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Seismic gap
Seismic gap
A part of an active fault where there have been no earthquakes in recent times. A seismic gap is a place where the fault is probably locked temporarily. When and if it should become active again, large earthquakes are the likely result.
Seismic waves
A technical term for the waves produced by an earthquake. There are two kinds of seismic waves: surface waves and body waves. All of the waves travel through
the ground at different speeds
and reach the surface at different times after the earthquake occurred. The compressional P waves travel fastest, at somewhere between
1.5 and 8km/sec. Shear S waves move at about two-thirds the speed of P waves. The main waves that cause damage are surface waves (Rayleigh waves and Love waves). They begin as body waves, but change character as they reach
the surface.
The difference in speed between waves accounts for the way they are spaced out along a trace on a seismogram.
About 90% of the earthquake’s energy is used in moving the rocks along a line of rupture. The remaining 10% is used in generating the shock waves.
(See also: Amplification; Reflection; Refraction; Seiche.)
0
Earthquake trace
Stylus
Post connected to ground
Rotating drum
Heavy weight on rod
Seismic zone
A region where the cause of earthquakes is the same. For example, where they share a common plate boundary.
Seismogram
The trace, or series of wavy
lines drawn on paper, produced
by a seismograph during an earthquake. (See also: Amplitude.)
 Seismogram—P, S, and surface waves reach a seismograph at different times.
P waves arrive first, then S waves, with surface waves being last. As the graph shows, the amplitude of the surface waves is far greater than the P or S waves, and they last far longer. That is why the surface waves are the most destructive part of an earthquake.
 Seismograph—A seismograph uses the principle of inertia to remain still while the earth is shaking.
Seismograph
An instrument designed to record the seismic waves produced by an earthquake.
A seismograph works on the principle that it takes a long time to set a suspended weight in motion. This means that the weight can be thought of as stationary even while the ground around it is moving violently.
One of the best ways to use
this principle is by means of a simple pendulum. When the ground shakes, the base and frame of the
P wave
S wave
Surface waves
0 500
1,000
1,500 2,000
2,500 3,000 3,500
Time in seconds
40
Magnitude of wave