Ground Motion Characteristics Questions and Answers

Answer: C

Correct answer: first and last exceedance of a threshold acceleration

Bracketed duration takes a threshold, conventionally 0.05 g, and measures the interval between the first and the last time the record crosses it. Everything before and after is disregarded as too weak to matter. The definition is arbitrary in its threshold but useful, since it isolates the part of the record that actually does damage.

Answer: D

The cumulative energy of the record is plotted and the interval between, typically, 5 and 95 per cent of the total is taken. Being based on energy rather than one threshold, it is more stable than the bracketed measure.

Answer: A

Correct answer: repeated cycles accumulate damage and degrade strength

Structural damage under earthquake loading is cumulative: each large cycle of inelastic deformation cracks concrete a little further, degrades stiffness and consumes some of the available ductility. A long record subjects the structure to many more such cycles at the same amplitude, so two motions with identical peaks can do quite different damage.

Answer: B

Correct answer: magnitude of the earthquake

Duration is governed chiefly by the length of time the fault takes to rupture, which grows with the size of the rupture and hence with magnitude. A magnitude 6 event may shake strongly for ten seconds, a magnitude 9 for several minutes. Distance and site conditions lengthen it further, but magnitude is the primary control.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the missing dimensions. Peak acceleration is a single ordinate from a single instant, and it says nothing about how long the shaking lasted or which frequencies carried the energy. Two records with the same peak can do very different damage, which is why a complete description needs amplitude, frequency content and duration together.

Answer: D

Correct answer: Fourier amplitude spectrum

The Fourier amplitude spectrum decomposes the record into its harmonic components and plots the amplitude of each against frequency, so it shows directly which frequencies the motion contains and how strongly. It is a property of the motion itself, unlike a response spectrum, which describes what oscillators do when driven by it.

Answer: A

Correct answer: the motion itself rather than the response of structures

The Fourier spectrum decomposes the record into its frequency components and involves no structure at all. A response spectrum is computed by driving a family of single-degree-of-freedom oscillators with the record and plotting their peak responses, so it depends on the assumed damping and describes the effect of the motion rather than its content.

Answer: B

Correct answer: energy is distributed with frequency

The power spectral density gives the mean square amplitude per unit frequency, so its integral over a band is the energy the motion carries in that band. It is the statistical counterpart of the Fourier amplitude spectrum and is used where the motion is treated as a random process, as in stochastic response analysis.

Answer: C

It is obtained by integrating the square of the acceleration over the whole record, so it grows with both amplitude and duration. That makes it a useful single parameter where both matter, as in liquefaction and slope stability assessment.

Answer: D

Correct answer: faster than low-frequency content

Short wavelengths are scattered and absorbed far more effectively by the heterogeneity of the crust, so the high frequencies are stripped out of the wave train as it travels. A record close to the source is rich in high frequencies and sharp in appearance; one at a few hundred kilometres is dominated by long-period motion, which is why distant great earthquakes threaten tall buildings rather than low ones.