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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
WHat are the 4 types of wave?
P-wave (primary) S-wave (secondary) Love and Rayleigh (surface waves)
26
How is the magnitude of deflection described?
Amplitude (m, cm or mm)
27
Will amplitude be positive or negative?
can be both
28
How is the time between adjacent peaks or troughs (a cycle) presented?
Period T (unit is s)
29
How is frequency present?
1/ frequency (number of cycles per second) | Unit - Hertz or Hz
30
What is the outer limit of the seismic wave ripple called?
The wavefront
31
What depth will body waves travel at?
Deep under the earths surface
32
What atre the general characteristics of body waves?
Not typically destructive but give a useful indiaction of EQ location
33
What are p-waves?
Primary waves
34
How do p waves travel?
As compression (+) and dilation (-) in the same direction Fastest travelling
35
What are the velocities of p-waves through different materials?
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
What is the movement like for s-waves?
Particle motion is side to side or up and down at 90 degrees to forward motion of wave
37
What are s-waves?
secondary waves (arrive second)
38
What material do s-waves not tarvel through?
Liquids
39
What are the velocities of s waves through different materials?
Air, water, melt 0 m/s (only solids) Sedimentary: 1000-2500 m/s Crust ~ 3-4 km/s Mantle > 4-5 km/s
40
What do s-waves define the speed of?
Crack proporgation
41
Why are p and s waves curved when they travel through the earth?
Due to varaitions in speed as they travel through the layers | Always arrive from the bottom
42
What will the travel plane be for surface waves?
just surface dont go deep
43
What is particle motion like depth wise for love and rayleigh waves?
Greatest motion at surface which dies out with depth Wave confined to upper few km
44
Why are surface waves so destructive?
Large motions at the surface
45
What is the movement of love waves?
strong side to side shaking at 90 degrees to forward motion of wave i.e. in transverse direction - orthogonal
46
What is the movement of rayleigh waves?
up-down (vertical) as well as forwards-backwards (radial) shaking. Slowest. Retrograde elliptical
46
What are the 3 directions of seismographs?
Vertical movement Horizontal (E-W) Horixontal (N-S)
47
What waves will be picked up by the veritcal seismograph?
P waves arrive steeply with some indication of S waves Rayleigh waves cause big disturbance
48
What is wanted from the 2 horizontal seismographs?
Want to find radial and transverse to locate direction of EQ
49
What is radial and transverse movements?
Radial - motions in direction of EQ Transverse - perpendicular to EQ location
50
What seismographs will s waves show on?
Vertical Radial Transverse
51
What seismogrpah will love waves show on?
side to side movement (transverse) so will show on transverse graph
52
What seismographs will Rayleigh show on?
Vertical and Radial
53
What is used to determine the distance from EQ?
S-P time interval
54
What was the global seismometer network originally set up to monitor?
Nuclear monitoring
55
What is distance around the earth measured in for tele seismic earthquakes?
Degrees around the earth
56
How many seismograph data points are needed to locate the diatacnce of EQ?
3 where the circle of diatnce all intercept= epicentre location
57
What is the double couple mechanism for earthquakes?
both sides of the fault spring forward during the quake, radiating energy in all directions
58
What are te three main types of fault?
Normal (hanging wall down, steep) Thrust fault (hanging wall up, gentle) Strike-slip (vertical)
59
What is needed for a fault to slip?
needs to overcome inherent strength and friction
60
What is a dextral fault?
right lateral - right moving fault
61
What is a sinistral fault?
left lateral - left moving
62
What direction will experince a postive wave first?
Those experncing compression
63
What area will experience a negative wave first?
Those from dilation
64
Whar are the 2 nodal planes and what happens?
One is the rupture other is the auxillary nodal plane with no clear first motion
65
What is the name for the diagram produced from the nodal planes?
focal mechanism or beachball
66
What makes beachballs difficult to interpret?
unless we know the orientation of the rupture we cannot distinguish easily between the fault plane and the auxiliary nodal plane
67
What determines which global EQ stations get compression or dilation first?
oreintation of ray compared to nodal planes
68
How do we work out which nodal plane is the fault plane?
Geology Surface breaks (line of visible rupture, often not seen unless >6) Isoseismals Aftershock distribution
69
What are isoseismals?
zones of equal damage/shaking elongate along direction of fault plane (like contour lines of equal shaking)
70
How can aftershocks help find the fault plane?
minor earthquakes that occur after the main earthquake and cluster around the main rupture

Geological natural hazards (21 decks)

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