Describe how seismic waves are used to probe Earth’s interior

Probing earth’s interior

Discovering the structure and properties of Earth’s deep interior has not been easy. Light does not
travel through rock,so we must find another ways to “see” into our  planet earth . The most accurate way to learn about Earth’s interior would be to dig or drill a hole and examine what is extracted. Unfortunately, this is only possible at shallow depths. The deepest a drill has ever penetrated is only 12.3 kilometers (7.5 miles), about 1/500 of the way to Earth’s centre—an extraordinary accomplishment due to the rapid increases in temp and pressure with depth we are going beneath the earth.

“Seeing” Seismic Waves
Fortunately for seismologists, many earthquakes are large enough that their seismic waves travel all the way through Earth and can be detected on the other side (Figure 2).

This property of seismic waves is similar to how medical X-rays image our bones and organs. There are about 100 to 200 earthquakes each year that are large enough (about M 6) to be recorded by seismographs around the globe. These large quakes provide the means to “see” into our planet and have been the source of much of the data that allow us to more fully understand Earth’s interior.

Seismic Velocities

Recall that the various seismic waves travel at different speeds. In addition, the speed at which P waves and S waves travel through Earth’s layers depends largely on the properties of the materials that transmit them. In general, seismic waves travel fastest when rock is stiff (rigid) or less compressible. These properties of stiffness and compressibility are used to interpret the composition and temperature of the rock.
For instance, when rock is heated, it becomes less stiff (imagine warming a frozen chocolate bar), and waves travel through it more slowly. When P waves enter the outer core, which is liquid, they slow dramatically while S waves are not transmitted  Likewise, waves travel at different speeds through Earth materials that have different compositions. For example, seismic waves travel faster through oceanic crust, which is composed of basalt, than through the continental crust, which has an overall composition akin to granite. Thus, the speed at which seismic waves travel through a layer can help determine both the type of material and its temperature.

Interactions between Seismic Waves and Earth’s Layers:

Interpreting the waves recorded on seismograms is challenging because seismic waves usually do not travel along straight paths. Instead, seismic waves interact with Earth’s layers and are reflected and refracted as they pass through our planet (Fig 3).

 You are familiar with reflected sound waves that we call echoes. When a seismic wave hits a boundary between different Earth materials, such as the boundary between the crust and the mantle, some of the waves are reflected back toward the surface (Fig 4).

The remaining energy passes though the boundary and is refracted (bent). This is similar to how light is refracted (bent) as it passes from air to water.
One of the most noticeable behaviour  of seismic waves is that they follow strongly curved (refracted) paths because their velocities generally increase with depth (Fig 5).

 Within a particular layer, the speed of seismic waves increases with depth because pressure increases and squeezes the rock into a more compact, rigid material. Within Earth’s mantle, where there are both distinct boundaries and gradual seismic velocity changes caused by changes in mineral properties, the pattern of seismic waves is complex.

Fig 6

shows what S waves look like when they travel from a deep earthquake through the mantle. Note how the single wave from the shock is soon broken into many different waves that appear on seismograms as separate signals.