Ground Motion Characteristics Questions and Answers

Answer: A

Averaging the shear wave velocity over the top 30 metres gives a single number that captures the stiffness of the ground the waves must pass through. Codes group sites into classes on that basis and assign amplification factors to each.

Answer: B

Correct answer: amplify the motion relative to rock

As a wave passes from stiff rock into softer soil its amplitude must increase to carry the same energy flux through a medium of lower impedance, and resonance within the soil column amplifies it further near the site period. Soft sites therefore shake harder and at longer periods than nearby rock sites, which is what devastated Mexico City in 1985.

Answer: C

Correct answer: stiffness falls and damping rises as strain increases

Soil is linear only at very small strains. As shaking intensifies the shear modulus falls, sometimes to a small fraction of its initial value, while hysteretic damping rises. Both effects lengthen the site period and limit the amplification, so a soft site that amplifies a weak motion greatly may amplify a strong one much less.

Answer: D

Properties are adjusted until they match the strains they produce, which captures much of the non-linearity without a full non-linear model. It becomes unreliable at very large strains, where a genuinely non-linear analysis is needed.

Answer: A

It integrates over all sources, magnitudes and distances to state, for example, the acceleration with a 10 per cent chance of being exceeded in 50 years. A deterministic analysis instead takes one chosen scenario and asks what it would produce.

Answer: B

Correct answer: considers a specific earthquake scenario

A deterministic analysis selects a particular source and magnitude - typically the maximum credible event on the nearest active fault - and computes the motion it would produce. A probabilistic analysis integrates over all sources, magnitudes and distances with their rates of occurrence, producing a motion with a stated annual probability of exceedance.

Answer: C

A 10 per cent chance of exceedance in 50 years gives about 475 years, the level at which a building is expected to be damaged but not to collapse. The maximum considered earthquake is a rarer motion, often near 2475 years.

Answer: D

Each period is treated separately in the hazard calculation, so the curve is an envelope of equal likelihood rather than the spectrum of any one event. That is also its limitation, since no single earthquake produces it.

Answer: A

It breaks the total hazard back down to show which combinations dominate, which is what guides the choice of records for a time history analysis. Without it a uniform hazard spectrum gives no scenario to select against.

Answer: B

Correct answer: the expected level of ground motion in a region

The zone factor in IS 1893 is the regional seismicity term, corresponding to the maximum considered earthquake for that zone. Soil is handled through the spectrum shape, importance through the importance factor and ductility through the response reduction factor - four separate terms in the design coefficient, each doing one job.