Ground Motion Characteristics Questions and Answers

Answer: A

Correct answer: peak acceleration, peak velocity and peak displacement

These are the three amplitude parameters, and each is dominated by a different part of the frequency range - acceleration by high frequencies, displacement by low, velocity by the middle. Magnitude and intensity describe the earthquake and its effects rather than the record, and duration and frequency content are separate categories of parameter.

Answer: B

Short-period structures respond to the high-frequency content that governs acceleration, so PGA is a reasonable index for them. Tall flexible buildings are driven by longer periods, where velocity or displacement is the better measure.

Answer: C

Correct answer: intermediate and long-period structures

Peak velocity is governed by the intermediate frequencies, which is where the energy that matters to medium and long-period structures lies. Peak acceleration is dominated by very high frequencies, whose brief spikes shake a stiff structure but carry little energy for a flexible one, so it correlates poorly with damage to taller buildings.

Answer: D

Integrating twice suppresses high frequencies and emphasises long periods, so displacement is a long-period measure. It is also the parameter most corrupted by baseline error, which is why it is the least reliably determined of the three.

Answer: A

Correct answer: Bracketed duration of strong shaking

Duration is its own category of parameter, describing how long the shaking lasted rather than how large it was. The three amplitude parameters are peak acceleration, peak velocity and peak displacement. Keeping the categories apart matters because a complete description of a motion needs one parameter from each: amplitude, frequency content and duration.

Answer: B

Correct answer: 2.94 m/s 2

\[ 0.30 \times 9.81 = 2.94\ \mathrm{m/s^2}. \] Peak acceleration is quoted as a fraction of \( g \) because that fraction is roughly the horizontal inertia force as a proportion of the structure’s weight, which is the form the designer wants. Dividing by \( g \) instead gives 0.031 and multiplying by 100 gives 29.4, both of which are offered as distractors.

Answer: C

For a rigid body the inertia force is the mass times the acceleration, which equals the weight times the acceleration expressed as a fraction of g, so 0.2 times 500 is 100 kN. This is the simplest form of the seismic coefficient method.

Answer: D

Correct answer: 100 Hz

The Nyquist frequency is half the sampling rate: \[ \frac{200}{2} = 100\ \mathrm{Hz}. \] Content above it is not merely lost but folded back into the record as spurious low-frequency energy, which is why an analogue anti-aliasing filter is applied before digitising rather than afterwards.

Answer: A

Correct answer: 2.5 Hz

Frequency is the reciprocal of period: \[ f = \frac{1}{T} = \frac{1}{0.4} = 2.5\ \mathrm{Hz}. \] The predominant period is where the Fourier spectrum peaks, and it matters because a structure whose natural period is near it will be driven towards resonance - which is why site period and building period are compared during design.

Answer: B

When the driving motion is concentrated at the structure's own period, successive cycles reinforce the response and displacements build up. Matching periods is what destroyed mid-rise buildings on soft ground in Mexico City in 1985.