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General Geology: Earthquakes

General Geology: Earthquakes. Instructor: Prof. Dr. Boris Natalin. Earthquake focus and epicenter. Earthquakes and faults. Rebound theory . Mechanism of earthquakes and focal mechanism solution. Elastic rebound theory Movement occurs instantaneously

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General Geology: Earthquakes

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  1. General Geology: Earthquakes • Instructor: Prof. Dr. Boris Natalin

  2. Earthquake focus and epicenter

  3. Earthquakes and faults • Rebound theory

  4. Mechanism of earthquakes and focal mechanism solution Elastic rebound theory Movement occurs instantaneously Shaking of the ground is caused by rock motions Rebound causes the motion of particles. Motion occurs in opposite direction on the opposite walls of a fault

  5. Vertical and horizontal motions The North Anatolian Fault reveals the horizontal motion

  6. Vertical and horizontal motions • Alaskan earthquake in 1964 resulted from vertical motion (see the fault scarp)

  7. Evolution of earthquakes • Foreshocks • Aftershocks • Both are weaker than the main earthquakes • Aftershocks are dangerous because buildings are already damaged Rupture and propagation • Most time faults are locked because of confining pressure • If stress exceeds the rock strength (frictional resistance) , rupture occur • The rupture starts at earthquake focus and propagates in horizontal and vertical direction along the fault plane • Slippage affects a local area • Slippage stops when rocks have not been sufficiently strained to overcome frictional resistance

  8. Seismology • Chinese seismograph designed by Chang Heng in 152 A.D.

  9. Seismographs Records horizontal ground motions Records vertical ground motions

  10. Seismic waves • Seismic waives radiation • Surface waves • Body waves- primary waves (P waves)- secondary waves (S waves)

  11. Seismic waves: P waves (push and pull)

  12. Seismic waves: S waves (shake or shear)

  13. Types of seismic waves: Body waves

  14. Types of seismic waves: Surface waves

  15. Seismic waves • P waves are resulted from a change of volume- propagate in solid, liquid, and gas • S waves are resulted from a change of shape- propagate only in solid • P waves are fast (e.g. 6 km/c in granite) • S waves 1.7 times slower

  16. Seismogram

  17. Locating earthquakes • Difference in P and S velocities is used

  18. Locating earthquakes

  19. Earthquake belts 95% earthquake energy is released along plate boundaries

  20. Earthquake depth • 5 to 700 km • Shallow (˂70 km), intermediate (˂ 300 km), and deep • Divergent plate boundaries - shallow • Subduction zones – deep • Wadati - Benioff zones

  21. Earthquake intensity and magnitude • Mercalli intensity scale (1902) is base on destruction • Intensity depends on:- energy released by an earthquake;- distance from epicenter;- surface material. • Magnitude is a measure of released energy

  22. Richter magnitude scale (1935) -Seismic record accounting for amplitudes of waves and distances to the focus.

  23. Richter magnitude scale (1935) • Based on seismic record accounting for amplitudes of waves and distances to the focus • Logarithmic scale of magnitudes

  24. Richter magnitude Each magnitude unit equates 32-fold energy release

  25. Richter magnitude Each magnitude unit equates 32-fold energy release

  26. Moment magnitude scale • The moment magnitude scale (abbreviated as MMS; denoted as MW) is used by seismologists to measure the magnitude of earthquakes in terms of the energy released. • The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped.

  27. Seismic moment is a quantity to measure the size of an earthquake. The scalar seismic moment Mo is defined by the equation Mo = μAD , where μ is the shear modulus of the rocks involved in the earthquake (in dyne / cm2) A is the area of the rupture along the geologic fault where the earthquake occurred (in cm2), and D is the average displacement on (in cm). • Thus, it has dimensions of energy, measured in dyne centimeters

  28. Earthquake magnitude and energy equivalence Thousands of moderate earthquakes are needed to decrease the chance for the occurrence of a major quake

  29. The analysis of the table shown on previous slide should dispel the notion that the moderate earthquake decrease the chance for the occurrence of the strong earthquake in the same region.

  30. Earthquake risk maps

  31. Earthquake risk maps

  32. Earthquake risk maps

  33. Earthquake destruction • Magnitude • Distance to the epicenter- 20-50 km is dangerous -beyond this region ground shaking deteriorates rapidly

  34. Seismic vibration • 15 second to 3-4 minutes

  35. Seismic vibration • Seismic vibration depends on: • Ground motion amplification. • Soft sediments amplify vibration. • Avcilar and central part of Istanbul.

  36. Seismic vibration • Seismic waves have more or less specific frequencies • The intervening rock material vibrate over wide range of frequencies but each type of the material has a natural period of vibration • The most dangerous case is when natural period of vibration works in unison with seismic frequency

  37. Liquefaction Unconsolidate sediments saturated with water

  38. Liquefaction

  39. Liquefaction Swimming pull near Sapanca Lake after 1999 earthquake

  40. Tsunami • Seismic sea waves

  41. Tsunami

  42. Predicting earthquakes • Short-range predictions- some were successful- false alarms

  43. Predicting earthquakes: long-term

  44. Predicting earthquakes

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