Why do S-waves fail to travel through Earth's outer core?
AThe outer core is too hot, and S-waves cannot propagate above a critical temperature
BShear waves require a material with a non-zero shear modulus; the liquid outer core has no shear resistance
CS-waves are refracted back to the surface by the density contrast before reaching the outer core
DThe outer core absorbs all wave energy due to its extremely high density
S-waves are shear waves — they deform material sideways relative to the direction of travel. This requires the medium to resist shear deformation (i.e., have a non-zero shear modulus). Liquids flow rather than resist shear, so their shear modulus is zero and S-waves cannot propagate. The observation of an S-wave shadow zone is one of the primary lines of evidence that Earth's outer core is liquid.
Question 2 True / False
Surface waves arrive at seismograph stations before P-waves and S-waves.
TTrue
FFalse
Answer: False
P-waves are the fastest seismic waves and arrive first; S-waves arrive second; surface waves (Love and Rayleigh) arrive last because they travel slowest. The name 'primary' (P) and 'secondary' (S) reflects arrival order. However, surface waves have the largest amplitude and longest period, so they cause the most structural damage — a common source of confusion between 'first' and 'most destructive'.
Question 3 Short Answer
How does the S–P arrival-time difference at a seismograph station allow seismologists to estimate the distance to an earthquake's epicenter?
Think about your answer, then reveal below.
Model answer: P-waves and S-waves travel at different, known speeds through Earth. The gap between their arrival times grows predictably with distance — the farther the station from the epicenter, the larger the S–P time difference. By reading this gap from a seismogram and consulting travel-time curves (which map S–P interval to distance), seismologists can estimate how far away the earthquake occurred. Three or more stations are then needed to triangulate the exact epicenter location.
This is analogous to estimating how far away lightning struck by timing the gap between the flash and thunder, since light and sound travel at very different speeds. The S–P method works because P and S velocities are well-characterized from decades of seismic data and the physics of elastic wave propagation in Earth's layered interior.