Description

An earthquake is one of the most terrifying phenomena that nature can dish up. We generally think of the ground we stand on as "rock-solid" and completely stable. An earthquake can shatter that perception instantly, and often with extreme violence.

Up until relatively recently, scientists only had unsubstantiated guesses as to what actually caused earthquakes. Even today there is still a certain amount of mystery surrounding them, but scientists have a much clearer understanding.

Interstate 880 in Oakland, California, damaged by 

7.1 quake that shook San Francisco area in 1989

Photo courtesy USGS

There has been enormous progress in the past century: Scientists have identified the forces that cause earthquakes, and developed technology that can tell us an earthquake's magnitude and origin. The next hurdle is to find a way of predicting earthquakes, so they don't catch people by surprise.

Basically, an earthquake is a vibration that travels through the earth's crust. Technically, a large truck that rumbles down the street is causing a mini-earthquake, if you feel your house shaking as it goes by, but we tend to think of earthquakes as events that affect a fairly large area, such as an entire city. 

All kinds of things can cause earthquakes:

• volcanic eruptions

• meteor impacts

• underground explosions (an underground nuclear test, for example)

• collapsing structures (such as a collapsing mine)

But the majority of naturally-occuring earthquakes are caused by movements of the earth's plates.

We only hear about earthquakes in the news every once in a while, but they are actually an everyday occurrence on our planet. According to the United States Geological Survey, more than three million earthquakes occur every year. That's about 8,000 a day, or one every 11 seconds!

Residential damage caused by the 1994 earthquake in Northridge, California.

Photo courtesy FEMA

The vast majority of these 3 million quakes are extremely weak. The law of probability also causes a good number of stronger quakes to happen in uninhabited places where no one feels them. It is the big quakes that occur in highly populated areas that get our attention.

Earthquakes have caused a great deal of property damage over the years, and they have claimed many lives. In the last hundred years alone, there have been more than 1.5 million earthquake-related fatalities. Usually, it's not the shaking ground itself that claims lives -- it's the associated destruction of manmade structures and the instigation of other natural disasters, such as tsunamis, avalanches and landslides.

Prince William Sound, Alaska, damage caused by a 1964 9.2 earthquake

Photo NGDC

SLIDING PLATES

The biggest scientific breakthrough in the history of seismology - the study of earthquakes - came in the middle of the 20th century, with the development of the theory of plate tectonics. Scientists proposed the idea of plate tectonics to explain a number of peculiar phenomenon on earth, such as the apparent movement of continents over time, the clustering of volcanic activity in certain areas and the presence of huge ridges at the bottom of the ocean.

The basic theory is that the surface layer of the earth - the lithosphere - is comprised of many plates that slide over the lubricating athenosphere layer. At the boundaries between these huge plates of soil and rock, three different things can happen:

• Plates can move apart - If two plates are moving apart from each other, hot, molten rock flows up from the layers of mantle below the lithosphere. This magma comes out on the surface (mostly at the bottom of the ocean), where it is called lava. As the lava cools, it hardens to form new lithosphere material, filling in the gap. This is called a divergent plate boundary.

   

• Plates can push together - If the two plates are moving toward each other, one plate typically pushes under the other one. This subducting plate sinks into the lower mantle layers, where it melts. At some boundaries where two plates meet, neither plate is in a position to subduct under the other, so they both push against each other to form mountains. The lines where plates push toward each other are called convergent plate boundaries.

   

• Plates slide against each other - At other boundaries, plates simply slide by each other -- one moves north and one moves south, for example. While the plates don't drift directly into each other at these transform boundaries, they are pushed tightly together. A great deal of tension builds at the boundary.

The San Andreas fault California, which marks the plate boundary between the Pacific oceanic plate and the North American continental plate, extends over 650 miles (1,050 km) of land  Photo courtesy USGS

Where these plates meet, you'll find faults -- breaks in the earth's crust where the blocks of rock on each side are moving in different directions. Earthquakes are much more common along fault lines than they are anywhere else on the planet.  Hence, along these fault lines we can expect to see most serious earthquakes.

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