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An earthquake is not an isolated event of ground shaking. Foreshocks and aftershocks occur with most earthquakes, and in this video lesson you will learn about them and how they are part of the overall process.

The Shaking Ground

Earthquakes are some of the most dangerous natural disasters humans have experienced.

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They often come with little warning and can cause serious damage. Buildings collapse, roads cave in, and landslides tumble downhill during an earthquake. People may be trapped in their homes or under debris, and the cost of rebuilding after such an event can be tremendous.An earthquake is not one isolated event. However, much of the ground shaking that comes from an earthquake, which is the shaking of the ground when rock below Earth’s surface breaks, comes in one large burst of energy. This is similar to a spring recoiling – when you push the spring together on itself it builds up energy; when you let go, the energy explodes outward in all directions from the center point.

The same thing happens underground when rock is stressed along plate boundaries. Rocks may be compressed when they’re pushed together, experience tension if they’re pulled apart, or generate friction if they’re rubbed along each other. If you have a bad day, and the stresses just keep building up all day long, you’re likely going to snap at some point. The stress becomes too much to bear and you explode when you reach a breaking point. Rock underneath Earth’s surface does the same thing along faults. These are fracture lines along broken rock that move. Movement along faults may not cause an earthquake, but if the movement builds up enough energy the stress causes the rocks to reach their breaking points and release that energy as an earthquake.

Foreshocks and Aftershocks

Because faults are not straight, smooth boundaries, ground shaking often occurs both before and after the main quake. Foreshocks are the energy release and ground shaking before an earthquake and aftershocks are the energy release and ground shaking after an earthquake. Foreshocks are before, aftershocks are after – makes sense!Foreshocks are less likely to do damage than aftershocks because they’re smaller in magnitude.

You might even think that they could be used to predict earthquakes, since we can measure seismic activity, the movement of ground, on a machine called a seismograph. Unfortunately, foreshocks are usually too small and too close to the time of the main quake to help us know that the main quake is coming.Aftershocks are sometimes just as hazardous as the main quake itself. In fact, aftershocks may be so strong that they’re stronger than the main quake. When this happens the aftershock will be renamed as the main quake, and the main quake will be considered a foreshock. While foreshocks occur around the same time of the main quake, aftershocks may not occur until days or weeks later!Aftershocks are fairly unpredictable. The main quake doesn’t tell us much about an aftershock.

We don’t know if or when it will come, where it will occur (since it doesn’t always occur in the same place as the main quake), or how strong it will be. Aftershocks may continue to do damage to an area that has been hit by the main quake because structures like buildings and roads are already rattled from the previous shaking event.

Lesson Summary

Earthquakes are events where the ground shakes from rock breaking below Earth’s surface. This often occurs along fault lines, which are the fracture lines along broken rocks that move. When these rocks push on each other, pull apart from each other, or rub together, we get seismic activity, which is the movement of ground.

Like a spring recoiling after being compressed and released, the energy that builds up along fault lines explodes outward once it reaches a breaking point. The stress becomes too much for the rock to bear, and it releases that energy as an earthquake. Because fault lines are not smooth, even planes, ground shaking can occur both before and after the main quake. When ground shaking occurs before the main event of an earthquake, we call these events foreshocks.

When ground shaking occurs after the main quake, we call these events aftershocks. Foreshocks are before, aftershocks are after.While foreshocks do occur before the main quake, they’re usually too small in magnitude and too close to the time of the main event to help us with prediction.

Aftershocks, however, are often so strong and so long after the main quake that they may be considered the main earthquake and the main quake may be re-categorized as a foreshock. Aftershocks can be quite dangerous because not only are they unpredictable, happening days or weeks later, but structures like roads and buildings are already damaged from the main quake and can’t handle any more shaking from the ground below.

Learning Outcomes

Once the lesson is complete, you should be able to:

  • Identify the causes of seismic activity
  • Distinguish between foreshocks and aftershocks
  • Recognize the damage done by earthquakes and its aftershocks

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