Why do P waves travel faster through the lithosphere through the asthenosphere?

As a geophysicist with expertise in seismology, I can explain the behavior of P waves in the Earth's interior. P waves, or primary waves, are a type of elastic wave that can travel through solids, liquids, and gases. They are the fastest moving waves produced by an earthquake and are the first to be detected by seismographs. P waves travel faster through the lithosphere because the lithosphere is the rigid, outermost shell of the Earth composed of the crust and the uppermost part of the mantle. The lithosphere is made up of less dense, cooler rocks that are more rigid due to their lower temperatures. The speed of P waves is influenced by the density and elasticity of the materials they pass through. In the lithosphere, the rocks are more rigid and less dense, which allows P waves to move more quickly. On the other hand, the asthenosphere is a part of the upper mantle located below the lithosphere. It is characterized by more ductile and hotter rocks that are more easily deformed under stress. The asthenosphere is less rigid than the lithosphere, and while it is still a solid, it behaves more like a very viscous fluid over long periods of time. The material in the asthenosphere is denser than that in the lithosphere, but the P waves do not travel faster through the asthenosphere than through the lithosphere. In fact, the speed of P waves decreases as they move from the lithosphere into the asthenosphere because the denser, more ductile material in the asthenosphere offers more resistance to the passage of the waves. To summarize, P waves travel faster through the lithosphere than through the asthenosphere due to the differences in rigidity, density, and temperature between these two layers of the Earth.

Seismic waves travel faster in the mantle than they do in the crust because it is composed of denser material. Thus, stations further away from the source of an earthquake received waves that had made part of their journey through the denser rocks of the mantle.