Seismic Waves
Earth has a diameter of 12, 740 km, yet humans have barely penetrated its surface. Most
mining operations are limited to the upper 7 km of the crust. Kola Well, a research well
in northern Russia, is only 12.3 km deep. How then, have scientists learned about the
composition of Earth’s interior?
Seismic Waves
Knowledge of Earth’s inner makeup has been gained by the study of seismic waves
produced by earthquakes and controlled nuclear explosions. During such an event,
seismic waves radiate out in all directions from a focus, or point of origin. They pass
through Earth and emerge at distant points. The emerging waves, which carry
information about the materials they pass through, are recorded and analyzes at
seismographic stations.
Seismic waves travel at different speeds and reflect or refract depending on the type and
properties of materials they encounter. Such behaviour is often evident at the boundaries
between two materials that differ physically or chemically. In general, seismic waves
travel fastest through materials that are rigid, yet elastic; that is, resilient solids.
Temperature and pressure also impact seismic waves, with pressure having the greatest
influence. Higher temperatures slow seismic waves, while higher pressures speed them
up. By measuring the reflection, refraction, and travel time of seismic waves, scientists
have been able to make discoveries about Earth’s interior.
Seismographs are instruments that are used to record seismic waves. Today, hundreds of
seismographs measure seismic activity all over the world.
P Waves
Also known as primary waves, P waves are a type of seismic wave that compress and
expand rock in the same direction that the wave is traveling. S waves temporarily change
the shape of the materials that they pass through. This means that S waves are
transmitted by solids only. Solids can elastically spring back to their initial shape after an
S wave has passed. Liquids and gases do not have this ability because they flow and lack
fixed shapes. S waves travel at about half the speed of P waves.
The Core-Mantle Boundary
Beno Gutenberg discovered the core-mantle boundary in 1914. He noted that P waves
diminish and die out roughly between 105 and 140 away from an earthquake. This area
is known as the P wave shadow zone. Gutenberg said that the shadow zone could be
explained by the existence of a central core within Earth that is composed of materials
that differ from the mantle. It was found that the core creates the shadow zone because it
causes P waves to refract. Later research revealed that S waves do not travel through the
core at all and P waves slow down by 40% in this region. Because P waves travel more
quickly in solids than liquids and S waves do not travel through liquids at all, scientists
concluded that at least part of the core must be liquid.
Questions:
1. What type of seismic wave is depicted in the illustration? Explain your reasoning.
2. Which of the following statements is true about P waves?
A.
B.
C.
D.
They travel through solids only.
They are the slowest of the seismic waves.
They travel along Earth’s outer surface only.
They change the volume of the substances they pass through.
3. What seismic evidence led Beno Gutenberg to discover the core-mantle boundary?
Earthquakes
An earthquake is a vibration of Earth due to the rapid release of energy. Powerful
volcanic eruptions and atomic explosions can cause earthquakes. However, most
earthquakes occur along faults at plate boundaries. As tectonic plates bump, jostle, and
slide along and over one another, they often become stuck. Here, tectonic forces build up
stress between two plates on either side of a fault. Over time, the stress builds and the
plates begin to deform or change shape. Eventually, the stress becomes so high that the
plates slip, and the deformed rocks snap back into their original shapes. One of the most
famous earthquake belts occurs along the Ring of Fire where tectonic plates converge.
An earthquake originates at a point called a focus, ranging in depth from 5 to almost 700
kilometres. Earthquakes are classified according to the depth of their foci as shallow,
intermediate, or deep focus. Shallow-focus earthquakes are generally the most
destructive type as they have foci within 70 km of the surface. Earthquakes that occur
along oceanic ridges where the crust is thin is always shallow focus, though never strong.
Intermediate-focus earthquakes are those with foci between 70 and 300 km of the surface,
while deep-focus earthquakes occur in subduction zones, namely along the Pacific coast,
or Ring of Fire. These are less destructive because the energy of seismic waves
dissipates with distance from their source.
Studies of earthquakes around sea trenches and subducting oceanic plates reveal that
focus depth increases with distance from the trench. Most shallow-focus quakes occur
near the trench, while most deep-focus quakes occur farther away on the landward side of
the trench. These zones of seismic activity are known as Wadati-Benioff zones. They
occur along the submerged portion of the subducting plate, nearly 700 km down, at
angles between 35 to 90 to the surface. As the plate descends, it interacts with the
overriding plate above it to produce shallow-focus earthquakes. Deeper-focus
earthquakes are generated as the plate dips farther into the mantle. The cause of deepfocus quakes are not fully understood today. Some scientists suggest that they are
triggered by large-scale phase changes of rock minerals induced by high pressures and
temperatures deep below the surface.
Questions
1.. How was the formation of the Andes (South America) different from the formation of
the Himalayas?
2. Where do deep-focus earthquakes occur?
3. Which land feature is associated with subduction?
A.
B.
C.
D.
Rift valley
Oceanic ridge
Hot spot chain
Continental volcanic arc