Earthquake Engineering Questions and Answers – Seismology – Local Site Effect

This set of Earthquake Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Seismology – Local Site Effect”.

1. The velocity of seismic waves is fastest in which of the following?
a) Hard rock
b) Soft rock
c) Hard Soil
d) Soft Soil

Explanation: The particles of the hard rock are confined and hence the celerity of the particles is maintained. Due to this the energy loss which may arise in soft soil due to celerity of particles reduces and the seismic waves are capable for traveling fastest in hard rock.

2. The earthquake effect are more felt in soft rock then in hard rock. This can be explained as an occurrence of which of these phenomena?
a) Impedance contrast
b) Damping in soil
c) Resonance
d) Trapping of waves

Explanation: Owing to a decrease in celerity of seismic waves while passing through hard to soft medium, they get bigger in amplitude to carry the same amount of energy. Therefore, with decrease in density and S-wave velocity of the medium, the particle velocity at the ground surface increases. Thus, shaking tends to be stronger at sites with softer soil layers.

3. When the frequency of seismic waves becomes equal to the natural frequency of soil, which of these phenomena does the soil undergo?
a) Resonance
b) Basin erosion
c) Impedance
d) Wave trapping

Explanation: When the frequency of seismic waves and soil become equal, the soil vibrates with its maximum fundamental natural frequency. This effect is called as resonance.

4. The damping of the soil is defined by which of these parameters?
a) Damping ratio
b) Magnification factor
c) Quality factor
d) Damping Index

Explanation: In a soil medium during particle-particle collision few collisions may not be perfectly elastic. Some amount of energy may be lost in undergoing deformation. Quality factor of soil is defined as the fractional loss of this energy per cycle of vibration.

5. Higher the frequency of vibration greater will be the energy loss. This implies that at higher frequencies the damping ________________
a) Decreases
b) Remains same
c) Increases
d) Increases and then reduces

Explanation: Damping is the energy loss in the system when it is subjected to harmonic excitation. When the frequency of vibration increases the energy loss occurs at a faster rate and hence the damping increases.

6. Basin edge induces which type of seismic waves?
a) Surface waves
b) Body waves
c) Bending waves
d) Shear waves

Explanation: Basin edges induce surface waves mainly because of their cone-like geometry. It leads to diffraction of seismic waves and energy focusing.

7. From past earthquake data, it has been found that the earthquake tremors at the base of a hilly terrain is _______________ the weathered peak.
a) More than
b) Less than
c) Same as
d) Independent of

Explanation: Narayan in 2003 reported on the basis of past earthquake data and simulation results that damage to the building environment occurs more at the ridge than at the base provided that the rock at the top is not weathered.

8. The seismic waves amplify because of presence of unfavourable geological condition.
a) True
b) False

Explanation: Seismic waves undergo trapping, resonance, impedance and focusing while travelling across the depth of the earth surface. They also amplify because of the presence of valleys, ridges and lateral discontinuities.

9. Which of these reasons may not be attributed to the increased duration of seismic waves in soft soils?
a) Trapping of waves
b) Presence of ridges
c) Impedance contrast
d) Damping of soil

Explanation: Presence of ridges is a topographical phenomenon. It has very little to do with the macroscopic composition of the soil or rock mass. Duration of seismic wave depend more upon the macroscopic composition and the interlocking of the soil mass. Hence presence of ridges can’t be directly associated with the increased duration of seismic waves in soft soils.

10. Which of the following statement is not true for local site effects caused due to earthquakes?
a) Damping of soil during earthquake causes differential settlement
b) Wave trapping is responsible for an increase in duration of the earthquake motion in a soil mass
c) There is more damage along the narrow zone of a soil mass with lateral discontinuities
d) Basin-edge induces strong seismic waves near the edges

Explanation: Due to soil damping the seismic energy is lost to a considerable extend and the soil particles vibrate with lesser amplitudes. Differential settlement is caused due to interference of multiple seismic waves. It is independent of the damping of soil.

Sanfoundry Global Education & Learning Series – Earthquake Engineering.

To practice all areas of Earthquake Engineering, here is complete set of Multiple Choice Questions and Answers.

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