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

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6 Earthquakes in subduction zones

7 Earthquakes in continental regions

8 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

9 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.

10 Friction in the fault plane

11 Cycles of the elastic rebound

12 Description of a fault plane

13 Three angles to characterize a fault plane and its slip

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15 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

16 Plate Boundaries

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

19 Lamia fault, Greece.

20 Strike-slip earthquake in Landers (California)

21 Surface traces of faults after erosion

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

23 The tectonic setting of the North-Anatolian fault

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

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

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

27 Thrust Fault Example

28 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

29 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.

30 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.

31 Seismic waves produced by earthquakes

32 The waves propagate away from the earthquake, also called source

33 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.

34 In the air or in fluids, we have pressure waves only. In queues also. In solids, we have pressure and shear waves: rth

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

37 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

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

39 Long period electromagnetic seismographs at ATD (Djibouti)

40 The entrance to the ATD station (Djibouti)

41 The electronic equipment at ATD :

42 The entrance of the tunnel to the KIP station (Hawai)

43 + one in the basement of the department Seismological stations in Norway

44 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

45 Seismic waves produced by earthquakes

46 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

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48 We can read the arrival time of the P wave t p. If we knew the origin time of the earthquake t 0, we could write: t p = t 0 + d / V p which implies for the distance: d = V p *(t p – t 0 )

49 The arrival times of the P and S waves are: t p = t 0 + d / V p t s = t 0 + d / V s 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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51 Wave paths for regional earthquakes

52 Wave propagation for distant earthquakes

53 Main layers in the Earth

54 P P S

55 Wavepaths for distant earthquakes

56 Paths of S waves in the mantle and in the core

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60 Note the time scale: long-period instruments are required to register these waves.

61 Surface waves: late, long-period and large amplitude waves

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63 Surface waves: late, long-period and large amplitude waves

64 R1 R2

65 Surface waves: late, long-period and large amplitude waves

66 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

67 The magnitude(s) measure the amplitude of the seismic waves and the energy of the earthquake. The intensity measures the destructions related to the earthquake.

68 The Richter magnitude of local earthquakes

69 The amplitude of the ground displacement increases by a factor of 10 each time the magnitude increases by 1. The energy increases by a factor of about 33 for a step of 1 in magnitude.

70 ML for local earthquakes (Richter magnitude adapted to local structure) Mb, Ms: measured on P waves or surface waves for distant earthquakes Moment magnitude Mw related to the seismic moment M 0 : a more accurate measurement which tells something about the total energy of the earthquake

71 The seismic moment M0 M 0 = μ S d μ is the rigidity around the fault zone S is the surface of rupture d is the length of slip along the fault plane We make a magnitude Mw out of it.

72 Bigger Faults Make Bigger Earthquakes

73 Bigger Earthquakes Last a Longer Time

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75 Earthquakes in Norway between the 4 th and 11 th of November 2004

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77 The intensity: a location dependent measurement of the destructions caused by an earthquake. From I (not felt) to XII (total destruction). Based on field observations and questionnaires.

78 ShakeMaps

79 Chap19: 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

80 Can we predict earthquakes? At long term: partly, at least along plate margins. At intermediate term: some recent results based on stress measurements and calculations At short term: no.

81 Long-term prediction based on the theory of the elastic rebound

82 Cycles of the elastic rebound

83 Seismic gaps at present time

84 Intermediate-term prediction: based on stress redistribution after an earthquake. Which fault is the next one to break in a complex fault system?

85 The North-Anatolian fault system close to Istanbul

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89 Short-term prediction: not possible yet Therefore, we have to take earthquake risk into account when we build.

90 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

91 The seismic hazard Measure how frequent and how strong are earthquakes in a given region The earthquakes have been recorded for only one century. Too short time period to give a good image of the frequency of large earthquakes in many regions.

92 For regions without strong recent earthquakes, it is possible to use the number of small earthquakes to evaluate how often we get a large one.

93 It is also possible to study the traces of very old earthquakes in sediments.

94 Definition of seismic hazard: 10% probability of acceleration larger than … within 50 years. But the wave period is important also…

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97 Then you need to take into account local effects like amplification in sediments to get more detailed maps which can be used for city planning for example.

98 The seismic risk In a deserted area, it doesn’t matter if there are strong earthquakes. In a region with a dam or a nuclear power plant, even a small earthquake can be a catastrophe. The seismic risk takes into account the type of building etc in the area in addition to the vibrations caused by earthquakes.

99 The only way to prevent damage from earthquakes at the present time is to build according to special rules called the seismic code.

100 Origin of damages by earthquakes Direct: ground shaking More indirect: landslides, sediment liquefaction, tsumanis Indirect: fire, water contamination, disease

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102 What an earth vibration does to a building?

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105 Indirect effects: Landslides and avalanches Sediment liquefaction Tsunamis

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109 Tsunamis

110 Tsunami propagation across the pacific Ocean

111 Lisbon earthquake, Nov 1.,1755.

112 Exercices on the web-page of the course for next week. This presentation on the web-page also.