How Earthquakes Work
 Photo courtesy USGS A section of Interstate 880 in Oakland, California, damaged by the magnitude 7.1 earthquake that shook the San Francisco area in 1989. See more earthquake pictures. |
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. In this article, we'll find out what causes earthquakes, and we'll also find out why they can have such a devastating effect on us.
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)
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!
 Photo courtesy FEMA Residential damage caused by the 1994 earthquake in Northridge, California. |
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.
 Photo courtesy NGDC Residential damage in Prince William Sound, Alaska, due to liquefaction caused by a 1964 9.2-magnitude earthquake. |
In the next section, we'll examine the powerful forces that cause this intense trembling and find out why earthquakes occur much more often in certain regions.
Sliding Plates
 Photo courtesy USGS One of the best known faults is the San Andreas fault in California. The fault, 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. |
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.
Click here for a great plate-boundary diagram.
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.
In the next section, we'll look at some different types of faults and see how their movement creates earthquakes.
Faults
Scientists identify four types of faults, characterized by the position of the fault plane, the break in the rock and the movement of the two rock blocks:- In a normal fault (see animation below), the fault plane is nearly vertical. The hanging wall, the block of rock positioned above the plane, pushes down across the footwall, which is the block of rock below the plane. The footwall, in turn, pushes up against the hanging wall. These faults occur where the crust is being pulled apart, due to the pull of a divergent plate boundary.
Normal fault
- The fault plane in a reverse fault is also nearly vertical, but the hanging wall pushes up and the footwall pushes down. This sort of fault forms where a plate is being compressed.
- A thrust fault moves the same way as a reverse fault, but the fault line is nearly horizontal. In these faults, which are also caused by compression, the rock of the hanging wall is actually pushed up on top of the footwall. This is the sort of fault that occurs in a converging plate boundary.
Reverse fault
- In a strike-slip fault, the blocks of rock move in opposite horizontal directions. These faults form when the crust pieces are sliding against each other, as in a transform plate boundary
Strike-slip fault
In all of these types of faults, the different blocks of rock push very tightly together, creating a good deal of friction as they move. If this friction level is high enough, the two blocks become locked -- the friction keeps them from sliding against each other. When this happens, the forces in the plates continue to push the rock, increasing the pressure applied at the fault.
If the pressure increases to a high enough level, then it will overcome the force of the friction, and the blocks will suddenly snap forward. To put it another way, as the tectonic forces push on the "locked" blocks, potential energy builds. When the plates are finally moved, this built-up energy becomes kinetic. Some fault shifts create visible changes at the earth's surface, but other shifts occur in rock well under the surface, and so don't create a surface rupture.
 Photo courtesy USGS Crop rows offset by a lateral strike slip fault shifting in the 1976 earthquake that shook El Progresso, Guatemala. |
The initial break that creates a fault, along with these sudden, intense shifts along already formed faults, are the main sources of earthquakes. Most earthquakes occur around plate boundaries, because this is where the strain from the plate movements is felt most intensely, creating fault zones, groups of interconnected faults. In a fault zone, the release of kinetic energy at one fault may increase the stress -- the potential energy -- in a nearby fault, leading to other earthquakes. This is one of the reasons that several earthquakes may occur in an area in a short period of time.
 Photo courtesy USGS Railroad tracks shifted by the 1976 Guatemala earthquake |
Every now and then, earthquakes do occur in the middle of plates. In fact, one of the most powerful series of earthquakes ever recorded in the United States occurred in the middle of the North American continental plate. These earthquakes, which shook several states in 1811 and 1812, originated in Missouri. In the 1970s, scientists found the likely source of this earthquake: a 600-million-year-old fault zone buried under many layers of rock.
The vibrations of one earthquake in this series were so powerful that they actually rang church bells as far away as Boston! In the next section, we'll examine earthquake vibrations and see how they travel through the ground.
Making Waves
When a sudden break or shift occurs in the earth's crust, the energy radiates out as seismic waves, just as the energy from a disturbance in a body of water radiates out in wave form. In every earthquake, there are several different types of seismic waves.
 Photo courtesy USGS Structural damage caused by vibrations from the 1964 Alaska earthquake |
Body waves move through the inner part of the earth, while surface waves travel over the surface of the earth. Surface waves -- sometimes called long waves, or simply L waves -- are responsible for most of the damage associated with earthquakes, because they cause the most intense vibrations. Surface waves stem from body waves that reach the surface.
There are two main types of body waves.
- Primary waves, also called P waves or compressional waves, travel about 1 to 5 miles per second (1.6 to 8 kps), depending on the material they're moving through. This speed is greater than the speed of other waves, so P waves arrive first at any surface location. They can travel through solid, liquid and gas, and so will pass completely through the body of the earth. As they travel through rock, the waves move tiny rock particles back and forth -- pushing them apart and then back together -- in line with the direction the wave is traveling. These waves typically arrive at the surface as an abrupt thud.
- Secondary waves, also called S waves or shear waves, lag a little behind the P waves. As these waves move, they displace rock particles outward, pushing them perpendicular to the path of the waves. This results in the first period of rolling associated with earthquakes. Unlike P waves, S waves don't move straight through the earth. They only travel through solid material, and so are stopped at the liquid layer in the earth's core.
Click the play button to start the earthquake.
When P and S waves reach the earth's surface,
they form L waves. The most intense L waves
radiate out from the epicenter.
Both sorts of body waves do travel around the earth, however, and can be detected on the opposite side of the planet from the point where the earthquake began. At any given moment, there are a number of very faint seismic waves moving all around the planet.
Surface waves are something like the waves in a body of water -- they move the surface of the earth up and down. This generally causes the worst damage because the wave motion rocks the foundations of manmade structures. L waves are the slowest moving of all waves, so the most intense shaking usually comes at the end of an earthquake.
In the next section, we'll see how scientists can calculate the origin of an earthquake by detecting these different waves.
