Earthquakes are well known for the damage and destruction they leave behind. But what is an earthquake? In this video lesson you will learn about what causes earthquakes and the various components involved in these powerful natural disasters.
What Is An Earthquake?
You know how when you have a really bad day and it seems like everything is going wrong? It’s just one thing after another, and you get so frustrated that by the end of the day you are ready to explode from frustration.
In fact, you may get so frustrated that you have a sudden outburst or release of all that built-up energy. And those unlucky enough to be around you when that happens should watch out!Earth also builds up and releases energy in this way. When this occurs, we have an earthquake, which is the shaking of the ground when rock below Earth’s surface breaks. Energy builds up underground and once it builds up enough it just can’t hold back any longer, and we get the same explosive release of energy as a person might at the end of a long, bad day.When the energy is released it radiates outward in all directions.
As the energy travels toward Earth’s surface from underground it shakes the ground, sometimes so much that it can cause damage above the ground. A seismograph is a machine that records ground movement from earthquakes. The information recorded tells us about the strength and speed of the energy traveling from the breaking point underground.
What Causes Earthquakes?
We now know what an earthquake is, but what exactly causes the rock underground to break? This is the result of stress along plate boundaries on Earth. The plates are dynamic, so they are always moving.
Sometimes they move enough that they push into each other or pull apart. Compressional stress occurs when rocks are pushed together – they’re pressed into one another. Tensional stress occurs when rocks are pulled apart – they’re being stretched farther than they would be otherwise.
Shear stress is when rocks slide past each other in opposite directions – it’s like rubbing your hands together; they don’t push or pull, but there’s a lot of friction there!When the stress gets to be too much, like all of the events building up on a bad day, the rock breaks and the ground begins to shake. If the rock splits into separate pieces, we get a fault, which is the line of fracture along the split rock. One of the most famous faults on Earth is the San Andreas Fault in California. This fault runs almost the entire length of the state of California, and is well known for causing frequent earthquakes in this area.
The Components of an Earthquake
Just like you may have some release of your tension at different times during your bad day, not all of the breaking of rock and energy release happens at once during an earthquake. There may be foreshocks and aftershocks, which are the energy released before and after the main quake.The point underground where the actual breaking of the rock occurs is called the focus.
It might help to remember this by thinking of it as the focal point of the earthquake. This is where the main event occurs underground. The point directly above the focus on the surface of Earth is called the epicenter.
This is where the ground shaking is usually the strongest. From this point on the surface, the waves of energy from below ground begin to travel outward, so you can think of this as the central point of shaking above ground. Because the shaking is strongest here, this is also where the most damage usually occurs.You may be surprised to learn that earthquakes don’t actually kill people; it’s all of the damage that occurs because of the shaking ground that causes people harm. Buildings crumble and fall, landslides and avalanches may be triggered, and roads and bridges can collapse. Falling objects are also quite likely to harm people during an earthquake, as items are shaken off of shelves, walls, and buildings.
Where Do Earthquakes Occur?
Since earthquakes are the result of plate boundary interactions, it’s no surprise that most earthquakes occur in just a few specific areas on Earth.
In fact, these areas are so famous for earthquakes and volcanoes, which also tend to occur where plates meet, it’s been nicknamed ‘The Ring of Fire.’ There is a major plate boundary along California, which is where the San Andreas Fault is found. Along the west coast of South America we also find another major plate boundary, and quakes are quite common in this region as well. The Pacific Rim is another area with lots of shaking activity, so places like Indonesia are also very familiar with this dangerous natural disaster.Many of the plate boundaries are underwater in oceans, and when earthquakes occur in these locations they may lead to tsunamis. These are huge water waves that occur with underwater earthquakes. Tsunamis are very dangerous when they reach coast lines because the waves have built up an incredible amount of energy as they traveled along.
One of the most deadly tsunamis occurred in December of 2004 from an earthquake in the Indian Ocean. More than 230,000 people were killed during this event, which is considered one of the deadliest natural disasters in history.
An earthquake is like Earth having a really bad day. Energy builds up underground when plate boundaries push, pull, or rub against each other. After a while, the stress becomes too much to handle and the rock underground breaks.
This releases a massive amount of energy, similar to an ‘explosion’ you might have when you’re just too frustrated at the end of a bad day. The shaking of the ground that occurs when rock below Earth’s surface breaks is called an earthquake.There are different types of stress that cause earthquakes, depending on how the rocks are moving. If the rocks are being compressed by pressing into each other, this is called compressional stress. If there is tension from the rocks being pulled apart, this is called tensional stress.
When the rocks rub together we get shear stress, which is similar to rubbing your hands together – not a lot of damage, but certainly a lot of friction! When rocks break into two pieces, faults are created. These are the lines of breakage between the rocks, and where we often find heavy earthquake activity, like along the San Andreas Fault in California.The point underground where the earthquake begins is called the focus. This is the focal point where the rock actually breaks. Directly above the focus on the surface is the epicenter, which is the central point from where all of the ground shaking extends outward on the surface. Because this is the central point, it’s also where we usually see the most damage from an earthquake. Earthquakes don’t always release all of their energy at one time.
Foreshocks and aftershocks are energy that is released before and after the main quake.Earthquakes cause a lot of damage because they do some serious ground shaking. Buildings, roads, and bridges may collapse, landslides and floods may occur, and people can be killed by falling objects or structures.
Because earthquakes mostly occur along plate boundaries, we find the most shaking activity along specific areas in the world. The ‘Ring of Fire’ is nicknamed as such because more earthquakes and volcanoes occur here than anywhere else. Many plate boundaries in the Ring of Fire are found in the ocean. But just because the earthquakes happens far out at sea doesn’t mean it doesn’t affect those on land. In December 2004, several countries along the Pacific Rim were hit by a very powerful tsunami, which is huge water waves that come from underwater earthquakes. This tsunami is considered one of the most deadly natural disasters in history because of its massive death toll and land damage.
You may be prepared to do the following after viewing this video lesson:
- Know the conditions that could lead to earthquakes
- Offer details about stress on the plates of the Earth
- Detail the various shocks caused by earthquakes
- Recall the characteristics of the ‘Ring of Fire’
- Understand how tsunamis are caused