Earthquakes

Question 1

What is an earthquake?

Answer 1

A release of elastic energy stored in the lithosphere, built up from gradual deformations.

Question 2

Where do earthquakes normally happen?

Answer 2

Near fault lines.

Question 3

What are earthquakes at faults parameterised by?

Answer 3

The slip vector, the slip angle and the area of the fault plane.

Question 4

What is the focus?

Answer 4

Point from which slip begins and from which the slip propagates.

Question 5

What is epicentre?

Answer 5

Closest point on the Earth’s surface to the focus point.

Question 6

What are 3 things we can observe from an earthquake?

Answer 6

• Damage afterwards
• Movement along fault
• Seismic waves

Question 7

What are the 4 types of fault?

Answer 7

• Thrust fault (compression)
• Normal fault (expansion)
• Strike-slip (horizontal movement)
• Oblique fault

Question 8

What is the modified mercalli scale?

Answer 8

Qualitative scale indicating damage to buildings and infrastructure.

Question 9

What is the equation or the seismic moment M0?

Answer 9

M0 = μA|u| [Nm]

Question 10

What is the magnitude scale?

Answer 10

Logarithmic scale to show size of the earthquake.

Question 11

What is the equation for the magnitude Mi?

Answer 11

Mi = (log(base 10) of E) + corrections, where E is some measurement such as the seismic moment etc

Question 12

What do we need to estimate the energy of the earthquake?

Answer 12

Measurement of the seismic waves and the location of the focus.

Question 13

How would you get the energy if you had the equation of a wave displacement, A = A(max)*cos(kx-ωt)?

Answer 13

• KE 1/2mv^2

- So = 1/2 * ρ*(dA/dt)^2

Question 14

What is the equation for the surface wave magnitude, M(s)? What are the letters?

Answer 14

M(s) = log(base 10) of (A(max)/T)+1.66*log(base 10) of Δ + 3.3

Δ = angular distance in degrees from epicentre
A = magnitude
T = seismic wave period

Question 15

What is the equation for body wave magnitude M(B)? What are the letters?

Answer 15

M(B) = log(base 10) of (A(max)/T) + Q(D,h), where Q is the correction term as a function of distance D from the epicentre, h the depth of focus

Question 16

What is the equation for the moment magnitude M(W)? What are the letters?

Answer 16

M(W) = 2/3*log(base 10) of M(0) - 10.7, where M(0) is the seismic moment

Question 17

What is the equation relating the energies of two earthquakes in terms of the seismic moment and the moment magnitude?

Answer 17

E1/E2 = M1(0)/M2(0) = 10^3/2 *(M1(W)-M2(W))

Question 18

What is the equation relating the energies of two earthquakes in terms of the surface wave magnitude?

Answer 18

E1/E2 = 10^1.44 *(M1(S)-M2(S))

Question 19

What does a change in 2 magnitudes equate to on the magnitude scale?

Answer 19

1000 times

Question 20

What is the equation for the frequency of the surface wave magnitude?

Answer 20

log(frequency(M(S))) = C-D*M(S), where C,D are some constants

Question 21

What can the poisson distribution give us an indication of?

Answer 21

The frequency of events.

Question 22

What is the equation for the poisson distribution?

Answer 22

P(x, λ, t) = (exp(-λt)*(λt)^x)/x!, where λ is the rate of events per unit time, t is the time interval we are interested in and x is the number of events we are interested in

Question 23

How far down can earthquakes occur?

Answer 23

0-700km

Question 24

What does the diagram of how we measure earthquake depth and location look like?

Answer 24

Earth curve with focus and then 3 curved x values going to different seismometers at different points on earths surface

Question 25

What information can a single seismometer provide us with?

Answer 25

Amplitude, arrival times and period/frequency

Question 26

What can an array of nearby seismometers give us?

Answer 26

Direction of propagation, polarity

Question 27

What can an array of widely spread seismometers give us?

Answer 27

Location, depth

Question 28

What is the equation for a(i), the arc length from the seismometer to the epicentre?

Answer 28

a(i) = Δi*R(earth) ~ x(i), the path length from the seimometer to the focus

Question 29

How can we use seimographs to distinguish nuclear tests from earthquakes?

Answer 29

M(B) < 2.87 + 0.6*M(S) = Earthquake

M(B) > 2.87 + 0.6*M(S) = Nuke

Question 30

How can we use north and south to determine earthquake location?

Answer 30

See time it took to get to north and south and draw appropriate circles centred on north and south. Earthquake could be at one of the two places the circles cross.