Lecture 5 - Earthquake Faulting and Seismic Waves

Question 1

What indicates that slip (relative displacement) has occurred along linear faults?

Answer 1

Surface ruptures usually generated by large earthquakes.

Question 2

How far can faulting in large earthquakes can be traced?

Answer 2

tens-to-hundreds of kilometres

Earthquakes do not occur at points or along lines

Question 3

Faults are ……

Answer 3

planar

rare exposures show striations and corrugations indicating the direction of slip

Question 4

What are the three types of faulting?

Answer 4

reverse/thrust
normal
strike-slip

Question 5

How do reverse or thrust faults rupture?

Answer 5

Due to horizontal compressive stress. One side is thrust over the other as the two blocks move together, leading to shortening (contraction) and crustal thickening.

Question 6

What are some examples of reverse/thrust faulting?

Answer 6

1999 M 7.3 Chi-chi earthquake, Taiwan.
Main Himalayan Thrust in Nepal - responsible for raising the Himalayas!
Cascadia megathrust fault.

Question 7

What is a fault scarp?

Answer 7

Where the thrust overhang has collapsed.

Question 8

Where do reverse/thrust faulting earthquakes usually occur?

Answer 8

Convergent plate boundaries: subduction zones and continental collision zones.

Question 9

How do normal faults rupture?

Answer 9

Due to horizontal tension stress. One side of the fault slides down the other as the two blocks move apart, leading to extension and crustal thinning.

Question 10

What are some examples of normal faulting?

Answer 10

1959 M 7.3 Hebgen Lake earthquake, Montana.

2016 M 6.6 Norcia earthquake, Italy.

Question 11

Where do normal faulting earthquakes usually occur?

Answer 11

Divergent plate boundaries: mid-ocean ridges and continental rifts. However, under certain circumstances they also occur in subduction zones and continental collisions.

Question 12

How do strike-slip faults rupture?

Answer 12

Due to simple shear. The two sides move laterally past one another. There is shortening in one direction and extension in another, but no crustal thickening or thinning.

Question 13

A strike-slip fault is left-lateral if….

Answer 13

the motion of the opposite side is to the left

Question 14

A strike-slip fault is right-lateral if….

Answer 14

the motion of the opposite side is to the right

Question 15

What are some examples of strike-slip faulting?

Answer 15

2019 M 7.1 Ridgecrest, CA earthquake (right-lateral).

2016 M 7.0 Kumamoto earthquake, Japan (right-lateral).

Question 16

Where do strike-slip faulting earthquakes usually occur?

Answer 16

On strike-slip plate boundaries, including oceanic transform faults and continental shear zones.
However, they also occur within continental collision zones and subduction zone fore-arcs.

Question 17

What are oblique faults?

Answer 17

Faults that have a component of reverse or normal slip and a component of strike-slip.

Question 18

What is stick-slip behaviour?

Answer 18

Faults exhibit stick slip behaviour. The fault is “stuck” between earthquakes, until stresses overcome friction and the sides “slip”.

Question 19

What is the earthquake cycle?

Answer 19

It describes how stress builds up on a fault over long periods of time – driven by plate tectonics – and is then released in an earthquake taking just a few seconds.

Question 20

What happens during the interseismic period of the earthquake cycle?

Answer 20

Steady motion occurs away from the fault but the fault itself is “locked”.

Question 21

What happens during the coseismic phase of the earthquake cycle?

Answer 21

The earthquake and elastic rebound.

Question 22

What happens during the earthquake cycle at subduction zone megathrusts?

Answer 22

It involves vertical as well as horizontal motions.
During the interseismic period, the lower plate slowly subducts and the megathrust is locked. At the trench the upper plate is dragged downwards, but at the coast it contracts and bulges upwards.
During the coseismic phase, these motions are reversed. The mega- thrust slips, the upper plate rebounds westwards, the sea-floor at the trench is suddenly uplifted and the coastline subsides.

Question 23

What are two types of body waves?

Answer 23

Primary compressional P-waves

Secondary shear S-waves

Question 24

What are two types of surface waves?

Answer 24

Love waves

Rayleigh waves

Question 25

How are seismic waves recorded?

Answer 25

By seismometers or seismographs, highly- sensitive instruments that measure and amplify ground motions.

Question 26

What is a seismogram?

Answer 26

A graphical representation of ground velocity at a point in space (the seismometer) as a function of time.

Question 27

How do p-waves and s-waves help to determine Earth’s interior structure?

Answer 27

Refraction and reflection of body waves by major boundaries within the Earth allows us to determine its interior structure.

Question 28

How can we pinpoint the epicentre of an earthquake?

Answer 28

By combining distances for three (or more) seismometers.

Question 29

How do we calculate the distance between a seismometer and an earthquake?

Answer 29

[VpVs(Ts- Tp)] / [Vp-Vs] = x
Where; Ts-Tp is the separation time between p-waves and s-waves, x is distance, Vp is the velocity of the p-waves and Vs is the velocity of the s-waves.

Question 30

How deep do fault planes typically reach?

Answer 30

around 15km

Normally the only part of the crust cool enough to be brittle.

Question 31

What is the seismogenic zone?

Answer 31

The area containing fault planes.

Question 32

What is the rupture area?

Answer 32

The part of the fault plane that slips in an earthquake.

Question 33

What is a surface rupture?

Answer 33

Where the rupture area intersects the surface.

Question 34

What is the hypocentre?

Answer 34

The point on the fault plane where slip starts.

Question 35

What is the epicentre?

Answer 35

The point at the surface directly above the hypocentre.

Question 36

When are body waves and surface waves released?

Answer 36

When slip in an earthquake releases elastic strain stored in the crust as seismic waves.
Body waves pass through the body of earth and surface waves travel around Earth’s surface.

Question 37

What are the characteristics of p-waves?

Answer 37

Particle motion in the direction of wave travel.
Very fast.
Cause little damage.

Question 38

What are the characteristics of s-waves?

Answer 38

Particle motion perpendicular to the direction of wave travel.
Slower than p-waves but still much faster than the surface waves.
Cause little damage.

Question 39

What are the characteristics of Love waves?

Answer 39

Particle motion perpendicular to the direction of wave travel.
Slower than body waves but much more damaging.

Question 40

What are the characteristics of Rayleigh waves?

Answer 40

Involve elliptical particle motion in the direction of wave travel.
Slower than body waves but much more damaging.