The period and the wavelength are related by a simple expression connecting the two with the speed of the wave wavelength = speed x period. The lower panel can be thought of as the motion of a single point for an interval of time.
The upper panel shows a snapshot of the wave at a single time. Waves are energy transmitting phenomena that have an amplitude and a wavelength. Another important term is frequency, which is the inverse of the period, or one divided by the peak-to-peak time between wave crests. If we record the ground motion at a single location for a range of time, we can measure the time between peaks in the motion, which we call the wave period. To measure the wavelength directly, we would need a group of instruments that measure the amplitude of the wave at the same time but at different locations. First, we use the term wavelength the refer to the peak-to-peak distance on a wave measured at a single time - like in a snapshot. To understand the difference between these seismic recording instruments, we need to discuss a little about the waves. We can detect ground motion in Missouri caused by increased surf activity as a hurricane or large storm system approaches the eastern coast of the lower 48 states. They can detect motions that are much smaller than the thickness of a sheet of paper or as tall as a room. The range of ground motion amplitudes that are of interest in earthquake studies is very large and seismometers we use are very sensitive. Seismic waves are generated by many different processes: Waves generated by large earthquakes can be detected throughout the world and are routinely recorded and analyzed by seismologists. These vibrations travel outward in all directions from their source. If you've felt Earth shake during an earthquake or explosion then you've felt seismic waves. The "disturbance" can be an alternating electromagnetic field strength (light), a variation in water height (ocean waves), a variation in material density (sound waves), or a distortion of the shape of the ground (seismic waves). Waves are very common in nature: light is a wave, sound is a wave, ocean surf is generated by waves, and even matter has wave-like properties. WavesĪ wave is a disturbance that transfers energy through a medium. We'll discuss waves in more detail in the next section, for now we need a few basics so that we can understand the variety of seismic signals and instruments. Careful observation of ground vibrations during the last 80 years or so have lead to our understanding these vibrations, which are caused by seismic waves. Seismograms come in many forms, on "smoked" paper, photographic paper, common ink recordings on standard paper, and in digital format (on computers, tapes, CD ROMs). The fundamental observations used in seismology (the study of earthquakes) are seismograms which are a record of the ground motion at a specific location. Seismograms : Broad-band, Long- Short-Period | Acceleration.
Seismograph Systems : Analog and Digital Records | Seismometers | Seismometer Responses.
Waves : Wavelength and Period | Multiple Frequency Waves.
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