L1 - Earthquake Source Seismology + P1

Question 1

What do we want to know about an earthquake when it happens? (4)

Answer 1

Location (lat/long)
Depth
Magnitude
Focal mechanism

Question 2

What sources of information are there for earthquakes? (3)

Answer 2

Seismology
Field work
Satellite geodesy

Question 3

Where does the rupture in an earthquake start? (1)

What is the epicentre? (1)

Answer 3

Hypocentre

Surface expression of the hypocentre

Question 4

How is a rough location of an earthquake found? (2)

Answer 4

Use multiple teleseismic stations

Use the difference in p and s arrival times

Question 5

Why can p and s arrival times not be used for an accurate earthquake location? (2)

Answer 5

Assumes velocity isotropy

Velocity anomalies of +/- 10% of V(s) in the Earth

Question 6

How can the accuracy of epicentre locations be improved? (2)

Answer 6

Use relative locations as nearby earthquakes have small distances that give different arrival times
Or have multiple local seismic stations

Question 7

Define teleseismic distance (1)

Answer 7

30-80 degrees

Question 8

Why are teleseismic distances used? (1)

Answer 8

The ray paths through lithospheric and upper mantle material, immediately below the earthquake are very similar

Question 9

Why is there a trade-off between the time and depth of earthquakes? (1)

Answer 9

Event could have been shallow at an early time or deeper at a later time when viewed from teleseismic distances

Question 10

How can the depth problem broadly be fixed in locating earthquakes? (2)

Answer 10

Local recordings

Waveform modelling

Question 11

Why do local recordings solve the depth problem in locating earthquakes? (2)

Answer 11

Ray paths to seismometers are different

Removes trade-off

Question 12

How can local recordings of earthquakes give very accurate lat/long/depth of earthquakes? (2)

Answer 12

If stations positioned X km apart, the depth can be accurately estimated deeper than X km
If the velocity structure is known, depth within 1km or better

Question 13

How were magnitudes defined in the past? (2)
What problem arose? (1)
Why? (1)

Answer 13

Body-wave magnitude using waves with 1s period
Surface-wave magnitude using waves with 20s period
Saturates for bigger events
Only sensitive to one part of the fault plane for large earthquakes

Question 14

How was the problem of scale saturation solved? (2)

Answer 14

Seismic Moment M(0)
Where M(0) = μAu

Question 15

Why were waves with 1s and 20s periods used to define magnitudes of earthquakes in the past? (1)

Answer 15

Filters out the 6s resonant period of oceans

Question 16

How does waveform modelling work? (1)

Answer 16

Filter seismogram to only long-period waves (>=15s)

Question 17

What are the benefits of waveform modelling? (3)

Answer 17

Wave λ > EQ fault plane so event looks like a point source to ignore rupture propagation/slip variation
Ignores short λ variations in velocity structure
Better signal-to-noise ratio as ocean resonance is 7s

Question 18

Which waves are used for waveform modelling? (2)

Answer 18

p, pP, sP for body waves

s, sS for surface waves

Question 19

What information can the received pulse generated by an earthquake be used to find? (3)

Answer 19

Pulse width depends on depth
Pulse shape depends on focal mechanism
Pulse amplitude depends on seismic moment

Question 20

What is a general rule that can be used if a fault breaks the surface? (1)

Answer 20

Fault depth ~ 2 x centroid depth

Question 21

What is the difference between the signal generated by an earthquake and a bomb? (2)

Answer 21

EQ: pure double-couple solution = compressional + dilatational
Bomb: all compressional

Question 22

What is the source time function? (1)

Answer 22

How long it takes the rupture to propagate across the fault plane

Question 23

Why is there a trade-off between time function and depth in waveform modelling? (1)

Answer 23

A broad pulse can be matched by a deeper centroid (increases time between direct and reflected pulses) or a longer time function (longer rupture)

Question 24

Why is there a trade off between depth and moment in waveform modelling? (2)

Answer 24

Direct and reflected phases generally have opposite polarities for dip-slip faults (phase reversal at the free surface)
Destructive interference at shallow depths and more moment needed to produce the observed amplitude

Question 25

When modelling the waveform of an earthquake with an unknown fault orientation using p waves, why can strike or rake not be constrained well but dip can be?

Answer 25

Unknown fault orientation means all stations in the middle of one quadrant
Changing strike and rake likely means the stations will still be in the middle of the quadrant
But changing dip moves the stations towards or away from the nodal plane, which affects amplitude