ACT (Enhanced)ScienceMedium
A geophysicist is studying seismic wave data to understand Earth's interior structure. They detect two distinct types of waves: P-waves that travel through both solids and liquids, and S-waves that only travel through solids. By analyzing the arrival times and paths of these waves at different seismic stations, the geophysicist infers the presence of a liquid outer core and a solid inner core. This inference is primarily based on which characteristic of seismic waves?
- ATheir speed and medium-dependent propagation.
- BTheir polarization and wavelength.
- CTheir amplitude and frequency.
- DTheir ability to refract and reflect at boundaries.
Show answer & explanationAnswer & explanation
Correct answer: A. Their speed and medium-dependent propagation.
The key information provided is that P-waves travel through both solids and liquids, while S-waves only travel through solids. This difference in propagation through different media, combined with their speed variations, allows geophysicists to map the Earth's interior layers. The arrival times are a direct measure of speed.
Why the other options are wrong
- B. Polarization and wavelength are wave properties, but the scenario's critical information revolves around the wave types' ability (or inability) to travel through different states of matter.
- C. While amplitude and frequency are properties of waves, the scenario emphasizes how different waves behave in different states of matter, which relates to propagation characteristics, not just these properties.
- D. Refraction and reflection occur at boundaries, but the core inference here is *why* S-waves disappear in certain regions, which is due to their inability to propagate through liquids.
Seismic Wave Propagation
The movement of P-waves and S-waves through Earth's interior, which provides critical information about the composition and physical state of Earth's layers.
- P-waves (primary) are compressional, faster, and travel through solids, liquids, and gases.
- S-waves (secondary) are shear, slower, and travel only through solids.
- Changes in wave speed and direction indicate changes in material properties.
Memory trick: P-waves push through all, S-waves stick to solids.