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Geological natural hazards > Earthquake principles > Flashcards

Earthquake principles Flashcards

1
Q

What is an earthquake?

A

Sudden and substantial brittle failure along a fault

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2
Q

How can a quote from legally blonde help describe earthquakes?

A

Bend and snap

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3
Q

What is the seismic moment and what is it dominated by?

A

Seismic energy released (Fracture and frictional sliding dominate)

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4
Q

What is the sliding from earthquakes like?

A

Stick-slip instead of stable sliding

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5
Q

What is the majority of the force opposing fault movement?

A

Friction

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6
Q

What happens to frictional stress prior to movement?

A

Stress increases until slip where static friction is exceeded

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7
Q

What is the differece between dynamic friction (when object moving) and static friction?

A

Dynamic is less then static

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8
Q

When will movement stop for a fault slipping?

A

Will move until force drops well below dynamic friction (braking takes a long time) then sticks

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9
Q

What happens during the stick phase of a fault?

A

Stress will increase until sudden release in next slip event

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10
Q

What is the timing like for the EQ duration and interseismic time?

A

EQ duration very short - seconds to minutes
Interseismic can be years, deacdes, millenia

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11
Q

What is elastic rebound?

A

The sudden release of the stored elastic energy is the earthquake and produces elastic or seismic waves that radiate outwards

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12
Q

What is the base of the seismogenic zone?

A

the lowest point which eathquakes occur

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13
Q

What is the hypocenter (focus)?

A

point within the earth where an earthquake rupture starts

Not always max point of displacement or middle of rupture

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14
Q

What is the epicenter?

A

point on the Earth’s surface directly above the hypocenter

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15
Q

What is the magnitude of an earthquake?

A

measure of earthquake’s power on a logarithmic scale

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16
Q

What is the rupture zone?

A

occurs between the base of the seismogenic zone (~ brittle crust) and surface. Only largeish earthquakes (6+) actually reach the surface => a surface break

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17
Q

What are aftershocks?

A

small event following main earthquake as fault zone readjusts to main slip event.

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18
Q

What can aftershocks help define?

A

Distribution can accurately define main rupture area

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19
Q

What are siesmic waves?

A

Energy radiating out from the rupture

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20
Q

What will snap be like at 0 seconds?

A

Rupture expands in a skewed circular sense on fault plane sending seismic waves all directions

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21
Q

What will snap be like at 5 seconds?

A

Fault cracks at surface
Rupture still expanding as crack on fault
Rocks at surface begin to rebound from defomred state

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22
Q

What will snap be like at 10 seconds?

A

Fault crack extends
Rupture front progresses down fault plane - reducing stress - energy continues radiating (waves)

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23
Q

What does a longer rupture mean for earthquakes?

A

LArger earthquakes which generate longer more complex shaking

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24
Q

What will snap be like after 20 seconds?

A

Rupture progressed along entire length of fault and EQ stops

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25
Q

WHat are the 4 types of wave?

A

P-wave (primary)
S-wave (secondary)
Love and Rayleigh (surface waves)

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26
Q

How is the magnitude of deflection described?

A

Amplitude (m, cm or mm)

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27
Q

Will amplitude be positive or negative?

A

can be both

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28
Q

How is the time between adjacent peaks or troughs (a cycle) presented?

A

Period T (unit is s)

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29
Q

How is frequency present?

A

1/ frequency (number of cycles per second)

Unit - Hertz or Hz

30
Q

What is the outer limit of the seismic wave ripple called?

A

The wavefront

31
Q

What depth will body waves travel at?

A

Deep under the earths surface

32
Q

What atre the general characteristics of body waves?

A

Not typically destructive but give a useful indiaction of EQ location

33
Q

What are p-waves?

A

Primary waves

34
Q

How do p waves travel?

A

As compression (+) and dilation (-) in the same direction
Fastest travelling

35
Q

What are the velocities of p-waves through different materials?

A

Air, dry soil ~ 300 m/s
Water, wet soil 1500 m/s
Sedimentary: 2000-5000 m/s
Crust ~ 6 km/s, Mantle >8 km/s

36
Q

What is the movement like for s-waves?

