Identifying the Earliest Seismic Phase at 120°

This question asks to identify the seismic phase that arrives first at a seismograph station located at an epicentral distance of $120^{\circ}$. Seismic phases are different wave paths that travel through or reflect off various layers of the Earth.

Seismic Phase Analysis

We need to compare the arrival times of the given seismic phases ($P_{diff}$, $P_cP$, $PKP$, $PPP$) at a distance of $120^{\circ}$.

• $PPP$: This is a P-wave that has reflected off the Earth's surface three times. Such multiply reflected waves typically have significantly longer travel times compared to waves that travel more directly through the Earth's interior or core.
• $P_cP$: This phase represents a P-wave that travels from the source, reflects off the outer core's boundary (core-mantle boundary), and then travels back to the surface. Reflections generally increase travel time.
• $PKP$: This is a P-wave that travels from the source, refracts (bends) as it enters the Earth's core (denoted by 'K'), travels through the core, refracts again as it exits the core, and finally reaches the station as a P-wave. This phase travels through the core.
• $P_{diff}$: This phase involves P-waves that are diffracted (bent) along the core-mantle boundary. It essentially travels around the edge of the core. This phenomenon is observed at specific distance ranges beyond the core's "shadow zone".

Determining the Earliest Arrival at 120°

Standard seismic travel-time curves and seismological models show the behavior of these phases:

• At distances like $120^{\circ}$, waves that travel through the core ($PKP$) or are diffracted along its boundary ($P_{diff}$) arrive much earlier than surface-reflected waves like $PPP$.
• The $P_{diff}$ phase is specifically observed as the earliest P-wave arrival in the epicentral distance range of approximately $100^{\circ}$ to $140^{\circ}$. It represents the wave that travels along the core-mantle boundary, arriving just before the direct $PKP$ waves in this distance range.
• While $PKP$ also passes through the core, the specific path and refraction geometry at $120^{\circ}$ result in $P_{diff}$ arriving slightly earlier. $P_cP$ arrives later than both $P_{diff}$ and the primary $PKP$ branches.

Therefore, the earliest seismic phase to reach a station at an epicentral distance of $120^{\circ}$ is $P_{diff}$.