Hello, I'm a geophysicist with a strong background in seismology. I'd be happy to explain the concept of shadow zones and why they exist. Shadow zones are regions on the Earth's surface where seismic body waves, specifically the primary or P-waves, are not detected after an earthquake. These zones are located at an angular distance of roughly 105 to 145 degrees from the earthquake's epicenter. The existence of shadow zones is due to the way seismic waves travel through the Earth and interact with its internal layers. When a seismic wave encounters the boundary between the Earth's mantle and its core, the wave's path is altered through a process known as refraction. The core of the Earth is primarily composed of iron and nickel and is in a liquid state, which has different physical properties from the solid mantle. Because of these differences, P-waves cannot travel through the liquid outer core; they are refracted around it. This causes the P-waves to be diverted away from certain areas on the Earth's surface, creating the so-called shadow zones where they are not detected. read more >>

At almost the opposite point on the Earth's surface, the "P" waves reappear. The shadow zone exists because the waves are refracted as they pass through the boundary between the mantle and the core and are diverted from their original paths.read more >>