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The Nature of Earthquakes

The Nature of Earthquakes. Earthquakes Are A Unique Catastrophe. Building Damage Is Affected by the Type of Earth Movement. The Earth’s Surface Floats On A Sea of Molten Rock. The lines created by the collision of the immense plates are called faults .

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The Nature of Earthquakes

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  1. The Nature of Earthquakes

  2. Earthquakes Are A Unique Catastrophe

  3. Building Damage Is Affected by the Type of Earth Movement

  4. The Earth’s Surface Floats On A Sea of Molten Rock • The lines created by the collision of the immense plates are called faults. • The surface plates are held in place by friction until the buildup of pressure causes sudden movement - an earthquake. The Earth’s surface plates are in constant motion - veryslow constant motion. Diagram Courtesy of J.H. Wiggins Co.

  5. How Does This Work? • Earthquakes move surface plates in any direction or any combination of directions. Each such movement causes unique damage. Diagram Courtesy of J.H. Wiggins Co.

  6. In Addition to Movement... • The earth can radiate waves that will shake structures without actually moving them. • Shaking is a major source of earthquake damage to structures. • The graphic illustrates a P-Wave - a form of compression and release. Similar to a slinky motion.

  7. S-Wave • S-Waves travel by vibrating up and down. • S-Waves can cause vertical cracks and change the elevation of parts of buildings. • The hammer in the illustration represents the point where energy is released.

  8. The Fault Plane • This thin line represents the fault line. Earth movement occurs along this line.

  9. Fault Movement Two Types of Fault Movement : • Strike-Slip A sideways movement. • Dip-Slip An up and down movement.

  10. Strike-Slip Fault Movement • Little change in elevation on either side of the fault plane. A very common type of earth movement. Lateral Movement

  11. Dip Slip Fault Movement • Graben - the fault-bounded valley. • Horst - the fault-bounded ridge. Horst Graben Vertical and Upward Movement

  12. Normal Fault Movement • The earth on the right side of the fault plane line drops.

  13. Thrust Fault Movement • Thrust Fault Movement [Reverse]The earth on one side of the fault plane rides up and over the earth on the other side of the fault plane. Normal Reverse

  14. Thrust Fault • Thrust FaultA reverse fault with gently dipping fault surface. Thrust Fault

  15. Left Lateral Strike Slip • The earth moves laterally along the fault line with primary movement along the left side of the fault. Diagram Courtesy of J.H. Wiggins Co.

  16. Other EQ-Related Earth Movement Landslides and “Poor Ground” • This type of movement is the result of earth-shaking versus actual direct fault-related movement.

  17. Typical Landslide Remediation Soil Nailing - Used to stabilize hillsides.

  18. Rockfall Landslide • Rock masses that fall through the air.

  19. Slump Landslide • Intact masses move with other large masses to cause instability and soil erosion.

  20. Debris Slide • Typified by broken masses of rock and earth, a debris slide moves on the underlying surface.

  21. Earthflow or Liquefaction • Soil moves like thick fluid, seeking its own level, generally caused by the vibrations (waves) that accompany earthquakes.

  22. Soil Types Prone to Liquefaction • Sand • Gravel • Soft soils • Dried lakebeds • High water table areas • River flood planes with high deposits of silt

  23. Evidence of Liquefaction • Sand Boils Sub-surface sand is liquefied by the vibration waves of the earthquake. The discharge to the surface resembles boiling water.

  24. Typical Liquefaction Damage • Note how foundation shows signs of sinking into the earth. • Following liquefaction, the soil around a building must be stabilized before commencing repairs.

  25. Illustrations of Non-EQ Earth Movement

  26. Soil Densification Soil Densification is the: • Compaction of unstable soil caused by earthquake vibration and shaking. • Creation of minor depressions in the earth’s surface. • Cause of serious damage to foundations and superstructures when beneath buildings.

  27. Voids Voids are created by: • Improper drain fields, • Erosion, • Improperly channeled roof runoff, • Leaking plumbing such as exterior faucets, evaporation, and surface water. Diagram Courtesy of J.H. Wiggins Co.

