Seismology

Question 1

what are the 3 ways that materials deform

Answer 1

change volume (pressure from all sides)
change shape (shear)
change length (compression)

Question 2

S wave velocity equation

Answer 2

Vs = sqrt (µ / rho)

µ = shear modulus
rho = density

Question 3

p wave velocity equation

Answer 3

Vp = sqrt ((K +4/3 µ) / rho))

K = bulk modulus
µ = shear modulus

Question 4

what is the order of typical seismic velocities

Answer 4

1-10 km/s

Question 5

what type of waves are faster and what rocks are faster to move through

Answer 5

p faster than s
igneous faster than sediments

Question 6

what are the 2 body waves and their characteristics

Answer 6

p waves
- pressure/primary
- longitudinal
- arrive first

s waves
- shear/secondary
- transverse (NOT in fluid)

Question 7

what are the 2 surface waves and their characteristics

Answer 7

love waves
- pure horizontal shear

Rayleigh waves
- similar to ocean waves
- elliptical partical motion
- volume change

both:
velocity 0.9 x body waves

Question 8

how do body vs surface waves decrease their amplitude with distance

Answer 8

body - decrease with 1/r^3
surface - decrease with 1/r^2

surface decrease less with distance therefore are more destructive

Question 9

order of seismic wave arrivals

Answer 9

p wave
s wave
love wave
Rayleigh wave

Question 10

when do standing seismic waves occur

Answer 10

after large earthquakes (>7.5), constructive interference around the earth creates standing waves of frequency = resonant frequency of earth.
shaking + destruction for longer time.

Question 11

what is the wave equation in 2D

Answer 11

∂2u/∂x2 = c^2 . ∂2u/dt2

Question 12

what are the factors influencing seismic velocities

Answer 12

stiffness ( k µ )
density
pressure

temperature
melt/volatiles (liquid µ = 0)

Question 13

what is Snell’s Law

Answer 13

sinθ1/v1 = sinθ2/v2 = constant

Question 14

formula for Ray Parameter / Horizontal Slowness

Answer 14

p = u . sinθ
= sinθ / v

(θ = angle to normal)

Question 15

what is the wave slowness, u, formula

Answer 15

u = 1 / v

(v=wave velocity)

Question 16

equation for critical angle reflection

Answer 16

sin θc = v1 / v2

(can find by subbing θ=90 into snells law)

Question 17

what is Huygens principle

Answer 17

an interface between mediums acts as a source of waves

Question 18

what are head waves, how are they generated, and how do they appear on a travel time graph

Answer 18

• generated along the interface upwards due to huygens principle
• appear after critical angle
• are fastest arrival after crossover distance

Question 19

What does the travel time - distance curve look like for reflected waves and what does this show

Answer 19

• starts at vertical 2 way travel time
• gets closer to direct wave over long distances

Question 20

What are the 3 components in the ray path

Answer 20

• P -> direction of ray path
• SV (shear vertical)
• SH (shear horizontal)
  -> hor/ver oscillations perp. to direction, orthogonal to each other

Question 21

what happens to P vs SV components of a wave at an interface

Answer 21

SV refracted at a smaller angle due to slower velocity

Question 22

what should a travel time curve look like on a spherical earth vs a flat reference frame

Answer 22

spherical = looks like getting quicker (gradient decrease) as distance across surface less than actually travelled

flat = straight line constant gradient

Question 23

how to find apparent velocity of a wave from a travel time curve

Answer 23

v = 1 / slope

Question 24

Discuss the concept of ‘turning depth’

Answer 24

• velocities increase into earth
• will refract away from the normal when moving into a faster velocity layer
• gradually curve closer to horizontal
• return up as head waves
• depth reached depends on ray parameter

Question 25

How does the ray parameter dictate geometry of wave path

Answer 25

small p = travel deeper

small p:
= small θ (near vertical), need to be refracted through more layers to get to horizontal

Question 26

what is the ray parameter for a spherical earth

Answer 26

p = r. u. sinθ

Question 27

how do anomalous changes in velocity at depth impact the travel time curves

Answer 27

Sudden increase -> triplications (waves taking different paths arrive at the same point at same time)
e.g. transition zone (olivine -> wadsleyite -> ringwoodite) = 2 sudden increases = 2 triplications

decrease (low velocity zone) -> refract other way -> gap in curve
e.g. liquid outer core

Question 28

what is evidence for a solid inner core

Answer 28

low amplitude waves being discovered in p wave shadow zone.
p waves refracted deeper into liquid, then reflect off boundary to inner core, changing direction, allowing p waves in shadow zone predicted by only liquid core.

Question 29

what has seismology revealed about the internal structure of the moon

Answer 29

has a fluid outer core and solid inner core like earth. Also has partial melt layer at base of mantle -> little change in density, existence revealed by drop in velocity

Question 30

what are traveltime residuals and what is generally their pattern across earth

Answer 30

∆T = T(obs) - T(pred)
continents = -Ve = faster
oceans = +ve = slower

Question 31

How can seismology image downgoing slabs and rising plumes in the earth

Answer 31

slabs = cold = fast velocity anomaly
plume = hot = slow velocity anomaly

anomalies seen in different wavelengths formed in standing waves across the earth in quakes -> fast = shorter wavelength