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GEO1011

GEO1011. Chap. 19 : Earthquakes. Chap 19: Earthquakes. What is an earthquake and its relation to plate tectonics The seismic waves How to locate an earthquake The sizes of an earthquake and how to measure them Earthquake prediction Seismic hazard and seismic risk. Chap 19: Earthquakes.

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GEO1011

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  1. GEO1011 Chap. 19 : Earthquakes

  2. Chap 19: Earthquakes • What is an earthquake and its relation to plate tectonics • The seismic waves • How to locate an earthquake • The sizes of an earthquake and how to measure them • Earthquake prediction • Seismic hazard and seismic risk

  3. Chap 19: Earthquakes • What is an earthquake and its relation to plate tectonics • The seismic waves • How to locate an earthquake • The sizes of an earthquake and how to measure them • Earthquake prediction • Seismic hazard and seismic risk

  4. Earthquakes in subduction zones

  5. Earthquakes in continental regions

  6. Earthquakes occur in the cold, brittle parts of the Earth: • the upper part (upper crust and upper part of the upper mantle) • the subducted lithosphere

  7. The theory of the elastic rebound Forces associated with plate motion act on plates, but friction inhibits motion until a given stress is reached. Then, slip occurs suddenly.

  8. Friction in the fault plane

  9. Cycles of the elastic rebound

  10. Description of a fault plane

  11. Three angles to characterize a fault plane and its slip

  12. Normal faults in extension regions like on mid-oceanic ridges, graben structures • Reverse faults in regions under compression, like subduction zones • Strike-slip faults along transform faults or in regions with shear

  13. Plate Boundaries

  14. Trace of the Fuyun earthquake (Mongolia) Fault trace 60 years after an M=8 earthquake

  15. Lamia fault, Greece.

  16. Strike-slip earthquake in Landers (California)

  17. Surface traces of faults after erosion

  18. Most fault systems are complex The North-Anatolian fault close to Istanbul

  19. The tectonic setting of the North-Anatolian fault

  20. Focus: where the slip starts at depth Epicenter: its projection on the surface

  21. The rupture propagates along the fault plane at a velocity of about 3km/s. The rupture lasts a few seconds for moderate earthquakes.

  22. Dimensions of earthquake fault planes: • largest dimensions: 1000km (Chile 1960) • smallest: no lower limit. Any small crack is an earthquake. Thrust Fault Example

  23. Thrust Fault Example

  24. Chap 19: Earthquakes • What is an earthquake and its relation to plate tectonics • The seismic waves • How to locate an earthquake • The sizes of an earthquake and how to measure them • Earthquake prediction • Seismic hazard and seismic risk

  25. Seismic waves Distinguish between the earthquake itself (some motion on a fault) and the vibrations that this sudden motion generates in the surrounding media: the seismic waves. Destruction come from the seismic waves associated with the earthquake.

  26. Seismic waves = vibrations • Equivalent to sound waves in the air or waves in the water. The earthquake is the stone you throw in the water.

  27. Seismic waves produced by earthquakes

  28. The waves propagate away from the earthquake, also called source

  29. Seismic waves propagate at velocities of a few km/s: much faster than water waves or sound waves in the air, for which the velocity is 0.3km/s. • At a few km from an explosion, the ground vibration will arrive before the sound.

  30. In the air or in fluids, we have pressure waves only. In queues also. • In solids, we have pressure and shear waves: http://www.whfreeman.com/understandingearth

  31. The periods of these waves: from around 0.01s (local earthquakes) to 53 mn (maximum on Earth)

  32. How are these waves registered? They are registered by seismographs. You have different types of seismographs: • Short-period: for rapid vibrations • Long period: for slow vibrations • Broadband: for all vibrations

  33. The principle of a seismograph:a damped pendulum. weight which can oscillate recording system + clock

  34. Long period electromagnetic seismographs at ATD (Djibouti)

  35. The entrance to the ATD station (Djibouti)

  36. The electronic equipment at ATD:

  37. The entrance of the tunnel to the KIP station (Hawai)

  38. Seismological stations in Norway + one in the basement of the department

  39. Chap 19: Earthquakes • What is an earthquake and its relation to plate tectonics • The seismic waves • How to locate an earthquake • The sizes of an earthquake and how to measure them • Earthquake prediction • Seismic hazard and seismic risk

  40. Seismic waves produced by earthquakes

  41. Velocities of waves: P waves: about 5.6 km/s in the crust (first few tens of km in the Earth) S waves: about 3.4 km/s in the crust

  42. We can read the arrival time of the P wave tp. If we knew the origin time of the earthquake t0, we could write: tp = t0 + d / Vp which implies for the distance: d = Vp*(tp – t0)

  43. The arrival times of the P and S waves are: tp = t0 + d / Vp ts = t0 + d / Vs which implies: ts – tp = d / Vs – d / Vp = d ( 1/Vs -1/Vp ) = d (Vp-Vs)/(VsVp) This gives: d = (ts - tp) Vs Vp / (Vp – Vs) or about d = 8 (ts-tp) for d in km and t in s and local earthquakes

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