Background for Teachers

Since many earthquakes are recorded around the globe, seismologists need to agree on a single way to identify times without confusion about different time zones ("7pm" in California is not the same as "7pm" in China). That's why scientists usually agree to report the time as if they all lived in the same time zone – that of Greenwich, England. In REV, the time at which an earthquake occurs is listed in GMT (Greenwich Mean Time), sometimes called Greenwich Meridian Time because it is measured from the Greenwich Meridian Line at the Royal Observatory in Greenwich, England (historically, the observatory housed a very accurate clock). You can determine how GMT relates to your local time by consulting the entry for GMT.

The similarity of the vocabulary terms seismometer, seismograph and seismogram can produce some confusion so it is recommended that you ensure you and your students become comfortable with proper usage as you move through the tutorial.

We study earthquakes by examining how much and in which direction the ground moves and shakes. A seismometer is the instrument that converts ground motion into a signal that can be recorded by a computer or other recording device. A seismograph is the word used to describe the seismometer plus its recording system together as a single unit, though today the terms seismometer and seismograph are often used interchangeably to describe seismic monitoring equipment. (See more information on how a seismograph works) Note: The web site has a good illustration of how seismographs work, but the label "modern seismograph" is slightly misleading – that method was modern in the early 1980's. A modern seismometer is fully digital.

The data output by a seismograph is called a seismogram. The lines and wiggles that you see when looking at a seismogram show the amount the ground shook at different points in time. For more information on how to read a seismogram consult: How to Read a Seismogram or REV Glossary of Terms.

There are several types of seismic waves. The different types of waves leave the fault surface at the same time during an earthquake, but they arrive at seismic monitoring stations at different times because they travel at different speeds. Surface waves travel near the Earth's surface, whereas body waves move through the interior of the Earth. Each type of wave shakes the ground in different ways. A P (Primary) wave, or compressional wave, is a body wave that shakes the ground back and forth in the same and opposite direction as the one in which the wave is moving. P waves travel faster than S waves and are generally felt first. They usually cause very little damage.

An S (Secondary) wave, or shear wave, is a body wave that shakes the ground back and forth perpendicular to the direction in which the wave is moving. S waves are slower and much more destructive than P waves (because they are usually higher amplitude and the direction of motion is harder for buildings to withstand). (See more information on how seismic waves move)

Any disturbance or force acting on the Earth's surface will generate tiny seismic waves, some of which are large enough for seismometers to record. Referred to as “noise” it can originate from traffic (like big trucks shaking your house as they drive by), construction, and other human activity. Nature also produces a lot of noise – ocean waves moving across shallow seas or crashing into the shoreline are believed to cause most of the constant noise seen on seismometers around the world. Coastal waves can be recorded by seismometers as much as 100 miles inland, and larger disturbances caused by storms in the open ocean generate seismic waves that travel across whole continents (vibrations from the effects of hurricanes in the Gulf of Mexico have been shown to be recorded as far away as Michigan and California). Some noise comes from the entire Earth continuing to ring like a bell for weeks following a large earthquake. Other non-earthquake events that can be recorded by the seismographs include landslides, avalanches, volcanic eruptions, and explosions – but many of these are much rarer than the day-to-day noise.

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