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.