Concise Summary of Seismic Waves

by Chris Chapman

 
When an earthquake occurs, two types of wave are generated. The faster compression P waves have their displacement in the direction of propagation. The slower S shear waves have their displacement perpendicular to their direction of propagation. Local frequency components up to a few hundred Hz may be recorded, but these ‘high frequency’ components are rapidly absorbed as the signals travel through the Earth. 


When these waves reach the surface of the Earth, they generate slower surface waves, which propagate radially outwards and are restricted to the outer layers of the Earth. The Love waves have the higher velocity and their displacement is perpendicular to the direction of propagation and in the plane of the surface. (Note: in the animation it may look like there is vertical motion, but the motion is all horizontal.)  The Rayleigh waves are slower and have a retrograde vertical motion in the direction of propagation.  For added details on these types of waves see Larry Braile's site:  http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm


The wave propagation is constrained by the transmission velocities in the various Earth layers, the crust, the mantle and the cores. The seismograph training manual from http://psn.quake.net/info/analysis.pdf contains many more details.  Due to layering and the curvature of the Earth, the seismic responses are divided into local (0 to 1.4 deg), near regional (1.4 to 6 deg.), regional (6 to 16 deg.) and teleseismic responses (16 to 180 deg.). The angle is that between the source and the observer at the centre of the Earth.


A 1 degree angle corresponds to 111.3 km, 69.2 miles or 60 nautical mile at the surface.


The P and S waves have frequencies between 0.5 and maybe 40 Hz, with the S waves having a bit less than ½ the period of the associated P waves. P waves from regional and teleseismic events may have frequencies between 1 and 3 Hz. The Love and Rayleigh surface waves may have periods up to several hundred seconds depending on the physical dimensions of the seismic slip, but for smaller quakes they are more typically between 10 and 30 seconds. The amplitude of Rayleigh waves of about 20 sec period are often measured to determine the surface-wave magnitude (MS) of a quake. For Local and Near Regional quakes, the maximum amplitudes of the P or S waves may be used to determine the Richter local magnitude (ML).


Visual computer displays of seismic waves can be made with the free SeismicWaves program of Alan Jones.  This program can be downloaded from: http://www.geol.binghamton.edu/faculty/jones/

While the surface waves radiate outwards in a fairly simple and predictable manner, the waves within the earth generate new wave pairs at every refraction and reflection. The response to a large quake depends on the depth of the quake as well as the angle between it and the observer and can get very complicated.
It a good idea to download and print a copy of the seismic time travel graph from http://neic.usgs.gov/neis/travel_times/ttgraph.html for future reference.

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