  28. Land subsidence Occurs when large amounts of ground water have been withdrawn from certain types of rocks, such as fine-grained sediments. Common to areas with underground mines, subsidence can occur when mines experience cave-ins (see diagram). Subsidence

  29. Subsidence • Location of maximum subsidence in U.S. as identified by researcher Joseph Poland. Signs show position of land surface in 1925, 1955, and 1977. • Subsequent use of surface water greatly reduced the rate of subsidence, but renewed ground-water pumping during the drought (1987-92) resulted in additional subsidence.

  30. Subsidence (continued) • Rocks and water are responsible for holding the ground up. When the water is withdrawn, the rock falls in on itself, resulting in subsidence.

  31. What Causes Land Subsidence: • Underground mining. • Natural compaction of soils. • Loss of water in organic soils. • Dissolving of subsurface limestone rock. • Withdrawal of ground water and petroleum. • First-time wetting of formerly dry, low-density soils.

  32. Common when ground water circulates through rock below the surface, then naturally dissolves it. Rocks such as: Limestone, Carbonate and Salt beds. Sinkholes

  33. Areas Prone to Subsidence and Sink Holes

  34. Mining Not limited to these areas. However, the most notable instances of mining-related soil problems are shown here. Diagram Courtesy of J.H. Wiggins Co.

  35. Underground Fluid Withdrawal Diagram Courtesy of J.H. Wiggins Co.

  36. Hydrocompaction Diagram Courtesy of J.H. Wiggins Co.

  37. Sinkholes Diagram Courtesy of J.H. Wiggins Co.

  38. Natural Compaction This area is a collection point for organic soils precipitating from the Mississippi River. As the soils continue to decompose and condense, the ground settles. Diagram Courtesy of J.H. Wiggins Co.

  39. Drainage of Organic Soils Also referred to as Erosion. Diagram Courtesy of J.H. Wiggins Co.

  40. Typical Damage from Frost • Note that attached structure does not have the same foundation as the main structure, resulting in frost heave movement.

  41. Soil Soaking and Drying • Soil can shrink and swell according to its level of moisture content. • Moisture extremes can cause damaging stress to foundations and concrete slabs. Diagram Courtesy of J.H. Wiggins Co.

  42. Soil Bearing Failure • Caused by improper support installation and related excavation. • Common to buildings with additions.

  43. Soil Stabilization A process used: • After an earthqauke to stabilize liquefied or liquefaction-prone soil, but • Prior to the actual repairing of a damaged structure.

  44. Soil nailing Slush grouting Ground Anchoring Dynamic Compaction Chemical additive stabilization Introduction of superlight materials Vibro-compaction and vibro-displacement Installation of vertical drains, retaining walls, or sub-terrainian perimeter containment walls around a building’s foundation Stabilizing Methods

  45. Slush Grouting • Involves pouring cement grout into cracks, fissures, or other earthquake-disturbed soil to fill surface fractures. • Includes pumping grout materials under pressure into sub-surface unstable soils.

  46. Deep Dynamic Compaction • Pounds the ground into a compact density by dropping a heavy compactor onto the surface. • Not used close to buildings. • Not an economical method of soil stabilization for small areas such as yards and drives.

  47. Soil Nailing • Used to hold soil of the unstable ground in place. • Long rods are drilled into metal or concrete surface plates. Diagram Courtesy of J.H. Wiggins Co.

  48. Soil Mixing • Stabilizing additives are mechanically mixed into the soil to improve viscosity and soil stability.

  49. Tie-Back Retaining Walls • To hold the wall in place, long metal rods are drilled into the hillside through the concrete or gunnite-coated wall.

  50. Soil Stabilization Involves Many Approaches • Be aware that all soil stabilization processes are not effective under specific circumstances. • Warning!Experience has shown that any soil stabilization effort must be closely examined by a qualified expert prior to implementation to avoid costly problems such as future failure or contamination.

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