Sound, ultrasound and infrasound

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Question 1
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The table in Table 1 outlines key properties of the two types of seismic waves produced during an earthquake: P-waves and S-waves.

Table 1

Wave TypeWave classificationRelative speedMedia it can travel through
P-wavesLongitudinalFastSolids and liquids
S-wavesTransverseSlowSolids only

Figure 1 shows the pathways of these seismic waves propagating from an earthquake epicentre through the Earth's interior, and the detection zones (Region X, Region Y, and Region Z) on the Earth's surface.

A cross-section diagram of the Earth showing an earthquake source at the top. Waves travel downward from the source. Through the mantle, the paths of both P-waves and S-waves curve gradually. At the core-mantle boundary, S-waves stop completely and do not pass through the core. P-waves refract (bend suddenly) at the core-mantle boundary, creating a shadow zone. Region X, spanning from 0 to 103 degrees on either side of the epicentre, detects both P-waves and S-waves. Region Y, from 103 to 142 degrees, is a shadow zone where neither direct P-waves nor S-waves are detected. Region Z, from 142 to 180 degrees at the bottom, detects only P-waves.

Explain how the wave paths shown in Figure 1 and the properties in Table 1 provide evidence for:

  1. The changing density within the Earth's mantle.
  2. The liquid state of the Earth's outer core.
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Sound, ultrasound and infrasound Questions

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