HOW EARTHQUAKES WORK
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How do earthquakes work?

Earthquakes are one of those natural phenomena that can make people shudder while still leaving them intrigued as to how to they work and how a single shake can bring buildings, trees, and other structures to shambles. Earthquakes can cause such terror at the mere thought because unlike other natural disasters such as forest fires, tornadoes, and hurricanes, which all occur on land or above the land, earthquakes actually move, shake and tear apart the very ground that we are standing on. It’s no wonder why people are so concerned when thinking of moving to high-risk areas for earthquakes such as California in the United States. But how much do we know exactly about earthquakes and is there any chance of removing some of their frightening mystery?

Scientists and researchers have come a very long way in understanding earthquakes and what causes them. Certainly, like many other scientific wonders, we are much farther in understanding them today than we were 10 years ago. However, even though scientists have good guesses at how earthquakes work and how they can be prevented, the next biggest scientific leap in this area will be discovering a way to predict earthquakes so that when they do occur, they will have minimal impact on the way we live our lives. This is something that has yet to be discovered but based on what we already know about how earthquakes work, science may not be that far off!

What is an Earthquake?

An earthquake is technically, anything that causes a vibration to be sent through the earth’s crust. Consider a large truck driving down the street. If the truck simply drives by and you know it only by seeing it, no earthquake occurred. However, if the truck was very large and heavy and caused your house to move slightly as it passed, this is technically an earthquake because the vibration from the truck caused the earth’s crust to move away from the truck until it reached your house, and possibly beyond. Although very small, this is considered an earthquake.

Although the ‘earthquake’ that occurs when the truck passes your house would never appear in the front pages of the newspaper, there are several (much more accurately-termed earthquakes) that happen all over the world every day. The United States Geological Survey actually estimates that there are approximately 8,000 earthquakes a day that occur on earth, which means that there is one happening every 11 seconds and adds up to 3 million earthquakes a year! Most of these are so small and weak that they are not worthy of reporting or are so small that they are barely felt.

Many of the larger quakes also often go unreported because they happen in remote areas of the earth where there is no civilization. However, when a massive quake hits, especially when it’s near a highly-populated area such as the one that China saw in May 2008, it definitely makes headlines and most often it’s because those earthquakes also cause a lot of devastation. Often much of the destruction that is reported isn’t even from the actual earthquake itself but come from the falling buildings and other structures as well as the tsunamis, hurricanes, and other natural disasters that are often a result of the initial earthquake. So what is this disastrous movement of the earth and just how do earthquakes work?

Earthquakes and Plate Tectonics

Many people know that earthquakes are caused by the earth’s tectonic plates shifting. However, that is the extent of most people’s common knowledge on the subject. While these plates are the main cause of earthquakes, there are also other less-common causes such as underground nuclear explosions and volcanic activity. However, it is the movement of the tectonic plates that is the main reason for most earthquakes and the idea of these plates has been around since the beginning of the 20th century, when the idea of such things were used to explain things such as the shifting and movement of the continents on earth. Seismology, which is the study of earthquakes, focuses mainly on these plates, the different ways they move, and the faults they have that cause earthquakes.

Tectonic plates are plates comprised of rock and soil that form the surface of the earth. This layer of earth that is made up of tectonic plates is known as the lithosphere and it covers the athenosphere, which is the lubricating layer of the earth. These plates are an absolute essential part of the earth’s construction but the problem is not within the plates themselves. Rather, the problem that causes earthquakes lies at the boundaries of these plates. There are a few different things that can happen at the boundaries of the tectonic plates.

Plates generally tend to lie side-by-side but because the plates are in just about constant motion, they move and shift away from each other or towards each other. When plates move away from each other, there is a gap in between them. This gap will be filled in with hot, molten rock from the lithosphere beneath the plates. This rock, or magma, will ooze out between the gap and will pour out over the tectonic plates, or the surface of the earth, which usually happens at the bottom of the ocean. As the magma begins to cool, it will also harden and form a new part of the lithosphere layer to fill in the gap. When this happens it’s referred to as a divergent plate boundary.

Just as tectonic plates move away from each other, they can also move towards each other and push up against each other. When this happens, most often one plate will push underneath, or subduct, the other. When the lower plate gets pushed underneath, it’s pushed into the molten layer and it then melts. The subducting of one of the plates must happen naturally and if it can’t the two plates will instead push up against each other. When this happens, instead of the plates being pushed down and melting, they will push up and remain masses of rock and soil. This is how mountains are formed. The lines where the two plates have met are known as convergent plate boundaries.

