Science of Earthquakes

What is an earthquake?

An earthquake is what happens when two blocks of the earth suddenly slip past each other. The surface slide is called the fault plane or missing. The location below the surface of the earth where the earthquake starts is called the hypocenter, and the location just above it on the surface of the Earth is called the epicenter.

Sometimes a preliminary earthquake tremors. These are smaller earthquakes that occur in the same place as the largest earthquake to follow. Scientists can not say that an earthquake is a foreshock until the larger earthquake. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that followed. These are smaller earthquakes that occur later in the same place as the main quake. Depending on the size of the main shock, aftershocks may continue for weeks, months and even years after the main shock.

What causes earthquakes and where do they happen?

The earth has four main layers: the inner core, outer core, mantle and crust. (Figure 2) The crust and upper mantle form a thin skin on the surface of our planet. However, this skin is not all in one piece is - which is composed of many pieces like a puzzle that covers the surface of the earth. (Figure 3) Not only that, but these pieces of the puzzle are still moving slowly, sliding over each other and collide. We call these puzzle pieces tectonic plates, and the edges of the plates are called plate boundaries. The plate boundaries are composed of many defects, and most of earthquakes worldwide occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate continues moving. Finally, when the plate has moved far enough, the UNSTICK edges in one of the faults and there is an earthquake.

Why does the earth shake when there is an earthquake?

While the sides of the faults are stuck together, and the rest of the block is moving, the energy normally blocks slide over one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and is off, all that stored energy is released. Energy radiates outward from the fault in all directions in the form of seismic waves like ripples in a pond. Seismic waves shake the earth as they move through it, and when the waves reach the surface of the earth, shaking the ground and anything on it, like our homes and us! (See P & S wave box).

How are earthquakes recorded?

Earthquakes are recorded by instruments called seismographs. The recording you make is called a seismogram. (Figure 4) The seismograph has a base set firmly on the ground, and a heavy weight hanging free. When an earthquake causes the ground to shake, based seismograph also shake, but the weight hanging does not. Instead, the spring or rope hanging from absorbs the movement. The difference in position between the part of the seismograph shaking and stationary part is what is recorded.

How do scientists measure the size of earthquakes?

The size of an earthquake depends on the size of the ball and the amount of slip on the fault, but that's not something that scientists can be simply measured with a tape measure from defects are many kilometers beneath the surface of the earth. So how do you measure an earthquake? They use recordings made on seismograms seismographs on the surface of the earth to determine how big was the earthquake (Figure 5). A short wavy line means not much wiggle a small earthquake, and a wavy line much longer wagging means a great earthquake. The length of the undulation depends on the size of the damage, and the size of the maneuver depends on the amount of slip.

The size of the earthquake is called its magnitude. It is a magnitude for each earthquake. Scientists also talk about the intensity of the movement of an earthquake, and this varies depending on where you are in the earthquake.

How can scientists tell where the earthquake happened?

Seismograms are useful for earthquake location also, and be able to see the P wave and S wave is important. He has learned from P & S waves at each ground motion in different ways as they travel through it. P waves are also faster than S waves, and this is what allows us to tell where it was an earthquake. To understand how this works, let's compare the P and S waves and lightning. Light travels faster than sound, so that during a thunderstorm I first see lightning and then hear thunder. If you are close to the lightning, the thunder will boom just after the lightning, but if you are away from the lightning may have several seconds before hearing thunder. The farther you are from the storm, the longer it will take between lightning and thunder.

P waves are like lightning, and S waves are like thunder. P waves travel faster and shake the ground where you are first. Then the S waves are and shake the ground as well. If you are near the earthquake, the P and S wave is one after the other, but if you are away, there will be more time between the two. Looking at the amount of time between the P and S wave on a seismogram recorded on a seismograph, scientists can know how far the earthquake was there. However, they may not know in which direction the earthquake seismograph was, but how far it was. If you draw a circle on a map around the station where the radius of the circle is the determined distance to the earthquake, they know that the earthquake is located somewhere in the circle. But where?

The scientists then used a method called triangulation to determine exactly where the earthquake was (Figure 6). It's called triangulation because a triangle has three sides, and takes three seismographs to locate an earthquake. If you draw a circle on a map in three different seismometers where the radius of each is the distance from the earthquake that station, the intersection of the three circles is the epicenter.

Can scientists predict earthquakes?

No, and it is unlikely to ever be able to predict them. Scientists have tried many different ways of predicting earthquakes, but none have succeeded. In any particular fault, scientists know there will be another earthquake at some point in the future, but have no way of knowing when it will happen.