Terms and Information on Earthquakes

Earthquake Terminology Earthquake: Is the vibration of Earth produced by rapid release of energy.
Fault: A break in a rock mass along which movement has occured.
S-wave (Secondary Wave): Measuring the up and down movement of an earthquake. They are the larger waves on the seizmograph.
P-wave (Primary Wave): Measuring the side to side movement of an earthquake. They are the small and faster moving waves on the seizmograph.
Elastic rebound: The sudden release of stored strain in rocks that results in movement along a fault.
Richter Scale: A scale of earthquake magnitude based on the motion of a seismograph.
Seismograph: An instrument that records earthquake waves.
Seismology: Th study of earthquakes an seismic waves

Information and Aspects of Earthquakes Earthquakes are recorded by a seismographic network. At each seismic station in the network it measures the movement of the ground at that site. The ground movement could be the result of an earthquake or a passing truck. Earthquakes usually result from fault movement between rock layers that releases energy in the form of waves. The resulting ground vibration pushes an adjoining piece of ground, causing it to vibrate, and therefore the energy travels out from the earthquake in a wave. As the wave passes by a seismic station, that piece of ground vibrates and a signal is recorded. Earthquakes produce two types of waves, surface and body. The body waves are divided into primary and secondary waves. The P-waves are compressional waves that travel fast but are not as large as the S-waves, shear waves that are slower but larger. The S-wave does most of the damage. Knowing how fast the waves travel through the earth, seismologist calculate what time and what location of the earthquake would give the pattern of shaking that was recorded. They measure the time of the wave that arrives first the wave traveling from the hypocenter which is the first part of the fault to slip. This particular process use to take hours to calculate but today with the use of computers it only takes minutes.

What's a Drum Recorder?

Drum recorders draw a seismogram on a piece of paper that is wrapped around a slowly revolving drum. As the drum revolves, the pen shifts across it, making a big spiral or helical record that eventually fills the entire page. When the paper is removed and laid flat, the record appears to be many horizontal lines. These mechanical recorders, popular 30 years ago, have largely been replaced by computers. The computers digitize the data (typically at 100 samples per second) and then store it in digital form. The digital data can then be displayed in a variety of ways by the computer.

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