Earthquakes can be a scary thing. It is hard to feel in control of anything when the very ground beneath your feet is moving! Earthquakes can have both natural and unnatural causes, and they can sometimes be very deadly. Here is a quick guide to how they work so that you can understand them better.
What is an earthquake?
An earthquake is a sometimes violent shaking or vibration that travels up through the Earth's crust or surface. This crust is composed of many different pieces called tectonic plates. These plates move around constantly, sliding against each other and bumping into one another, usually without much of a problem. Sometimes, though, the rough edges of one plate get caught on another plate. While the rest of the plate keeps moving, that rough edge is stuck in place until suddenly it unsticks at what is called a fault line (a divide between two tectonic plates), and the shuddering vibration this causes can be felt sometimes for hundreds of miles, and can lead to the destruction of buildings and other structures.
What causes an earthquake?
Earthquakes are all about energy. When the plates get stuck to each other, as described above, the energy that the plates would usually use to keep sliding past each other is stored up. It builds and builds until finally the plate unsticks and all of that energy is released in seismic waves that act like ripples on a pond, spreading out in all directions and shaking the earth beneath our feet. The normal movement of tectonic plates is not the only thing that can cause an earthquake, however. They can also be caused by volcanic eruptions, meteors crashing into the Earth, the collapsing of underground mines, or even underground nuclear weapons testing.
Foreshocks, mainshocks, & aftershocks
If you have heard anything about earthquakes before, you have probably heard about aftershocks, which are vibrations that come after an earthquake, like an echo, and can still be very harmful. What you may not have heard of, though, are foreshocks, which are smaller earthquakes that occur in the same location as the main earthquake that is about to begin. Unfortunately, these cannot be used as a predictor for the main earthquake, because it is difficult to tell that they are foreshocks until the main earthquake (or, in scientific terms, the "mainshock,") happens. It is unclear how long foreshocks continue before the mainshock occurs, but aftershocks can continue for several weeks, or even years, after the main event has finished.
Can earthquakes be predicted?
Earthquakes cannot be predicted. Scientists and researchers can keep an eye on fault lines that seem to be more active, but they cannot estimate when a seismic event will occur. They can record the earthquakes with a seismograph, though, which consists of a base set in the ground, above which hangs a weight on a string or spring. When the earth moves, the string absorbs the movement while the base moves with the earth. The difference between the shaking parts of the device and the parts that remain motionless is recorded, and this gives scientists a record of how large or small the earthquake was.
How many earthquakes have there been?
It is difficult to say how many earthquakes there have been, as they are happening all the time. According to geologists, there are more than 900,000 earthquakes per year with a magnitude greater than 2.0 on the Earthquake Magnitude Scale. Earthquakes at this magnitude are usually not felt, but they can still be picked up by a seismograph. Earthquakes have been occurring for thousands of years (if not longer), with the most lethal one ever recorded taking place in 1556 in Shensi, China. This one had an 8.0 magnitude and caused the death of around 830,000 people as it leveled the city.