Seismic waves are vibrations that travel through the Earth, typically generated by events like earthquakes. These waves carry energy away from their source, much like ripples expanding on a pond after a stone is thrown. Scientists use the behavior of these waves to explore the planet’s hidden interior. Seismic waves do not travel in straight lines; instead, they follow distinctly curved paths as they propagate through the Earth.
The Earth’s Layered Structure
The Earth is not a uniform sphere but is structured into several distinct layers, each with different physical properties. The outermost layer is the crust, a relatively thin, rocky shell. Beneath the crust lies the mantle, a much thicker layer primarily composed of solid rock, constituting about 84% of the Earth’s volume. Deeper still is the core, subdivided into a liquid outer core and a solid inner core, both largely made of iron and nickel.
These layers exhibit variations in properties like density, pressure, and rigidity. As depth increases, the pressure on the material rises due to the weight of the overlying layers. Density also increases with depth, the inner core being the densest layer. While temperature increases with depth, its effect on wave speed is often outweighed by the increasing pressure and rigidity.
How Material Properties Affect Wave Speed
The speed of seismic waves depends directly on the physical properties of the material they pass through. Two primary types of body waves, P-waves (compressional) and S-waves (shear), respond differently to these properties. P-waves, which involve compression and expansion, can travel through solids, liquids, and gases. Their speed generally increases with greater incompressibility and rigidity, while decreasing with higher density.
S-waves involve a shearing motion perpendicular to their direction of travel. These waves can only propagate through solid materials because liquids and gases lack the rigidity necessary to transmit shear stress. S-wave speed increases with greater rigidity and decreases with higher density. Although density appears in the denominator for both wave speed equations, increased rigidity and incompressibility with depth typically causes wave speeds to increase in denser, deeper materials.
The Physics of Wave Bending
Waves change direction when they move from one medium to another where their speed changes, a phenomenon known as refraction. This bending occurs because one part of the wavefront enters the new medium and changes speed before the rest of the wavefront. Imagine a car driving from a paved road onto a muddy field at an angle; the tire that hits the mud first slows down, causing the car to turn toward the mud.
The amount of bending depends on the angle at which the wave approaches the boundary and the degree of change in wave speed. This principle, described by Snell’s Law, applies to all types of waves, including light, sound, and seismic waves. If a wave enters a medium where its speed increases, it bends away from the normal, an imaginary line perpendicular to the surface. Conversely, if its speed decreases, it bends toward the normal.
The Formation of Curved Paths
The continuous variation in the Earth’s material properties with depth leads to curved seismic wave paths. Rather than distinct boundaries causing abrupt changes in speed, the density, pressure, and rigidity of Earth’s interior generally increase gradually and continuously. Because wave speed generally increases with depth in the mantle, seismic waves are constantly refracting upwards, back towards the surface. This ongoing bending, rather than a single sharp turn, results in the smooth, curved trajectories observed for seismic waves as they travel through the Earth. Studying these curved paths allows seismologists to deduce information about the Earth’s internal structure and composition.