Seismic Wave Refraction and Shadow Zones Explained

Statement I Analysis

Statement I discusses the behavior of seismic wave phases PKP and PKIKP at Earth's core boundaries.

• PKP phase: This seismic wave travels through the Earth's mantle (P), outer core (K), and back through the mantle (P). As it enters and exits the core, it refracts at the mantle-core boundary. The statement correctly implies that the path involves interactions (refractions) at this boundary.
• PKIKP phase: This specific phase of the P-wave passes through the solid inner core (I), meaning its path is P -> Outer Core (K) -> Inner Core (I) -> Outer Core (K) -> Mantle (P). It experiences refraction when entering the outer core, again when entering the inner core, and subsequent refractions when exiting the inner core and outer core. The statement correctly identifies that the PKIKP phase is refracted at the inner-outer core boundary (twice, upon entering and exiting the inner core).

Therefore, Statement I is considered true.

Statement II Analysis

Statement II claims that both PKP and PKIKP phases are absent from the seismic shadow zone.

• Seismic Shadow Zone: This is a region where seismic waves are not detected due to refraction and reflection caused by Earth's structure, primarily the core. The P-wave shadow zone exists roughly between 103° and 142° from the earthquake source.
• PKIKP Observation: The PKIKP phase, by definition, travels through the inner core. Its detection is crucial for seismology. While the mantle-core boundary refracts P-waves significantly, creating a shadow zone, the PKIKP waves emerge on the opposite side of the Earth, filling the region beyond ~142°. They are essential for understanding core structure and are observed in specific regions beyond the main P-wave shadow zone, effectively defining its outer limits for certain paths. They are not entirely excluded from all areas considered within shadow zones.

Since the PKIKP phase *is* recorded and observed in regions beyond the primary shadow zone, Statement II is considered false.

Conclusion

Based on the analysis, Statement I is true, and Statement II is false.