2.6 Seismic evidence for the Earth's structure

Question 1

Define the term stress as it relates to geology.

Answer 1

the force per unit area acting on or within a body

Question 2

Define the term strain as it relates to geology.

Answer 2

the change in shape of a body in response to stress

Question 3

What is a seismometer?

Answer 3

a device which receives seismic vibrations and converts them into a signal which can be transmitted and recorded

Question 4

What is a seismogram?

Answer 4

the paper or electronic record created by a seismograph

Question 5

What is a seismograph?

Answer 5

a device which receives and records seismic vibrations

Question 6

Which is the first of the seismic waves to arrive at a seismic station?

Answer 6

P-waves

Question 7

What are the key characteristics of P waves?

Answer 7

Primary - travel fastest and arrive first
Push - longitudinal and compressional waves (the vibration of the rock particles is back and forth, like sound waves)
Pressure - the particles alternately move together (compression) and apart (rarefaction) in the direction of travel of the wave

Question 8

What are the key characteristics of S waves?

Answer 8

Secondary - travel more slowly and arrive after P waves but before surface waves
Shear - the movement of the particles is sideways, in a shearing motion, at right angles to the direction of travel

Question 9

What are the key characteristics of L waves?

Answer 9

Long - the wavelength of the wave is longer than for P and S waves
Last - they are the slowest of the three main waves and arrive last

Question 10

Which factors affect the speed of a P wave?

Answer 10

density
incompressibility
rigidity

Question 11

Which factors affect the speed of a S wave?

Answer 11

density
rigidity

Question 12

What are L waves?

Answer 12

surface waves (Love waves) where the rock particles oscillate in a circular motion so the waves lose energy quickly with distance

Question 13

What is meant by the term earthquake focus?

Answer 13

the origin of the earthquake, where energy is relased

Question 14

What is meant by the term earthquake epicentre?

Answer 14

the point on the Earth’s surface directly above the focus

Question 15

At what angle from the epicentre are P waves not seen (P wave shadow zone)?

Answer 15

103° - 142°

Question 16

At what angle from the epicentre are S waves not seen (S wave shadow zone)?

Answer 16

103°

Question 17

What is the reason for the P wave shadow zone?

Answer 17

P waves are refracted at the core-mantle boundary due to the decrease in rigidity (outer core is a liquid) and therefore decrease in speed

Question 18

What is the reason for the S wave shadow zone?

Answer 18

the outer core is a liquid and S waves cannot pass through a liquid

Question 19

Why do surface waves not have a shadow zone?

Answer 19

they travel along the surface and are therefore not affected by the physical state of the Earth’s layers

Question 20

What is the name for the discontinuity found at around 35 km depth?

Answer 20

the Moho
(the Mohorovičić Discontinuity)

seismic velocities decrease from 7.2 to 8.1 km s^-1

Question 21

What is the name for the discontinuity found at around 2900 km depth?

Answer 21

the Gutenberg Discontinuity

P wave velocities decrease significantly and S waves disappear

Question 22

What is the name for the discontinuity found at around 5100 km depth?

Answer 22

the Lehmann Discontinuity

P wave velocities increase

Question 23

What causes the change in speed of seismic waves at the Moho discontinuity?

Answer 23

change in composition and density of the rocks of the crust and the mantle

Question 24

What causes the change in speed of seismic waves at the Gutenberg discontinuity?

Answer 24

change in state from the solid mantle to the liquid outer core

Question 25

What causes the change in speed of seismic waves at the Lehmann discontinuity?

Answer 25

change in state from the liquid outer core to the solid inner core