Seismology

Question 1

Types of Seismic wave:

Answer 1

Primary waves - propagations parallel to travel
Secondary waves - a shear wave, propagations perpendicular to travel
Surface waves - the deformation is at the surface. They are strong if the event is near the surface.
-> Love waves - sideways deformation
-> Rayleigh waves - up and down, similar to water waves

Question 2

How does a p- or s-wave behave when crossing a boundary?

Answer 2

the wave gets split into its p- and s- components, with one s and one p wave being reflected and one s and p being refracted.

Question 3

What do these seismic wave notations mean: P S K I J pP PP c i PKiKP

Answer 3

• P - primary wave through the mantle/crust
• S - secondary wave through the mantle/crust
  
  • K - p-wave through the outer core (s-waves cannot pass through outer core)
  • I - p-wave through the inner core
  • J - s-wave in the inner core (rare since must have been converted within the inner core)
  • pP - bounces off the surface above the hypocentre then away
  • PP - bounces away once
  • c - reflects off outer core boundary
  • i - reflects off inner core boundary
    
    • PKiKP is a p-wave that goes through the mantle, outer core then gets bounced off the inner core to come back through the outer core and mantle.

Question 4

What is a Moho T-T curve

Answer 4

Plots time vs offset of waves. There are two clear p- and s- trends, although there are many branches as they get reflected and refracted.

Question 5

How can the crust-mantle boundary be found with seismic waves?

Question 6

What is the transition zone and the Low velocity zone?

Answer 6

TZ - An area 400-600km deep where wave speed increases greatly due to phase changes. this means it is possible to have triplications, where 3 p-waves that have not bounced can reach the same place.
LVZ - an area between 80-200km deep where wave speed is decreased due to partial melts.

Question 7

How do seismic waves show the outer core is liquid?

Answer 7

s-waves cannot travel through liquids, and since these are not detected more than about 90o from an event, they cannot travel through this outer core.

Question 8

How can the hypocentre be found?

Answer 8

By comparing travel times of waves.

Question 9

What are ‘beach balls’?

Answer 9

Diagrams showing boundary movement. Black areas have been compressed and white been dilated.

Question 10

Seismic moment equation?

Answer 10

M0 = µAs (scalar moment)
A : area of fault plane
s : average slip
µ : shear modulus

Question 11

Moment magnitude equation?

Answer 11

Mw =2/3 log10(M0) − 6.1

Question 12

Where is the p-wave shadow?

Answer 12

Between 98o and 145o