When an earthquake fault ruptures, it causes two types of deformation:
static; and dynamic. Static deformation is the permanent displacement of
the ground due to the event. The earthquake cycle progresses from
a fault that is not under stress, to a stressed fault as the
plate tectonic motions driving the fault
slowly proceed, to rupture during an earthquake and a newly-relaxed but
Typically, someone will build a straight reference line such as a road, railroad,
pole line, or fence line across the fault while it is in the pre-rupture stressed
state. After the earthquake, the formerly stright line is distorted into a
shape having increasing displacement near the fault, a process known as
The second type of deformation, dynamic motions, are essentially sound waves
radiated from the earthquake as it ruptures. While most of the plate-tectonic
energy driving fault ruptures is taken up by static deformation, up to 10% may
dissipate immediately in the form of seismic waves.
The mechanical properties of the rocks that seismic waves travel through
quickly organize the waves into two types. Compressional waves, also known
as primary or P waves, travel fastest, at speeds between 1.5 and 8 kilometers
per second in the Earth's crust. Shear waves, also known as secondary or
S waves, travel more slowly, usually at 60% to 70% of the speed of P waves.
P waves shake the ground in the direction they are propagating, while S waves
shake perpendicularly or transverse to the direction of propagation.
Although wave speeds vary by a factor of ten or more in the Earth, the ratio
between the average speeds of a P wave and of its following S wave is quite
constant. This fact enables seismologists to simply time the delay between
the arrival of the P wave and the arrival of the S wave to get a quick and
reasonably accurate estimate of the distance of the earthquake from the
Just multiply the S-minus-P (S-P) time, in seconds, by the factor 8 km/s to
get the approximate distance in kilometers.
The dynamic, transient seismic waves from any substantial earthquake will
propagate all around and entirely through the Earth. Given a sensitive enough
detector, it is possible to record the seismic waves from even minor events
occurring anywhere in the world at any other location on the globe. Nuclear
test-ban treaties in effect today rely on our ability to detect a nuclear
explosion anywhere equivalent to an earthquake as small as
Richter Magnitude 3.5.
Seismographs and Seismograms
Sensitive seismographs are the principal tool of scientists who
study earthquakes. Thousands of seismograph stations are in operation
throughout the world, and instruments have been transported to the Moon,
Mars, and Venus. Fundamentally, a seismograph is a simple pendulum.
When the ground shakes, the base and frame of the instrument move with it,
but intertia keeps the pendulum bob in place. It will then appear to move,
relative to the shaking ground. As it moves it records the pendulum
displacements as they change with time, tracing out a record called a
One seismograph station, having three different pendulums sensitive to
the north-south, east-west, and vertical motions of the ground, will record
seismograms that allow scientists to estimate the distance, direction,
Richter Magnitude, and type of faulting of the
earthquake. Seismologists use networks of seismograph stations to
determine the location of an earthquake, and better estimate its other
parameters. It is often revealing to examine seismograms recorded at a
range of distances from an earthquake:
On this example it is obvious that seismic waves take more time to arrive
at stations that are farther away. The average velocity of the wave is just
the slope of the line connecting arrivals, or the change in distance divided
by the change in time. Variations in such slopes reveal variations in the seismic
velocities of rocks. Note the secondary S-wave arrivals that have larger
amplitudes than the first P waves, and connect at a smaller slope.
While the actual frequencies of seismic waves are below the range of human
hearing, it is possible to speed up a recorded seismogram to hear it.
You can click on this earthquake recording
to hear a seismogram from the 1992 Landers earthquake in southern
California, recorded near Mammoth Lakes in an active volcanic caldera
by the USGS.
The original record, 800 seconds long, has been speeded up 80 times so
that you hear it all within 10 seconds.
75 kb u-law;
149 kb WAV;
75 kb Quicktime
The clicks at the beginning of the recording are the sharp, high-frequency
P waves, followed by the rushing sound of the drawn-out, lower-frequency
S waves. This recording is also interesting because of the small, local
earthquakes within the Mammoth caldera that sound like gunshots.
The passage of the S wave from the magnitude 7.2 Landers event through the
caldera actually triggered a sequence of small earthquakes there.
The triggered earthquakes are similar to a burst of creaks and pops you
hear from your house frame after a strong blast of wind.
Landers triggered earthquakes up to magnitude 5.5 throughout eastern
California and Nevada, and in calderas as far away as Yellowstone.
Listen to more earthquakes with:
The pricipal use of seismograph networks is to locate earthquakes.
Although it is possible to infer a general location for an event from the
records of a single station, it is most accurate to use three or more stations.
Locating the source of any earthquake is important, of course, in assessing
the damage that the event may have caused, and in relating the earthquake
to its geologic setting.
Given a single seismic station, the seismogram records will yield a measurement
of the S-P time, and thus the distance between the station and the event.
Multiply the seconds of S-P time by 8 km/s for the kilometers of distance.
Drawing a circle on a map around the station's location, with a radius equal
to the distance, shows all possible locations for the event. With the S-P time
from a second station, the circle around that station will narrow the possible
locations down to two points. It is only with a third station's S-P time that
you can draw a third circle that should identify which of the two previous
possible points is the real one:
This example uses stations in Boston, Edinborough, and Manaus. With the
distances shown, all three circles can intersect only at a single point on
the Mid-Atlantic Ridge spreading center.
J. Louie, 7 Oct. 1996
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