# Earthquake Engineering Questions and Answers – Seismometer

This set of Earthquake Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Seismometer”.

1. When does the relative displacement of the oscillator mass approaches base displacement?
a) When acceleration is zero
b) When frequency of the oscillator increases
c) When frequency of the oscillator decreases
d) Never

Explanation: As the frequency of oscillator decreases, the relative displacement of the oscillator mass approaches base displacement. This is not dependent on the damping value. Thus the casing of the instrument moves with the vibrating body while the oscillator mass remains stationary.

2. Which of the following statement is true for Seismometers?
a) Instruments with very high natural frequency in comparison to the frequency of vibration
b) Instruments with natural frequency equal to the frequency of vibration
c) Instruments with natural frequency independent of the frequency of vibration
d) Instruments with very low natural frequency in comparison to the frequency of vibration

Explanation: Seismometers are the instruments whose natural frequency is very low in comparison to the frequency of vibration that is to be measured. The tuning ratio (ω/ωn) should be large. Depending upon the operating range of frequencies, the transducers are classified as seismometers and accelerometer.

3. Seismometers are often small in size due to requirement of very low natural frequency.
a) True
b) False

Explanation: Seismometers are often large in size due to requirement of very low natural frequency. The dimensions of seismometer are also governed by the peak maximum displacement of the vibrating body which is same as the peak displacement of the vibrating base.

4. When are the displacement pickups used?
a) When there’s a fixed reference point available
b) When resonance occurs
c) When there’s no fixed reference point available
d) When magnitude of earthquake is above 8

Explanation: The displacement pickups are used to pick up the vibration of a body when a fixed reference point is not available. For example, in determining the movement of the chassis of a vehicle. Therefore, it is required that the seismic mass behave as though it was fixed in space.

5. A heavy seismic mass attached to an extremely flexible spring results in which of the following?
a) System with very low time period
b) System with very low natural frequency
c) System with very high natural frequency
d) System with zero base displacement

Explanation: When a heavy seismic mass is attached to an extremely flexible spring, it results in a system with very low natural frequency. For frequency of vibration above the natural frequency of pickup, the displacement of the seismic mass relative to the casing is same as the displacement of casing with a phase difference of 180 degrees.

6. What is the value of natural time period (T) if the natural circular frequency (ω) is 40 rad/sec?
a) 0.057 sec
b) 0.105 sec
c) 0.157 sec
d) 0.215 sec

Explanation: Given,
natural circular frequency (ω) = 40 rad/sec
Natural time period (T) = 2π / ω
= 2π / 40
= 0.157 sec

7. High sensitivity seismometers are not suitable during a strong earthquake shaking.
a) True
b) False

Explanation: Seismometers are highly sensitive instruments and can pick up smallest of vibrations. However, during a strong earthquake shaking, the large ground displacements are not accommodated in these highly sensitive seismometers making them unsuitable for strong earthquake shaking.

8. Which of the following is true for velocity pickups?
a) Generated voltage is proportional to the induced electro-motive force
b) The induced electro-motive force is proportional to the velocity of vibrating body
c) Generated voltage is proportional to the rate of intersection of the magnetic field
d) Generated voltage is proportional to the velocity of vibrating body

Explanation: The generated voltage is proportional to the velocity of vibrating body. This is because the induced electro-motive force is proportional to the rate of intersection of the magnetic field. The transducer is a coil of wire carried by the seismic mass which is suspended in a radial magnetic field.

9. Which of the following is not a component of velocity pickup?
a) Diaphragm
b) Coil former
c) Speedometer
d) Nuts and washers

Explanation: Speedometer is not a component of a velocity pickup. Velocity pickups contain a central rod with its associated nuts, washers and coil former. @ flexible diaphragms are connected to the rod, whose stiffness forms the spring. The coil former suspends the coil in a narrow annular slot in a cylindrical magnet.

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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