Introduction to Engineering Seismology
Assignment 3 – Hints and Solutions
1. Underground nuclear explosion is detected based on the fact that these explosions
should produce all _________ waves. (Compressive)
A sphere of compressive waves is generated and it propagates outwards.
2. A ray is travelling in a rock with a 1.00 km/s seismic velocity. The ray encounters an
interface at a 30.0° angle with a rock velocity of 1.50 km/s on the other side. At what
angle with respect to the normal to the interface does the ray leave the interface? Give
the answer in degrees. (47-50 degrees)
Assuming the required angle be θ
(sin30° )/1.0 = (sinθ)/1.5
0.5 * 1.5 = 0.75 = sinθ
θ = sin-10.75 = 48.59°
3. Assuming P and S wave travel through the crust at 5 km/sec and 2.5 km/sec
respectively. If epicentre forms an equilateral triangle with two points A (10, 10) km
and B (35, 10) km on the ground surface. Calculate the difference in the arrival time
of P and S waves at A in sec. (4.9-5.1s)
Distance between A and B = 25km = distance of epicentre from A
TP = 25/5 sec, TS = 25/2.5 sec TS – TP = 25(2/5 – 1/5)sec = 25/5 sec = 5sec
4. Consider that the earth has a constant velocity everywhere (uniform). Calculate the
travel time ratio for an epicentral angle of 90° and 60°. Give the answer to the nearest
single decimal. (1.4-1.5)
For uniform velocity everywhere, all the travel lines will be straight. Hence, from
epicentre A to point B, the travel time will be inversely proportional to travel length
AB.
Case 1: Case 2:
TAB1 = AB/Velocity TAB2 = AB/Velocity
= R√2 /V = R/V
TAB1/TAB2 = √2 = 1.414
5. We know that no direct P and S waves are observed within the shadow zone. Use this
concept to calculate the approximate radius (in km) of the core of the Earth. (3900-
4000km)
We know that shadow zones start from epicentral angle of 103°
Approximate radius of the earth R = 6400km
Angle ACD = 103°/2 = 51.5°
cos51.5° = r/R = r/6400km
r = 6400 * cos51.5° = 3984km
6. Variation in strain of rocks affects the extent of fault rupture. (True)
More strained sections of a fault are more susceptible to rupture.
7. Rayleigh waves cause more damage than the body waves because of ___________
motion. (rolling)
8. If the velocity of a wave is constant and its frequency is tripled, the wavelength will
be _______ times the previous. Give answer to the nearest single decimal. (0.3)
Velocity = (Wavelength) * (Frequency)
= (Wavelength * 1/3) * (3*Frequency)
9. Among the body waves _________shadow zone is smaller. (P-wave)
Both P- and S-wave shadow zones start from 103°, but P-wave shadow zone ends
after 140°.
10. Chemical processes involving water movement may cement the ruptures, thus
subsiding the local disruptive forces, turning the fault inactive. (True)
11. The wave path SKKS passes through the Earth's core. (True)
12. Which of these opposes the rupture extension? (Asperitis, Soft rock zones)
