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(a) Draw the fault-plane solutions on the focal spheres for earthquakes 1 and 2.
(b) Indicate the fault types which generated earthquakes 1 and 2. From the tectonic
setting of each earthquake, explain how it is possible to infer which plane is likely
to be the fault plane. List the strikes and dips of the probable fault planes and the
probable slip vectors.
(c) Sketch the bathymetric profiles along sections A-A
, B-B
and C-C
.
(d) In this tectonic setting, over what depth range would you expect the earthquakes
to occur? (Cambridge University Natural Sciences Tripos IB, 1983.)
15. The boundary between the African and Eurasian plates, between the Azores Triple
Junction and Gibraltar, strikes approximately east-west. What is the nature of this
boundary? Sketch fault-plane solutions for the following:
(a) earthquakes along this plate boundary west of Gibraltar; and
(b) earthquakes east of Gibraltar in the Mediterranean.
16. A 200-m thick layer of sediment with P-wave velocity 2 km s
−
1
overlies material
with velocity 3 km s
−
1
. Calculate the critical distance for the refracted wave from the
second layer. If you had to determine the velocities and thickness of a structure such
as this by shooting a seismic-refraction experiment, indicate how many seismometers
youwould require and over what distances you would deploy them.
17. The first-arrival time-distance data from a seismic-refraction experiment are as fol-
lows:
refraction velocity 6.08 km s
−1
,
intercept 1.00 s
refraction velocity 6.50 km s
−1
,
intercept 1.90 s
refraction velocity 8.35 km s
−1
,
intercept 8.70 s
(a) Calculate the velocity-depth structure.
(b) Now assume that there is a hidden layer: a refractor with velocity 7.36 km s
−
1
and
intercept 5.50 s is a second arrival at all ranges. By how much is the total crustal
thickness increased?
18. The first-arrival travel times and distances obtained from a refraction-experiment shot
along a mid-ocean-ridge axis are tabulated below.
Offset distance (km)
First-arrival travel time (s)
2.0
4.10
3.0
4.45
4.0
4.85
5.0
5.20
6.0
5.40
7.0
5.60
8.0
5.80
9.0
5.95
10.0
6.15
12.0
6.50
(
cont.
)
