Imagine this: You're sitting watching television in your living room when you hear a dull roar. The room starts to shake, pictures fall off the wall, and for a few seconds you feel off balance. Suddenly, all is quiet. Was it an explosion? Was it an asteroid? A television news bulletin lets you know that you have just experienced an earthquake.
An earthquake happens when two parts of the earth's thick crust slide past, away from, or into each other. The earth's surface is made up of many large slabs of crust called plates, which glide on semi-fluid rock below them. Heat from deep within the earth creates the pressure to move these plates around. As the earth's plates split up and come together, they leave millions of scars called faults. Many of these old faults never move, but once in a while stress builds up, causing a fault to rupture and an earthquake to occur.
For residents of California, earthquakes are fairly common. But could it happen anywhere? The good news is that earthquakes tend to occur in some places more than others. The bad news is that scientists do not know how to predict them.
Yet, while earthquakes strike without warning, scientists can use information about previous earthquakes and geological data to determine the probabilities of future earthquakes. These probability models allow individuals, businesses, and governments to determine how to deal with the risk of a potential earthquake. For example, scientists estimate that over the next 30 years, the probability of a major earthquake occurring in southern California is 60%.
How do they know? They look at the buildup of stress in the fault found there, as well as the frequency of earthquakes.
Stress buildup takes place when fault movement increases the stress in the rocks, resulting in an earthquake. By measuring how much the fault moves each year, and the total amount of stress build-up, scientists can determine the amount of time between major quakes on a particular fault.
In addition, scientists can estimate the probability of an earthquake by studying past earthquakes, including their scale, the location of their epicenter along the fault, and the frequency with which they happen.
There is a never-ending battle between science and nature. Despite scientific progress, in the battle between scientists and earthquakes, earthquakes appear to be winning.