P-wave Diffraction Epicentral Distance

Seismic waves generated during earthquakes travel through the Earth's layers. P-waves, also known as primary waves, are the fastest type of seismic wave. They are compressional waves, meaning they travel by compressing and expanding the material they pass through, similar to sound waves.

Diffraction is a phenomenon where waves bend or spread out when they encounter an obstacle or an opening. In seismology, this bending can occur due to variations in the Earth's density and structure, particularly around large structures like the Earth's core.

Understanding P-wave Behavior with Distance

The path seismic waves take depends heavily on the distance from the earthquake's epicenter. P-waves travel directly through the Earth's mantle and core. When they encounter boundaries between different materials, like the mantle and the liquid outer core, they refract (bend).

• Direct P-waves: Travel directly from the source to the station.
• Refracted P-waves: Bend as they pass through different layers (e.g., mantle-core boundary).
• Diffracted P-waves: Bend around obstacles or edges, effectively travelling into regions where direct or simply refracted waves wouldn't reach.

The Earth's core significantly affects P-wave paths. P-waves cannot travel through the liquid outer core as shear waves but do travel through it, albeit with a change in speed and direction. This interaction creates a "P-wave shadow zone," an area on the Earth's surface where direct P-waves are not detected. This shadow zone typically begins around an epicentral distance of $96^\circ$ and extends to about $103^\circ$.

Weak P-wave Diffraction Range

The observation of weak P-wave diffraction is particularly noted in the epicentral distance range of approximately $103^\circ$ to $120^\circ$. Here's why:

• At distances just beyond $103^\circ$, P-waves that have travelled through the core emerge at the surface.
• Waves in the range of $103^\circ$ to around $142^\circ$ are often diffracted around the edge of the solid inner core or have passed through the outer core and are detected as PKP waves (P-waves that have traversed the core).
• The term "weak diffraction" in this context refers to the less intense signals observed as P-waves graze the core-mantle boundary or are diffracted around the core, compared to the stronger direct or refracted waves observed at shorter distances. The complexity of wave paths near the core boundary leads to these diffracted arrivals.

Understanding these specific distance ranges is crucial for seismologists to interpret seismic records accurately and learn about the Earth's internal structure.