The Movement of Seismic Waves Through the Earth
Wave Fronts and Travel Times
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| The propagation of earthquake waves. |
The energy released by an earthquake moves through rock in the form of waves, just as waves propagate outward from the impact point of a pebble on the surface of a pond. The boundary between the rock through which a wave has passed and the rock through which it has not yet passed is called a wave front. In 3-D, a wave front expands outward from the earthquake focus like a growing bubble. We can represent a succession of waves in a drawing by a series of concentric wave fronts. The changing position of an imaginary point on a wave front as the front moves through rock is called a seismic ray. You can picture a seismic ray as a line drawn perpendicular to a wave front; each point on a curving wave front follows a slightly different ray (figure above a). The time it takes for a wave to travel from the focus to a seismometer along a given ray is the travel time along that ray.
The ability of a seismic wave to travel through a certain material, as well as the velocity at which it travels, depends on the character of the material. Factors such as density (mass per unit volume), rigidity (how stiff or resistant to bending a material is), and compressibility (how easily a material’s volume changes in response to squashing) all affect seismic wave movement. Studies of seismic waves reveal the following:
- Seismic waves travel at different velocities in different rock types (figure above b). For example, P-waves travel at 8 km per second in peridotite (an ultramafic igneous rock), but at only 3.5 km per second in sandstone (a porous sedimentary rock). Therefore, waves accelerate or slow down if they pass from one rock type into another.
- Seismic waves travel more slowly in magma than in solid rock of the same composition, and more slowly in molten iron alloy than in solid iron alloy (figure above c).
- Both P-waves and S-waves can travel through a solid, but only P-waves can travel through a liquid (figure above d).
Reflection and Refraction of Wave Energy
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| Refraction and reflection of waves. |
Shine a flash light into a container of water so that the light ray hits the boundary (or interface) between water and air at an angle. Some of the light bounces off the water surface and heads back up into the air, while some enters the water (figure above a). The light ray that enters the water bends at the air water boundary so the angle between the ray and the boundary in the air is different from the angle between the ray and the boundary in the water. Physicists refer to the light ray that bounces off the air-water boundary and heads back into the air as the reflected ray, and the ray that bends at the boundary as the refracted ray. The phenomenon of bouncing off is reflection, and the phenomenon of bending is refraction. Wave reflection and refraction take place at the interface between two materials, if the wave travels at different velocities in the two materials.
Seismic energy travels in the form of waves, so seismic waves reflect and/or refract when reaching the interface between two rock layers if the waves travel at different velocities in the two layers. For example, imagine a layer of sandstone overlying a layer of basalt. Seismic velocities in sandstone are slower than in basalt, so as seismic waves reach the boundary, some reflect and some refract. The amount and direction of refraction at a boundary depend on the contrast in wave velocity across the boundary and on the angle at which a wave hits the interface. As a rule, if waves enter a material through which they will travel more slowly, the rays representing the waves bend down and away from the interface. For example, the light ray in figure above b bends down when hitting the air-water boundary because light travels more slowly in water. This relation makes sense if you picture a car driving from a paved surface diagonally onto a sandy beach the wheel that rolls onto the sand first slows down relative to the wheel still on the pavement, causing the car to turn toward the sand. Alternatively, if the ray were to pass from a layer in which it travels slowly into one in which it travels more rapidly, the rays representing the waves would bend up and toward the interface (figure above b).
Credits: Stephen Marshak (Essentials of geology)
