. At some location of Mexico City where most of the damages are found, soil deposits extend to about 7500 ft (2300 m). • Therefore, earthquake vibrations in this areas of Mexico city were catastrophically amplified. Original Water Level Building Structures 7500’ Weak soil deposits

10. Tectonics

11. This is a region where the Cocos Plate is being subducted underneath Mexico and is the most active subduction thrust fault in the western hemisphere.

12. Tectonics • The Cocos Plate made one of its all too frequent downward movements. • This time a 200 km long front, inclined 18 degrees east, thrust downward and eastward about 2.3 m in two distinct jerks about 26 sec apart.

13. Fig. 4.5

14. Last century , Mexico had 42 earthquakes with magnitudes greater than 7.

15. Effects of the 1985 Earthquake in Mexico City

16. Building Structural Damages: • Large number of the collapsed buildings were frame structures. • Large number of which did not meet the building standards. • Debris falling onto parked cars • Liquefaction. • Loss of Communication Media.

17. Structural Damages

18. Bottom-level Failure Due to Weak First Floor

19. Collapsed School Building School buildings are exposed to risk form earthquakes since they lack adequate stiffening in shear walls of large classroom areas.

20. Fig. 4.7 The amplitude of the shaking increases up the building Building with long axes perpendicular to ground motion suffer more shaking Building with different heights sway at different frequencies and bank into each other A building with different height tend to break apart

21. Fig. 4.8

22. http://www.frame3d.caltech.edu/animations/b1tabxabsmov.gif

23. Collapsed Floors Punctured by Load-Bearing Column Severe resonance oscillations of the buildings caused strain at the juncture between columns and ceiling slabs. The vertical columns were punched through the heavy floors that collapsed around them.

24. Total Collapse of Juarez Hospital Over four hundred medical personnel and patients were trapped in the maternity wing of the Juarez Hospital. Survivors were still being retrieved from the structure as late as ten days after the earthquake.

25. Collapsed 21-Story Office Building Buildings such as the one standing in the background met building code requirements. Obviously the collapsed office building did not.

26. Car Demolished by Debris Thousand of vehicles were destroyed, like the one in this picture, by falling debris.

27. Collapsed and Undamaged Office Buildings The 44-floor Torre Latino-Americana office building in the background on the right, remained almost totally undamaged, as it did in a 1957 earthquake. The building is a symmetrical steel frame structure built to resist earthquakes

28. High Rise Building This building twisted excessively in the earthquake, forming the X-shaped cracks. The earthquake subjected the building to shear, bending, tensional forces, and compression. The formation of the X-shaped cracks is evidence the energy from the earthquake dissipated in the shear walls.

29. Liquefaction

30. Liquefaction Mexico City is a very dangerous in terms of its local geology. It is currently sitting on a 800 meter (2625 ft) lake bed made up of silt and volcanic clays that create two problems that must be address here. Liquefaction is a type of ground failure in which water saturated sediment turns from a solid to a liquid as a result of shaking, often caused by an earthquake or even a volcanic eruption. The intense shaking causes the strength of the soil to become weak and the sand and water begin to flow.

31. Building Sank into Liquefied Soil This residential and commercial building sank more than three feet into the partially liquefied soil.

32. Loss of Communication Media

33. Central Communications Center

34. 1985 Mexico City Earthquake Tally: • 10,000 people lost their lives. • 100,000 people were left homeless. • 400 buildings are destroyed. • 3200 buildings were damaged in the most populous area of Mexico City. • Total Costs = $4 billion dollars in damages.

35. Government’s Proposal for Future Preparedness

36. Revisions to the building codes such as: • More Funding for researching dynamic behavior of soil deposits • Strict evaluation of lateral force design coefficients, story drifts, and change of natural period of vibration due earthquake induced structural decay • Improve Emergency Operations: • Emergency Operations should be clear with respect to rescue • Shelter for the homeless • Lifeline recovery and infrastructure repair • Deployment of volunteer rescue support and forces need to maintain order • Use of rescue equipment trainings • Impose a more strict licensing control of professional registration for Engineers and Scientists.

37. Conclusion: • Most metropolitan areas are ill prepared for an earthquake of this intensity. Mexico’s expectations are to advance that level of preparedness.

38. Fig. 3.26

39. Fig. 3.27

40. Fig. 3.28

41. Fig. 3.29

42. Fig. 3.30

43. Fig. 3.31 The Office of Disaster Preparedness in San Diago County is housed in a 2 story building (7000 ft sq) It is sitting on top of 20 lead impregnated rubber supports ( base isolators) That each weigh 1 ton

44. Fig. 3.33a

45. Fig. 3.33b

46. Fig. 3.34a

47. Damage to the buildings in Mariana district of San Francisco resulting from 1980 earthquake.

48. An apartment building in San Francisco's Marina District was thrown from its foundation and crushed a car in the 1989 earthquake. The Marina District was the scene of voracious fires caused by broken gas lines.Photo by Vince Maggiora

49. In the Santa Cruz mountains, the 1989 earthquake caused deep cracks. "I can't stop shaking," said one resident. "I guess I'm surviving, but I'm scared."Photo by Deanne Fitzmaurice

50. Fig. 4.7 The amplitude of the shaking increases up the building Building with long axes perpendicular to ground motion suffer more shaking Building with different heights sway at different frequencies and bank into each other A building with different height tend to break apart