Übung zu Globale Geophysik I 07 Questions
Übung zu Globale Geophysik I: Wednesday, 16:00 – 18:00, Theresienstr. 41, Room C419
Lecturer: Heather McCreadie
Companion class to Globale Geophysik I: Monday, 12:00 – 14:00, Theresienstr. 41, Room C419
Lecturer: Rocco Malservisi
1. What is the seismic evidence that the boundary between the core and the
mantle is liquid?
2. How would you distinguish between a Love wave and a Raleigh wave if you
were given an earthquake record from a WWSSN station?
[5]
[5]
3. Your foot is trapped under a steel railway sleeper. You feel the rail vibrate and
then 20 seconds later you hear a faint whistle. Estimate how much time you
have to free yourself.
[10]
Velocity of sound in air at sea level is approximately 333ms-1.
Velocity of sound in steel at sea level is approximately 5000ms-1.
4. Calculate the normal incidence P-wave reflection coefficients for the interface
Sandstone (α=3.0kms-1 and ρ=2.2 x 103 kgm-3) above a limestone (α=4.1kms-1
and ρ=2.2 x 103 kgm-3).
[10]
5.
Suggestion for question 4
Calculate the refraction angle of a P-wave passing through the interface of Sandstone
(α=3.0kms-1 and ρ=2.2 x 103 kgm-3) above a limestone (α=4.1kms-1 and ρ=2.2 x 103 kgm-3).
Which value of incidence angle is the critical angle? What would be the amplitude of a
refracted wave in the case of a wave passing from a layer of limestone to a layer of
sandstone?
Suggestion for question 1
Can we add the following? Based on your previous answer, would this imply that we can
not identify a solid body inside the core? How can we know that the inner core is solid?
On question 3 , can we add also the following? Can we use the same principle for early
warning of seismic regions? To which seismic wave would correspond the vibration, the
whistle and the train?
Question 5
Given the Gutenberg-Richter law (the relation between number of earthquakes and
magnitude)
Log10 N = a – b Ms
what is the average b value? What is the meaning of a small than average b values? What is
the meaning of a and how can it be used for hazards mapping?
(the solution is at page 123 of the new fowler)
Answers:
Question 1
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Übung zu Globale Geophysik I 07 Questions
The boundary between the core and the mantle is the most profound seismic discontinuity in the
Earth. It produces a P-wave shadow at epicentral angles between 105° and 142° and no Swaves at epicentral angles >142°. The absence of S-waves indicates that the outer core has a
shear modulus of zero, characteristic of a liquid.
Question 2
Love waves involve transverse motion and generally arrive before Raleigh waves.
Question 3
Velocity = distance/time. Therefore, time = distance/velocity.
The difference between the time you felt the vibration and heard the whistle is the difference in
the velocity of the waves in the different mediums (air and steel). Thus if x is the distance to the
train,
x
x

 20
333 5000
 x  7135m  7.1km
If the train was travelling at 60 km/hr then it would take 7/60 hrs to reach you: which is
approximately 7 minutes.
If the train was travelling at 100 km/hr then it would take 7/100 hrs to reach you: which is
just over 4 minutes.
Question 4
The ratio of the reflected amplitude to the incident amplitude is called the reflection coefficient.
   11
reflection coefficient = 2 2
 2 2  11
In this case since  2  1
reflection coefficient =
 2  1 4.1  3

 0.155
 2  1 4.1  3
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