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How do you measure seismic waves?
The instruments that measure seismic waves are called seismometers, and the records they make are called seismograms. By studying seismograms, seismologists can determine where an earthquake occurred. By studying seismograms over many years, they can determine where earthquakes are likely to occur.

Over Earth's long history, forces deep within the planet have moved huge sections of the earth's crust from one place to another. These sections are called tectonic plates, and the process by which they are moved is called plate tectonics. Earthquakes generally happen along the boundaries of the tectonic plates in the earth's crust that are moving apart, that are moving past one another horizontally, or that are moving toward one another in a process that forces one plate deep into the earth.

There are many different types of seismometers, but they all measure the distance that the ground moves — either vertically or horizontally — as a result of some seismic motion. Seismic motion can be measured in two basic ways. One way is with a weight suspended from a spring — this measures vertical motion by noting changes in the distance between the bottom of the weight and the base of the frame before the ground shakes and while the ground is shaking. The second way measures horizontal motion with a weight on a pendulum that can swing sideways. This marks the changes between the side of the weight and the sides of the frame before and while the ground is shaking.

Seismologists often use a weight on a spring to measure vertical motions. Plus they use swinging pendulums set up East to West and North to South to measure horizontal motions. All three systems are needed because the ground can move in any direction during an earthquake.

Seismometers must be very sensitive because the seismic motions from distant earthquakes are often very small. A medium-sized earthquake in Alaska, for example, will produce a ground motion in New York of less than one millionth of an inch when the P-waves arrive. If sensitive seismometers are placed in a large city or near an ocean beach, traffic or ocean waves produce vibrations that will interfere with the detection of distant earthquakes. In areas that are seismically active, strong-motion seismometers are often used. These instruments can measure very intense seismic motions that would cause sensitive seismometers to go off the scale.

Three of the ten largest earthquakes in the 20th century were in Alaska.

How do you locate an earthquake?
Thousands of seismometers are now operating at stations all around the world. When an earthquake occurs somewhere on Earth, many of these will record the motions of the sound waves, shear waves, and surface waves that are produced. By measuring the size of these waves and the times at which they reach each seismic station, seismologists can determine where the earthquake occurred and how large it was.

One way seismologists can locate the source of the waves is to interpret all the seismic motions recorded at each station and to backtrack along the path the motions traveled to find the common point from which all the seismic waves originated.

Another way to locate an earthquake is to study more than one type of seismic wave, taking into account the fact that different waves travel at different speeds. Both P-waves and S-waves, for example, start out at the same time like runners in a race. P-waves travel faster, however, and the farther they travel, the greater the time difference between when they arrive at a seismometer and when the S-waves arrive. By measuring this time difference, seismologists can tell how far away the earthquake is from each station. They can then locate the source of the earthquake by studying the different times at which the waves reached different seismic stations.

The place on the earth's surface where an earthquake occurs is called the epicenter. This will usually be the place where the shaking is strongest. The place where the seismic waves actually originate is called the hypocenter. Lying below the epicenter, the hypocenter is the place where rock actually breaks along a buried fault. It is quite rare for an earthquake to occur on a fault that breaks through to the surface of the earth. However, such surface faulting does sometimes occur. Most earthquakes occur in the earth's crust only a few miles below the surface. But about 10 percent of all earthquakes are deep, occurring more than 60 miles below the surface. Some hypocenters are as deep as 450 miles.

Florida and North Dakota have the smallest number of earthquakes in the US.

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