Questions: Seismic Waves: Body Waves and Surface Waves

Question 1

Seismograph stations on the opposite side of the Earth from a major earthquake record P waves but fail to record S waves at certain angular distances. What does this S-wave shadow zone tell us about Earth's interior?

• A. The earthquake was too weak to generate S waves that could travel the full distance
• B. S waves are absorbed by the mantle while P waves pass through, indicating a partially molten mantle
• C. Earth's outer core is liquid; S waves require a solid medium and cannot propagate through it
• D. S waves were converted to P waves at the core-mantle boundary and arrived as P waves

Answer: (C)

S waves are shear waves that require a medium capable of resisting shear deformation — liquids cannot sustain shear stress and therefore cannot transmit S waves. When seismologists mapped the angular distances where S waves are completely absent (the S-wave shadow zone, roughly 105°–140° from the epicenter), they concluded that S waves must be passing through a region that cannot transmit shear — Earth's liquid outer core. P waves (compressional) can travel through both solids and liquids, so they continue through the outer core, though refracted. The S-wave shadow zone is the primary seismological evidence for the liquid outer core.

Question 2

During an earthquake, which wave type typically causes the most structural damage to buildings?

• A. P waves, because they arrive first and cause instantaneous compression damage
• B. S waves, because their shear motion acts perpendicular to the propagation direction
• C. Surface waves, because they carry the most energy and produce the largest, most sustained ground displacements
• D. Rayleigh waves exclusively, since Love waves cancel out within structures

Answer: (C)

Surface waves are the most destructive. They travel more slowly than body waves but carry substantially more energy and generate far larger ground displacements. Rayleigh waves produce a rolling elliptical motion particularly damaging to structures; Love waves cause horizontal shearing. Because surface waves are confined near the surface where buildings exist, their energy is concentrated where it causes maximum damage. P waves produce a brief compression jolt, and S waves shake the ground transversely, but surface waves sustain large-amplitude shaking for a longer duration as the slower wave train arrives.

Question 3

S waves cannot propagate through Earth's outer core because that region is liquid and has no resistance to shear deformation.

• T. True
• F. False

Answer: True

True. Shear waves require that the medium resist shear deformation — the restoring force that sustains transverse oscillation. Liquids flow rather than restoring to their original shape under shear stress; there is no restoring force to propagate a shear wave. Therefore S waves cannot travel through liquids at all. The existence of a global S-wave shadow zone maps the size of Earth's liquid outer core precisely, because S waves reaching this region are blocked while those that miss the core arrive normally on the far side.

Question 4

P waves cause more ground shaking and structural damage than surface waves during an earthquake.

• T. True
• F. False

Answer: False

False. P waves arrive first, often producing a brief thump or jolt, but they carry relatively little energy compared to surface waves and produce small ground displacements. Surface waves arrive later (traveling more slowly) but bring far larger amplitudes and sustained shaking. Most building damage and seismic hazard in earthquake-prone areas is attributed to surface wave motion. This is why earthquake engineering focuses on how structures respond to the low-frequency, large-amplitude shaking of surface waves rather than the initial P-wave arrival.

Question 5

Explain how recording P and S wave arrival times at multiple seismograph stations allows geologists to determine where an earthquake occurred.

Model answer: P waves always travel faster than S waves, so the P–S arrival time gap at any station grows with distance from the earthquake. Measuring this time gap at one station gives the distance to the epicenter (but not the direction). With three or more stations at different locations, geologists draw a circle of calculated radius around each station; the earthquake epicenter is the unique point where all circles intersect (triangulation). Focal depth is constrained by comparing body-wave travel times with surface-wave arrivals and by the directional pattern of wave amplitudes.

This method works because P and S wave speeds in different rock types are known from studying thousands of previous earthquakes. The P–S time gap is like hearing thunder 10 seconds after lightning — it tells you the storm is ~3 km away, but not in which direction. Three stations provide three distance circles with a unique intersection. Modern seismological networks use hundreds of stations and sophisticated waveform inversion to locate earthquakes within kilometers of their true epicenter and depth, providing near-real-time location estimates within minutes of any significant earthquake.