UNIVERSITY OF BRISTOL
January 2015 Examination Period
School of Earth Sciences
Level M/7 Examination
EXAM PAPER CODE EASCM0018(J)
SEISMOLOGY
Time allowed:
2 hours
This paper contains 13 questions.
All students must answer all questions.
The marks available for each question is indicated in brackets.
OTHER INSTRUCTIONS
Clearly indicate the question number you are answering.
Remember to put your candidate number on the front of each booklet
used and on the extra sheet. You do not need to use a separate
booklet for each question.
ADDITIONAL INFORMATION
Non-programmable and non-graphing calculators are permitted in this examination.
TURN OVER ONLY WHEN TOLD TO START WRITING
Short Answer Questions (25 marks)
(1) Discuss the contrast in material properties across the core-mantle boundary.
(6 marks)
(2) How did seismologists deduce that the outer core is liquid and that the inner core
is solid? It may be useful to mention the seismic phases imaging these regions.
Which seismologist discovered the Inner Core?
(2 marks)
(3) What is the difference between a ‘microseism’ and a ‘microseismic’ event?
(1 mark)
(4) What is Fermat’s principle?
(2 marks)
(5) Why is the SOFAR (sound fixing and ranging) channel so effective in transmitting
sound energy in the oceans?
(2 marks)
(6) What does the term teleseismic mean? Why is this a useful definition?
(7) What is a seismic ‘b-value’ and how can this be used as a fluid indicator?
(2 marks)
(2 marks)
(8) Describe how ‘receiver functions’ are used to assess crustal thickness? Some
sketches would be very helpful in your explanation.
(3 marks)
(9) What is the mathematical representation of Snell’s law in a spherical Earth? A ray
that leaves an earthquake at an angle 30 degree from the vertical has a ray
parameter of 531. At a depth of 660 km the P-wave velocity of the mantle is 10.0
km/s – what is the angle of ray propagation at this depth? At the base of the
mantle (2890 km deep) the P-wave velocity is 14.0 km/s – what is the angle of ray
propagation at this depth?
(3 marks)
(10) For the stress tensor:
find the traction (a) on the surface with a normal (1,1,1) (b) on the x-y plane.
(2 marks)
Long Answer Questions (45 marks)
(11) A seismic station has been installed in Chile and the station operator is a bit
overwhelmed with seismic events on the first day of the job.
(a) The station records two earthquakes that have the exact same magnitude, but
one is 5000 km away from the station, the other 5 km away. Neglecting
radiation pattern effects, describe at least 4 differences between the
seismograms recorded at the stations.
(4 marks)
(b) The seismologist is surprised to see a recording of a seismic event from a
location 1000 km away. After some thought and analysis that includes data
from other stations in the global network, he/she deduces that a nuclear bomb
has been detonated in the region. How would they have made this deduction?
(4 marks)
(c)
Figure 1 shows the power spectral amplitude for the P-wave recorded from the
event. Determine the magnitude (Mw) of the event. You can assume a P-wave
velocity of 7 km/s, a density of 3000 kg/m3 and a radiation correction of 0.52.
(4 marks)
Figure 1
(12) Figure 2 (see separate sheet) is a seismogram of an earthquake that occurred
beneath the Sea of Okhotsk, and at depth on the Kuril arc. These data are
bandpass filtered between 0.001 and 0.1 Hz. The earthquake is recorded at a
station in the village of Butajira, in Ethiopia, roughly 96 degrees from the source.
The uppermost trace is the radial component, the middle trace is the transverse
component, the lowermost trace is the vertical component.
(a) On the diagrams of a cross section of the Earth (Figure 3), sketch the raypaths
for the first 8 phases that are visible and marked on the seismogram (i.e., the
last one is SKS). The inner circle is the inner core, the middle circle marks the
core-mantle boundary, and the outer circle is the Earth’s surface. Make sure
that you note whether the ray is propagating as a P-wave or S-wave (this can
change along the raypath).
(8 marks)
(b) Why does the P phase arrive at the same time as PcP (similarly, S at the same
time as ScS)?
(1 mark)
(c)
What is the unmarked phase just before pPKiKP, which arrives at roughly 1100
seconds? How did you deduce this?
(2 marks)
(d) A swarm of microseimic events occurred in the Butajira geothermal field
during the arrival of this event. Why are these earthquakes not visible on these
seismograms?
(1 mark)
(e) Discuss any indications that the uppermost mantle beneath the Butajira station
is anisotropic. What other analysis would you perform to confirm your
deductions?
(6 marks)
(13) A seismologist working on the exploitation of shale gas in the Barnett shale in
Texas has been asked to interpret a seismicity dataset resulting from hydraulic
fracture stimulation.
(a) Describe 3 methods for locating seismic events using seismic arrays, either in
boreholes or on the surface. Describe when each work best.
(6 marks)
(b) A simple velocity model is then developed to locate the events. A seismic
survey reveals that Vp1 = 2000 m/s; Vp2 = 4000 m/s; Vs1 = 1000 m/s; Vs2 = 2500
m/s, where the subscripts 1 and 2 refer to the upper and lower layers. Given an
incident P-wave striking the interface at an angle of 300 from the vertical: What
is the angle from vertical of the reflected S-wave? What is the angle from
vertical of the transmitted P-wave? At what incident angle from vertical will
the P-wave generate an S-wave head wave?
(4 marks)
(c)
The seismologist notices that the seismic wavefield recorded by borehole
instruments is very different from that recorded at the surface. Discuss the
affects of attenuation and anisotropy on these seismic phases. What further
analysis would you recommend to better quantify these effects?
(5 marks)
Figure 2.
Seismogram to be used in Question 12.
CANDIDATE NUMBER ……………………..
Figure 3.
Earth Outline – to be used in Question 12