Seismologists observe that S-waves from deep earthquakes never arrive at recording stations on the opposite side of the Earth. The most direct inference is:
AS-waves are absorbed by the high-density iron of the lower mantle before reaching the core
BS-wave velocities are too slow to traverse the full Earth within the observation window
CThe outer core is liquid, and shear waves cannot propagate through fluids
DThe inner core reflects all S-waves back toward the source hemisphere
S-waves (shear waves) require a solid medium for propagation — they deform material in a direction perpendicular to travel, which fluids cannot sustain. The absence of S-waves beyond ~103° from the epicenter (the S-wave shadow zone) is the definitive evidence that the outer core is liquid. P-waves, which compress material longitudinally, can travel through liquids — and they do arrive at the other side (though refracted), confirming the core is there. This is not an assumption; it is directly inferred from seismic wave behavior.
Question 2 Multiple Choice
In a seismic velocity-depth profile, the low-velocity zone between approximately 80 and 200 km depth is caused by:
AThe compositional boundary where continental crust transitions to oceanic crust
BThe Mohorovičić discontinuity, where crustal granitic rock gives way to mantle peridotite
CPartial melting and elevated temperatures in the asthenosphere reducing the shear modulus
DPressure-induced phase transitions in olivine crystal structure at those depths
Below the lithosphere, temperatures in the asthenosphere are high enough to partially melt mantle rock (~1–2% melt fraction). Even a small amount of melt dramatically reduces the shear modulus (G), which lowers both Vp and Vs despite the fact that pressure is increasing. This creates an anomalous velocity decrease — a 'low-velocity zone' — that is the seismological signature of the weak, partially molten asthenosphere. Option B (the Moho) occurs at ~35 km (continental) and ~7 km (oceanic) depth — far shallower. Option D (olivine phase transitions) occurs at 410 and 660 km — far deeper, and those transitions increase velocity.
Question 3 True / False
Seismic tomography expresses its results as velocity perturbations — percentage deviations from a reference model — rather than absolute velocities.
TTrue
FFalse
Answer: True
True. Reference models like PREM define a one-dimensional baseline velocity-depth profile representing the average Earth. Tomographic studies compare observed seismic travel times to those predicted by the reference model and invert for velocity anomalies — regions that are faster (typically colder, e.g., subducting slabs) or slower (typically hotter, e.g., mantle plumes) than average. Expressing results as perturbations rather than absolute velocities removes the baseline and highlights the three-dimensional heterogeneity that is the actual scientific target.
Question 4 True / False
Seismic velocity generally decreases with depth throughout the Earth's mantle because increasing temperature progressively lowers the elastic moduli.
TTrue
FFalse
Answer: False
False. The dominant trend in the mantle is a *velocity increase* with depth, not a decrease. Although temperature rises with depth and acts to lower elastic moduli, the effect of increasing pressure — which stiffens rock by compressing it — outpaces the temperature effect throughout most of the mantle. Velocity only decreases locally in the low-velocity zone (asthenosphere, ~80–200 km) where partial melting is sufficient to reduce the shear modulus. Below this zone, velocity resumes its upward trend through the transition zone and lower mantle.
Question 5 Short Answer
Why is a velocity-depth model necessary before a seismologist can locate an earthquake, and what would happen to location estimates if the model were wrong?
Think about your answer, then reveal below.
Model answer: Earthquake location works by measuring the difference in arrival times of seismic waves at multiple stations and inverting for the source position. This requires knowing how fast the waves travel along each path — which depends on the velocity structure of the Earth they pass through. If the velocity model is wrong, the predicted travel times will be wrong, and the best-fit source location will be systematically displaced from the true location. In regions where the velocity model is poorly known (e.g., subduction zones with anomalous structure), earthquake location errors of tens of kilometers are common, which matters enormously for understanding fault geometry and seismic hazard.
This question connects the abstract concept of velocity models to their practical function. Students often treat velocity models as background knowledge rather than active tools. The key insight is that every seismological result — earthquake locations, depth estimates, fault mechanisms — is model-dependent. Improving velocity models (via tomography) directly improves every downstream analysis.