Why are S waves unable to propagate through Earth's outer core?
AS waves are absorbed by the high temperatures in the outer core
BS waves require a solid medium to transmit shear stress, and the outer core is liquid
CS waves travel too slowly to penetrate that depth before being refracted
DS waves are converted entirely to surface waves at the core-mantle boundary
Shear waves require a material with a non-zero shear modulus — in other words, the medium must resist deformation by shearing. Liquids cannot sustain shear stress, so their shear modulus is zero and S waves cannot propagate. The discovery of the S-wave shadow zone was the key evidence that Earth's outer core is liquid.
Question 2 True / False
P waves travel faster than S waves in the same material because P waves have longer wavelengths.
TTrue
FFalse
Answer: False
Wavelength does not determine wave speed in this context. P-wave velocity depends on the bulk modulus (resistance to compression) plus the shear modulus, divided by density: Vp = sqrt((K + 4G/3) / ρ). S-wave velocity depends only on the shear modulus: Vs = sqrt(G / ρ). Because the bulk modulus adds to the numerator for P waves, Vp > Vs regardless of wavelength. The speed difference is a consequence of the physical restoring forces, not wave geometry.
Question 3 Short Answer
A seismograph records a P-wave arrival at time 0 and an S-wave arrival 50 seconds later. What does this S-P time interval tell a seismologist, and what additional information would be needed to locate the earthquake?
Think about your answer, then reveal below.
Model answer: The 50-second S-P interval gives the distance from the seismograph to the earthquake source (epicentral distance), since P and S waves travel at known but different speeds. To locate the earthquake, you need S-P intervals from at least three seismograph stations; the intersection of three distance circles (trilateration) pinpoints the epicenter.
The S-P interval is a distance measurement, not a direction. Knowing that the earthquake was, say, 400 km away only places the source somewhere on a circle of radius 400 km around that station. Three such circles from three stations intersect at a unique point, yielding the epicenter. This is the basis of classical earthquake location methods.