Earthquakes Flashcards
What is an earthquake?
A release of elastic energy stored in the lithosphere, built up from gradual deformations.
Where do earthquakes normally happen?
Near fault lines.
What are earthquakes at faults parameterised by?
The slip vector, the slip angle and the area of the fault plane.
What is the focus?
Point from which slip begins and from which the slip propagates.
What is epicentre?
Closest point on the Earth’s surface to the focus point.
What are 3 things we can observe from an earthquake?
- Damage afterwards
- Movement along fault
- Seismic waves
What are the 4 types of fault?
- Thrust fault (compression)
- Normal fault (expansion)
- Strike-slip (horizontal movement)
- Oblique fault
What is the modified mercalli scale?
Qualitative scale indicating damage to buildings and infrastructure.
What is the equation or the seismic moment M0?
M0 = μA|u| [Nm]
What is the magnitude scale?
Logarithmic scale to show size of the earthquake.
What is the equation for the magnitude Mi?
Mi = (log(base 10) of E) + corrections, where E is some measurement such as the seismic moment etc
What do we need to estimate the energy of the earthquake?
Measurement of the seismic waves and the location of the focus.
How would you get the energy if you had the equation of a wave displacement, A = A(max)*cos(kx-ωt)?
- KE 1/2mv^2
- So = 1/2 * ρ*(dA/dt)^2
What is the equation for the surface wave magnitude, M(s)? What are the letters?
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
What is the equation for body wave magnitude M(B)? What are the letters?
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
What is the equation for the moment magnitude M(W)? What are the letters?
M(W) = 2/3*log(base 10) of M(0) - 10.7, where M(0) is the seismic moment
What is the equation relating the energies of two earthquakes in terms of the seismic moment and the moment magnitude?
E1/E2 = M1(0)/M2(0) = 10^3/2 *(M1(W)-M2(W))
What is the equation relating the energies of two earthquakes in terms of the surface wave magnitude?
E1/E2 = 10^1.44 *(M1(S)-M2(S))
What does a change in 2 magnitudes equate to on the magnitude scale?
1000 times
What is the equation for the frequency of the surface wave magnitude?
log(frequency(M(S))) = C-D*M(S), where C,D are some constants
What can the poisson distribution give us an indication of?
The frequency of events.
What is the equation for the poisson distribution?
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
How far down can earthquakes occur?
0-700km
What does the diagram of how we measure earthquake depth and location look like?
Earth curve with focus and then 3 curved x values going to different seismometers at different points on earths surface
What information can a single seismometer provide us with?
Amplitude, arrival times and period/frequency
What can an array of nearby seismometers give us?
Direction of propagation, polarity
What can an array of widely spread seismometers give us?
Location, depth
What is the equation for a(i), the arc length from the seismometer to the epicentre?
a(i) = Δi*R(earth) ~ x(i), the path length from the seimometer to the focus
How can we use seimographs to distinguish nuclear tests from earthquakes?
M(B) < 2.87 + 0.6*M(S) = Earthquake
M(B) > 2.87 + 0.6*M(S) = Nuke
How can we use north and south to determine earthquake location?
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.
