What are the principal layers of the Earth? What happens to earthquake waves when they reach the boundary between layers?

The Earth is composed of three principal layers, which are the crust, mantle, and core. The crust is the outermost layer, followed by the mantle, and then the core which is divided into two parts (outer core and inner core). 1. Crust: This is the Earth's outermost layer and consists of solid rock. It's divided into two types: the continental crust (which comprises the landmasses) and the oceanic crust (which lies beneath the ocean floors). The thickness of the crust varies from 5 km to 70 km. 2. Mantle: This layer lies beneath the crust and is a semi-solid layer made up of hot, flowing rock material. Its thickness is around 2,900 km. 3. Core: The core is the innermost layer of the Earth and is subdivided into the outer core and inner core. The outer core is predominantly composed of liquid iron and nickel, while the inner core is a solid sphere made of iron and nickel alloys. The outer core has a thickness of approximately 2,200 km, and the inner core has a radius of about 1,200 km.

When earthquake waves, also known as seismic waves, encounter the boundary between two layers of the Earth, their behavior can be described in terms of reflection, refraction, or both. 1. Reflection: When the seismic waves reach a boundary between two layers with different properties, some portion of the waves' energy is reflected back into the first layer. This is similar to how light is reflected when it hits a smooth surface like a mirror. 2. Refraction: As seismic waves pass from one layer to another with different density or material properties, their speed changes, and they are refracted. This bending of the waves is analogous to how light bends when it passes through different media like air and water. For example, as waves pass from the Earth's crust into the mantle, they refract due to the difference in density and velocity between the crust and mantle. Additionally, some types of seismic waves, called S-waves (shear waves), cannot travel through liquid layers like the Earth's outer core. When S-waves encounter the barrier between the mantle and the outer core, they are effectively stopped and absorbed, a phenomenon known as the S-wave shadow zone. P-waves (compressional waves), on the other hand, can travel through both solid and liquid layers, but they refract and change direction due to the density differences between the layers. In conclusion, earthquake waves' behavior as they reach the boundary between the principal layers of the Earth can involve reflection, refraction, and absorption, depending on the seismic wave type, material properties, and density differences in the various layers.