Understanding PKS and SKP Seismic Phases

The question asks about the arrival time and path of PKS and SKP seismic phases originating from a surface focus earthquake, as observed at an observatory. These specific seismic phases are defined by their interactions with the Earth's core.

• PKS phase: This phase represents a P-wave that travels through the Earth's mantle, interacts with the core-mantle boundary (CMB), passes through the Earth's core as a P-wave, interacts with the CMB again, and finally travels through the mantle to the observatory.
• SKP phase: This phase begins as an S-wave in the mantle. Upon reaching the CMB, it converts into a P-wave. This P-wave then travels through the Earth's core, interacts with the CMB, and propagates through the mantle as a P-wave to the observatory.

Comparing Paths and Arrival Times

The distinct nature of the PKS and SKP phases leads to specific characteristics regarding their paths and arrival times:

• Different Paths: The PKS and SKP phases inherently follow different geometrical trajectories (ray paths) through the Earth's interior. The PKS phase involves P-wave propagation through the mantle and core. In contrast, the SKP phase involves an initial S-wave segment in the mantle and a crucial S-to-P wave conversion at the core boundary, followed by P-wave propagation through the core. These differing wave types and conversion processes result in distinct paths to the observatory.
• Same Arrival Time: Although P-waves generally travel faster than S-waves, the specific complexities associated with the PKS and SKP ray paths—including their precise geometry and the varying velocities encountered within the mantle and core—result in equal travel times. The unique path characteristics for these two phases cause them to arrive simultaneously at the observatory.

Therefore, the PKS and SKP phases reach an observatory at the same time, travelling along different paths.