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Plate Tectonics > Earthquake Mechanisms > Flashcards

Earthquake Mechanisms Flashcards

1
Q

What are earthquakes?

A

A sudden release of elastic energy stored in the lithosphere.

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2
Q

What’s the epicenter?

A

The point on the surface above the origin of the earthquake (hypocenter).

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3
Q

What are shortcomings of the Richter scale function to quantify earthquake magnitude?

A

The function becomes non linear at large distances and for large magnitudes.

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4
Q

What are the two types of seismic body waves?

A

P-wave (compressional)

S-wave (shear)

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5
Q

What are the two types of surface waves?

A 
Rayleigh wave (circular motion)
Love wave (across surface motion)
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6
Q

How are tectonic regimes usually defined?

A

By terms of their relative magnitudes of vertical stress (Sv) and two mutually perpendicular horizontal stresses (Shmax & Shmin).

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7
Q

What are the earthquake “beach balls” called?

A

Focal mechanisms - give you an idea of fault orientations and the direction seismic waves move in (on the balls, they move from white to black). Different colours indicate different depth.

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8
Q

What determines the amount of energy released in an earthquake?

A

The slip area, and the slip vector. The large the slip area, the more energy will be released.

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9
Q

Is it possible to determine absolute stress magnitude values at an earthquake?

A

No. While the relative magnitude can be determined by fault type, orientation and motion, there is no measure for the absolute magnitude.

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10
Q

What does this Earthquake Wave Notation mean?

Pg or P

A

P - Direct wave

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11
Q

What is the Wadati Benioff zone?

A

A planar zone of seismicity that corresponds with the downgoing slab at a subduction zone, and develops beneath volcanic island arcs and continental margins above subduction zones.

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12
Q

Why is the Wadati Benioff zone useful?

A

Provides 3D map of the downgoing slab

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13
Q

How deep can the Wadati Benioff zone be?

A

Up to 670km deep

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14
Q

What does this Earthquake Notation mean?

Pn or Sn

A

Head wave in the mantle (the refracted wave)

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15
Q

What does this Earthquake Notation mean?

PmP

A

PmP: Reflected wave from Moho

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16
Q

What does this Earthquake Notation mean?

PP, SS, PS

A

PP, SS, PS: surface reflections that do not pass through boundary

17
Q

What does this Earthquake Notation mean?

PcP, ScP etc

A

Reflected from the core boundary

18
Q

What does this Earthquake Notation mean?

PKP

A

Refracted wave through the core

19
Q

What does this Earthquake Notation mean?

PKIKP

A

Refracted wave through the inner core

20
Q

What are the maximum and minimum stress orientations of:

- reverse fault

A 
Maximum stress (S1): Horizontal
Minimum stress (S3): Vertical
21
Q

What dip angle does Anderson’s Faulting Theory predict for a reverse fault?

A

~ 30 degrees

22
Q

What dip angle does Anderson’s Faulting Theory predict for a normal fault?

A

60 degrees

23
Q

What dip angle does Anderson’s Faulting Theory predict for a strike-slip fault?

A

90 degrees

24
Q

What are the assumptions of Anderson’s Faulting Theory?

A
  • Surface of earth is unconfined, and no shear stresses act on it
  • Homogenous rocks
  • Coloumb behaviour
25
Q

What are the maximum and minimum stress orientations of:

- normal fault?

A 
Maximum stress (S1): vertical
Minimum stress (S3): horizontal
26
Q

What are the maximum and minimum stress orientations of:

- strike slip fault?

A

Maximum (S1): Horizontal

Minimum (S3): Horizontal

27
Q

What are exceptions to Anderson’s Faulting Theory?

A
  • Thrust faults

- Low angle faults

28
Q

What do nodal planes show?

A

Orientation of the fault (from north) - always gives two possible solutions

29
Q

What do auxiliary planes show?

A

The dip angle of the fault

30
Q

Where does the peak of the emitted energy occur?

A

At 45 degrees to the source (in all directions)

31
Q

What is the change in composition from the upper mantle to lower mantle?

A

In the upper mantle, change from olivine to spinel at the 410km discontinuity, then at the 660km discontinuity with the lower mantle it changes again to perovskite and magnesio-wuestite

32
Q

Where do extensional earthquakes occur at a subduction zone?

A

Shallow crustal zone, on the subducting plate

33
Q

What is Snell’s Law of Refraction?

A

Sin theta 1/ sin theta 2 = v1/ v2

Plate Tectonics (10 decks)

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