Question

A seismic wave is traveling through the Earth's mantle. The deeper into the mantle the wave goes, the greater the pressure is.



As the pressure in the mantle increases,
A.
the wave's frequency increases.
B.
the wave's speed increases.
C.
the wave's speed decreases.
D.
the wave's frequency decreases.

Answer 1

B. the wave's speed increases.

Reason: seismic wave speeds depend on the elastic moduli and density (e.g. vp = sqrt((K + 4/3 μ)/ρ), vs = sqrt(μ/ρ)). Increasing pressure raises the rock's stiffness (bulk and shear moduli) more than it raises density, so wave speeds increase with depth. The wave's frequency is set by the source and does not change as it propagates (though wavelength changes when speed changes).