By Robert Hazen, Ph.D., George Mason University
The Earth is a layered planet. Through the process of differentiation, its core, mantle, and crust were formed. If you look at the Earth’s surface, there are many signs of change. An earthquake is a sudden shaking of the ground, and it is one of the most violent and destructive events that can occur on the Earth’s surface.
The Cause of an Earthquake
An earthquake is caused by a sudden crack or break in rocks that then slide past each other. When this rock suddenly shears or breaks, then you have a violent motion of the ground. It sends, basically, a sound wave, a shockwave through the ground. When this occurs, the ground can actually move up and down like a wave, if it’s a particularly violent earthquake.
Of course, most earthquakes are undetectable, but the most severe earthquakes can be absolutely devastating. There was an earthquake in 1976 in Tangshan, China, that killed 250,000 people.
An earthquake results from a gradual buildup of stress in two blocks of rock that are in contact with each other. Stress builds up because rock surfaces are irregular; they don’t just smoothly slip by each other, but they’re locked into place. As the stress builds up, and builds up, and builds up some more, suddenly you have a release of energy, as the two rocks slip by each other.
When they slide by each other—sometimes by as much as several meters; you can have a slip that’s, say, 10 feet or more, in some extreme cases—that releases huge amounts of energy.
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Are Earthquakes Predictable?
The energy that’s released during an earthquake is quite analogous to what happens in many everyday situations; for example, when you tightly stretch a rubber band, and it snaps, or you blow up a balloon, and it bursts. If you take a pencil and put it between your fingers, you can apply stress to it. Now, you know at some point it’s going to break—the question is, when? It bends farther and farther; you hear little pops, and then, suddenly, it breaks.
One of the problems with earthquakes is how do you predict when it’s going to happen? That sudden release of energy causes the ground to oscillate; it causes things to shake; it causes tremendous stresses on buildings; yet nobody knows when exactly it’s going to happen.
Sometimes, as in the case of the pencil, you might hear a little premonitory cracking, little tremors that occur before the earthquake. That’s one way of perhaps leading up to saying, well, sometime in the next week, or month, or year, you might have a big earthquake; but to say the exact day or the hour is almost impossible.
Learn more about plate tectonics.
In characterizing earthquakes, one of the important things is to know the severity of the earthquake, and there are a couple of different scales that are used to describe how severe an earthquake phenomenon has been.
One of these is the Richter scale; this was devised by the California seismologist Charles Richter in 1935. Richter’s scale records the amount of ground movement in factors of 10; a magnitude 2 earthquake causes 10 times more ground movement than a magnitude 1.
If you have a magnitude 5 earthquake, there is 100 times more ground movement than a magnitude 3. What’s really scary about this scale is that the amount of ground movement, in factors of 10, corresponds to energy changes in factors of about 33; so each increase of 1 in the Richter scale is an increase of 33 times the amount of energy.
So, it’s a very large scale, and the most intense and destructive earthquakes have very, very large energy contents indeed.
Mercalli Intensity Scale
Another scale that’s used is called the Mercalli Intensity Scale, and it records the destructiveness of an earthquake. You measure an earthquake not by how much the ground moves but by how many buildings fall down, in effect. It’s a scale from I to XII; a level I Mercalli earthquake is not felt by people at all, and you’d need a very sensitive seismometer to detect it.
A level XII, on the other hand, completely destroys every human structure in the area. The San Francisco earthquake in 1906 is estimated to have been a Mercalli XI earthquake.
Indeed, the big concern in regions near fault zones is not if an earthquake is going to occur, but when the next one is going to occur because big earthquakes are just a matter of life; they occur over and over again, over the centuries.
Learn more about geochemical cycles.
How Earthquakes Cause Changes Over Time
Earthquakes can cause large-scale changes over time. Each time an earthquake occurs, two blocks of rock move with respect to each other. One block may move slightly to the north, one block slightly to the south, with respect to each other.
Each earthquake causes only a few feet movement, maximum; but if you continue this process year after year after year, decade after decade, integrating over thousands and then millions of years, it can cause very large changes.
Small changes, integrated over long periods of time, end up causing large changes on the surface of the Earth. All kinds of small, incremental changes in geology, integrated over long periods of time, lead to big changes.
Common Questions about Causes and Long-term Effects of Earthquakes
An earthquake is caused by a sudden crack or break in rocks that then slide past each other. When this rock suddenly shears, or breaks, then there is a violent motion of the ground. It sends a sound wave, a shockwave, through the ground. When this occurs, the ground can actually move up and down like a wave, if it’s a particularly violent earthquake.
The Richter scale was invented by the California seismologist Charles Richter in 1935. Richter’s scale records the amount of ground movement in factors of 10.
Mercalli Intensity Scale records the destructiveness of an earthquake. It measures an earthquake by how many buildings fall down, in effect. It’s a scale from I to XII; a level I Mercalli earthquake is not felt by people at all, while a level XII one completely destroys every human structure in the area.