Earthquake Mechanisms

Question 1

What are earthquakes?

Answer 1

A sudden release of elastic energy stored in the lithosphere.

Question 2

What’s the epicenter?

Answer 2

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

Question 3

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

Answer 3

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

Question 4

What are the two types of seismic body waves?

Answer 4

P-wave (compressional)

S-wave (shear)

Question 5

What are the two types of surface waves?

Answer 5

Rayleigh wave (circular motion)
Love wave (across surface motion)

Question 6

How are tectonic regimes usually defined?

Answer 6

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

Question 7

What are the earthquake “beach balls” called?

Answer 7

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.

Question 8

What determines the amount of energy released in an earthquake?

Answer 8

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

Question 9

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

Answer 9

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

Question 10

What does this Earthquake Wave Notation mean?

Pg or P

Answer 10

P - Direct wave

Question 11

What is the Wadati Benioff zone?

Answer 11

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.

Question 12

Why is the Wadati Benioff zone useful?

Answer 12

Provides 3D map of the downgoing slab

Question 13

How deep can the Wadati Benioff zone be?

Answer 13

Up to 670km deep

Question 14

What does this Earthquake Notation mean?

Pn or Sn

Answer 14

Head wave in the mantle (the refracted wave)

Question 15

What does this Earthquake Notation mean?

PmP

Answer 15

PmP: Reflected wave from Moho

Question 16

What does this Earthquake Notation mean?

PP, SS, PS

Answer 16

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

Question 17

What does this Earthquake Notation mean?

PcP, ScP etc

Answer 17

Reflected from the core boundary

Question 18

What does this Earthquake Notation mean?

PKP

Answer 18

Refracted wave through the core

Question 19

What does this Earthquake Notation mean?

PKIKP

Answer 19

Refracted wave through the inner core

Question 20

What are the maximum and minimum stress orientations of:

- reverse fault

Answer 20

Maximum stress (S1): Horizontal
Minimum stress (S3): Vertical

Question 21

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

Answer 21

~ 30 degrees

Question 22

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

Answer 22

60 degrees

Question 23

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

Answer 23

90 degrees

Question 24

What are the assumptions of Anderson’s Faulting Theory?

Answer 24

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

Question 25

What are the maximum and minimum stress orientations of: | - normal fault?

Answer 25

``` Maximum stress (S1): vertical Minimum stress (S3): horizontal ```

Question 26

What are the maximum and minimum stress orientations of: | - strike slip fault?

Answer 26

Maximum (S1): Horizontal | Minimum (S3): Horizontal

Question 27

What are exceptions to Anderson’s Faulting Theory?

Answer 27

- Thrust faults | - Low angle faults

Question 28

What do nodal planes show?

Answer 28

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

Question 29

What do auxiliary planes show?

Answer 29

The dip angle of the fault

Question 30

Where does the peak of the emitted energy occur?

Answer 30

At 45 degrees to the source (in all directions)

Question 31

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

Answer 31

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

Question 32

Where do extensional earthquakes occur at a subduction zone?

Answer 32

Shallow crustal zone, on the subducting plate

Question 33

What is Snell’s Law of Refraction?

Answer 33

Sin theta 1/ sin theta 2 = v1/ v2