While plates are always moving, they don’t always gravitate towards or away from each other. Sometimes they are simply on their own path and they slide up against each other. While this doesn’t necessarily melt or shoot the plate up, it does create an enormous of pressure buildup where the boundary is and these are called transform boundaries. The most important thing to remember about where the boundaries lay on tectonic plates is that it is along these boundaries where the plates’ faults will lie and it is these faults that are responsible for most of the earthquakes that occur on earth.

Tectonic Plate Faults

There are 4 main different types of faults that can occur along the boundaries of the tectonic plates. These different types of faults are categorized by how the fault plane lies, which is the break in the rock and the movement of the rocks.

A normal fault, which may occur when two plates move away from each other, will show a fault plane that lies vertically. The hanging wall, which is the piece of plate that lays just above the fault plane, presses down and across the footwall, which is the plate that lies below the fault plane. The footwall then presses up against the footwall. This can also happen in reverse and is so appropriately named, a reverse fault. These types of faults also have vertical fault planes but the difference is that the hanging wall will push up while the footwall pushes down. This will occur when the plates become compressed and being pulled against each other instead of away from each other.

A thrust fault works in much the same way as a reverse fault does however the fault plane is nearly horizontal. These are the types of faults that will occur in converging plate boundaries and the hanging wall will actually be pushed right up on top of the footwall. These types of faults also occur when the plates become compressed. The plates also move horizontally in strike-slip faults, when the plates move in opposite directions but are still moving and pressing up against each other.

Whichever type of fault you’re referring to when you’re talking about the main causes of earthquakes, they all have a couple of things in common. The first is that it is often the initial reason for the fault, or the break, that causes an earthquake. The other thing that all faults have in common is that when the faults and the plates are pushed up against each other, this in itself is not enough to cause an earthquake. However, when plates are pushed up against each other, there is a significant amount of friction between them. While this friction is always present, there are times when it builds up and creates so much pressure that it locks the plates together, making them unable to move.

Once so much pressure has been built up from the friction, one of the plates will be thrust forward on top of the other. This is because the energy that is being built up by the friction is potential energy and when it is released by the forward movement of the plate, it is released as kinetic energy. If there is enough energy to create a large enough force, the effects will be able to be seen on land. However, these plate shifts are often so small and subtle that no movement is ever visible.

Most earthquakes occur around the boundaries of the plates, because this is where the movement of the plates and the fault lines are felt most intensely. Also along the boundaries of the plates is where the fault zones can be found, which are multiple faults that are all connected to each other. If one of these faults releases its pressure and this results in an earthquake, it could set off the surrounding faults around it to also snap and release their energy. This is the reason why there are often many earthquakes in one area within a short period of time.

Seismic Waves

Once the plates have shifted, moved, and broken, energy will be released in the form of seismic waves. These waves are very similar to the waves that can be found in a body of water when it becomes disturbed. Every earthquake will radiate out some form of energy in different kinds of seismic waves. The two different types of seismic waves are body waves and surface waves. Body waves occur within the inner part of the earth while surface waves are simply body waves that have reached the surface of the earth. Surface waves are also called long waves, or L waves, and they are responsible for most of the damage that is caused by earthquakes because they have the most intense vibrations.

There are two main different types of body waves and those are Primary waves, also known as P waves or compressional waves, and the second type is secondary waves, also called S waves or shear waves. Primary waves move at about 1 to 5 miles per second, making them the fastest of the body waves, and they can also move through solid, liquid, or gas, making them even more powerful and intense. As P waves move through the rock, they move the tiny particles of rock in whichever direction the wave is moving and at a very fast rate. Because of their speed, they are usually the first waves that are felt at the location of the earthquake and because of their power, they are often felt with a great thud.

Secondary waves travel a bit more slowly than P waves and they also can only move through solids so they will come to a stop once they reach the liquid center of the earth’s core. Because of this they don’t move straight through the earth and as the waves move, they push the pieces of the rock perpendicular to the path of the waves. It is this movement that causes the rolling effect that is often felt just before an earthquake hits. Both P waves and S waves are body waves and can be detected from the opposite end of the earth from which they are happening. Body waves are almost constantly happening somewhere on the planet.

Surface waves are also quite common (as we mentioned, an earthquake happens every 11 seconds on earth!) but they are also generally much more intense than body waves and cause most of the damage that earthquakes leave behind because they actually move the surface of the earth and move the structures on earth. This is why the most powerful effects of an earthquake are often felt at the end of it.

 

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