Seismic Waves and Measurement Questions and Answers

Answer: A

Correct answer: P wave

The P wave is a compressional wave and travels faster than any other seismic phase, at about 6 km/s in the crust against roughly 3.5 km/s for the S wave. It is therefore the first to arrive at a station, which is what the P stands for - primary. Surface waves are slower still, which is why the order of arrival is always P, then S, then the surface waves.

Answer: B

Correct answer: longitudinal compressional waves

A P wave propagates by alternate compression and dilatation of the material, and the particles move back and forth along the direction of travel, exactly like a sound wave. Because it needs only volume change and not shape change, it can pass through solids, liquids and gases alike. The S wave, by contrast, is transverse: its particle motion is perpendicular to the direction of travel.

Answer: C

Correct answer: liquids

An S wave propagates by shearing the material, and a liquid has no rigidity - its shear modulus is zero - so it cannot transmit shear at all. From

the velocity vanishes when \( \mu = 0 \). This is why S waves are absent beyond about 103 degrees from an epicentre, and that shadow zone is the principal evidence that the earth’s outer core is liquid.

Answer: D

The ratio depends on Poisson's ratio and comes out near 0.6 for common rock, so a P wave at about 6 km/s goes with an S wave near 3.5 km/s. The growing gap between arrivals with distance is what makes the S minus P interval useful.

Answer: A

Correct answer: have larger amplitude and longer period

Surface waves are confined to a thin layer near the surface, so their energy spreads in two dimensions rather than three and attenuates far more slowly with distance. They therefore arrive with large amplitude and long period, and their periods often fall close to the natural periods of buildings, which produces resonance. They also arrive last, being the slowest, and they last longer.

Answer: B

Correct answer: a retrograde elliptical path in the vertical plane

In a Rayleigh wave each particle traces an ellipse in the vertical plane containing the direction of travel, moving backwards at the top of its path - retrograde, the reverse of the orbital motion in a water wave. The amplitude decays rapidly with depth. The Love wave is the other surface wave, and its motion is purely horizontal and transverse to the direction of travel.

Answer: C

They exist only where a surface layer overlies faster material and they shake the ground sideways, which makes them particularly hard on structures with little lateral resistance. Rayleigh waves are the vertically elliptical ones.

Answer: D

Correct answer: Love wave

Body waves travel through the interior of the earth, and there are only two: the P wave and the S wave - the third option being simply another name for the P wave. The Love wave is a surface wave, guided along a low-velocity surface layer and dying away with depth, so it does not travel through the body of the earth at all.

Answer: A

Correct answer: 1 and 3 only

Statements 1 and 3 are correct: P waves pass through solids and liquids alike, and surface waves are confined near the ground surface. Statement 2 has it exactly backwards. S waves travel through solids and not through liquids, since a liquid cannot sustain shear - which is the reason for the S wave shadow zone and the evidence for a liquid outer core.

Answer: B

Correct answer: 2, 3, 1

The order of arrival follows from the velocities: P waves at about 6 km/s, S waves at about 3.5 km/s, and surface waves slower still. The interval between the P and S arrivals grows with distance and is what allows a single station to estimate its epicentral distance. Surface waves arrive last but usually with the largest amplitude, which is why the most damaging shaking comes after the first tremor.