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Difficulty: HardInferences from the Text

In 1936, Danish seismologist Inge Lehmann analyzed travel-time graphs of faint seismic compressional waves (PP-waves) recorded at stations situated within the deep 'shadow zone'—an angular distance between approximately 103∘103^\circ and 142∘142^\circ from an earthquake's epicenter where direct PP-waves were theoretically absent due to refraction at the mantle-core boundary. Prior models, established primarily by Beno Gutenberg, treated the Earth's core as a single homogeneous liquid shell that bent PP-waves downward, creating this shadow zone. Gutenberg and his contemporaries attributed anomalous PP-wave arrivals detected within the shadow zone exclusively to lateral diffraction around the core's outer perimeter or to instrumental noise. Lehmann demonstrated that diffraction could not account for the observed sharpness and high frequencies of these shadow-zone phases. Instead, she hypothesized a concentric, higher-velocity inner core possessing a distinct physical boundary that refracted and reflected PP-waves into the shadow zone. Although Lehmann refrained from definitively asserting the physical state of matter of this inner sphere, her calculations showed that a sudden velocity increase across a sharp discontinuity within the core matched empirical arrival times.

Which of the following statements can be properly inferred from the passage? (Select all that apply)

  1. Under Gutenberg's model, direct compressional waves were presumed to follow paths that bypassed surface monitoring stations situated between 103∘103^\circ and 142∘142^\circ from an epicenter.Answer
  2. Diffracted seismic waves typically exhibit lower frequencies or less distinct arrival profiles than the shadow-zone phases Lehmann evaluated.Answer
  3. C
    Lehmann conclusively demonstrated that the newly hypothesized inner core had to be in a solid state because liquids cannot transmit compressional waves.
  4. D
    Seismic monitoring stations located outside the 103∘103^\circ to 142∘142^\circ angular range were incapable of recording reflected seismic phases from the inner core boundary.
  5. E
    Gutenberg's contemporaries modified seismograph sensitivity parameters to suppress spurious readings caused by ambient mechanical vibrations.

Answer

The statements indicating that Gutenberg's model presumed direct compressional waves bypassed surface stations between 103∘103^\circ and 142∘142^\circ, and that diffracted waves typically exhibit lower frequencies or less distinct arrival profiles than the observed phases, are both logically supported.
The inference regarding Gutenberg's model is entailed by the definition of the shadow zone as an angular distance (103∘103^\circ to 142∘142^\circ) where direct waves were theoretically absent due to core refraction. The inference regarding diffracted waves is logically entailed because Lehmann's rejection of diffraction rested specifically on diffracted waves being incapable of producing the sharpness and high frequencies observed.

Step-by-Step Solution

1
Analyze Gutenberg's model and the definition of the shadow zone from the text.
The text states that in Gutenberg's model, refraction at the mantle-core boundary bent direct PP-waves downward, creating a shadow zone between 103∘103^\circ and 142∘142^\circ where direct arrivals were absent.
Establishing what Gutenberg's model predicted validates the statement regarding direct waves bypassing those angular stations.
2
Examine Lehmann's refutation of the diffraction hypothesis.
Lehmann showed that diffraction could not account for the observed sharpness and high frequencies of the anomalous waves.
This necessitates that diffracted waves do not share these sharp, high-frequency characteristics, confirming the second valid inference.
3
Evaluate and eliminate distractors against textual boundaries.
Claims regarding a proven solid state, inability of outside stations to detect phases, and alterations to instrument sensitivity are all unsupported by the text.
Distractors introduce unwarranted extrapolations or contradict explicit qualifications in the passage.

Key Concept

Drawing strictly bounded deductive inferences from textual premises without overextending beyond explicit quantifiers and qualifications.
Estimated Time:2m 30s
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Inferences from the Text

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