A

Particle motion is side to side or up and down at 90 degrees to forward motion of wave

37
Q

What are s-waves?

A

secondary waves (arrive second)

38
Q

What material do s-waves not tarvel through?

A

Liquids

39
Q

What are the velocities of s waves through different materials?

A

Air, water, melt 0 m/s (only solids)
Sedimentary: 1000-2500 m/s
Crust ~ 3-4 km/s
Mantle > 4-5 km/s

40
Q

What do s-waves define the speed of?

A

Crack proporgation

41
Q

Why are p and s waves curved when they travel through the earth?

A

Due to varaitions in speed as they travel through the layers

Always arrive from the bottom

42
Q

What will the travel plane be for surface waves?

A

just surface dont go deep

43
Q

What is particle motion like depth wise for love and rayleigh waves?

A

Greatest motion at surface which dies out with depth
Wave confined to upper few km

44
Q

Why are surface waves so destructive?

A

Large motions at the surface

45
Q

What is the movement of love waves?

A

strong side to side shaking at 90 degrees to forward motion of wave i.e. in transverse direction - orthogonal

46
Q

What is the movement of rayleigh waves?

A

up-down (vertical) as well as forwards-backwards (radial) shaking. Slowest.
Retrograde elliptical

46
Q

What are the 3 directions of seismographs?

A

Vertical movement
Horizontal (E-W)
Horixontal (N-S)

47
Q

What waves will be picked up by the veritcal seismograph?

A

P waves arrive steeply with some indication of S waves
Rayleigh waves cause big disturbance

48
Q

What is wanted from the 2 horizontal seismographs?

A

Want to find radial and transverse to locate direction of EQ

49
Q

What is radial and transverse movements?

A

Radial - motions in direction of EQ
Transverse - perpendicular to EQ location

50
Q

What seismographs will s waves show on?

A

Vertical
Radial
Transverse

51
Q

What seismogrpah will love waves show on?

A

side to side movement (transverse) so will show on transverse graph

52
Q

What seismographs will Rayleigh show on?

A

Vertical and Radial

53
Q

What is used to determine the distance from EQ?

A

S-P time interval

54
Q

What was the global seismometer network originally set up to monitor?

A

Nuclear monitoring

55
Q

What is distance around the earth measured in for tele seismic earthquakes?

A

Degrees around the earth

56
Q

How many seismograph data points are needed to locate the diatacnce of EQ?

A

3 where the circle of diatnce all intercept= epicentre location

57
Q

What is the double couple mechanism for earthquakes?

A

both sides of the fault spring forward during the quake, radiating energy in all directions

58
Q

What are te three main types of fault?

A

Normal (hanging wall down, steep)
Thrust fault (hanging wall up, gentle)
Strike-slip (vertical)

59
Q

What is needed for a fault to slip?

A

needs to overcome inherent strength and friction

60
Q

What is a dextral fault?

A

right lateral - right moving fault

61
Q

What is a sinistral fault?

A

left lateral - left moving

62
Q

What direction will experince a postive wave first?

A

Those experncing compression

63
Q

What area will experience a negative wave first?

A

Those from dilation

64
Q

Whar are the 2 nodal planes and what happens?

A

One is the rupture other is the auxillary nodal plane with no clear first motion

65
Q

What is the name for the diagram produced from the nodal planes?

A

focal mechanism or beachball

66
Q

What makes beachballs difficult to interpret?

A

unless we know the orientation of the rupture we cannot distinguish easily between the fault plane and the auxiliary nodal plane

67
Q

What determines which global EQ stations get compression or dilation first?

A

oreintation of ray compared to nodal planes

68
Q

How do we work out which nodal plane is the fault plane?

A

Geology
Surface breaks (line of visible rupture, often not seen unless >6)
Isoseismals
Aftershock distribution

69
Q

What are isoseismals?

A

zones of equal damage/shaking elongate along direction of fault plane (like contour lines of equal shaking)

70
Q

How can aftershocks help find the fault plane?

A

minor earthquakes that occur after the main earthquake and cluster around the main rupture

Geological natural hazards (13 decks)

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