Reading a seismogramThis is the actual seismogram recorded by a seismometer during a magnitude 5.0 e

Reading a seismogramThis is the actual seismogram recorded by a seismometer during a magnitude 5.0 earthquake on Jan Mayen island in 1995. There’s a lot of information in a graph like this if you know how to read it.A classic seismogram is made by a pen held by a weight in a fixed position on top of a spinning roll of paper. During a quake, the inertia of the weight keeps the pen from moving, while the paper drum moves due to the waves. The pen then marks the drum as it moves back and forth. Modern instruments detect these waves electronically, but the principle is the same.In this plot, time moves from left to right; the left side starts and the right side finishes. 3 different features stand out. The first hint of the quake is a small pulse of energy that decays away with some fine structure to it. Then, there’s a second pulse of energy, slightly less intense than the first. Just after this second pulse is when the biggest, most intense shaking starts.This is the classic sequence of waves created in an earthquake. The fastest moving waves are called p-waves, a pressure wave moving through the earth. The next to arrive is an s-wave, a shear wave also moving through the earth. Finally, the largest pulse of energy arrives in the form of surface waves known as Rayleigh and Love waves. The surface waves are where the largest ground motions take place and when most of the damage is done. They also have a lower frequency than the p and s waves, meaning that the wave arrivals become more spread out.The time between waves arriving at a seismic station depends on how far the station is from the quake. The farther away the station is, the bigger the difference in arrival times. The difference between the p wave arrival and surface wave arrivals can be as little as a fraction of a second for earthquakes very nearby and as long as tens of minutes for earthquakes far away. Each wave bounces and refracts off slightly different regions within the earth, leading to the complex structure in each portion. That complexity can be used by computers as a way of measuring actual differences inside the Earth. -JBBImage credit: https://www.flickr.com/photos/rockbandit/309794495Read more:http://www.geo.mtu.edu/UPSeis/reading.htmlhttp://tinyurl.com/o74xor4 -- source link

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