Animation of the Month
Seismograph
Activities
Introduction
A seismograph detects, amplifies, and records earthquakes as well as other ground motion. The word seismograph is often used synonymously with seismometer, but there is a distinction. The seismometer is the ground-motion detector part of the seismograph system. In our simple animation the seismometer includes the inertial mass and the spring, and the seismograph is the entire assembly that amplifies the slightest movements in the earth and records the signal (the seismogram). [More about seismographs.]
Vertical Seismograph
Animation of the principles of a drum-style vertical seismograph station that records up-and-down movement. Seismographs are designed so that slight earth vibrations move the instrument. The suspended mass remains at rest as the spring absorbs some of the motion. The stylus (pen) affixed to the mass, records the relative motion between itself and the rest of the instrument, thus recording the ground motion.
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Animation of the vertical seismograph above slowed down to show compressive behavior of the P wave in the direction the wave is traveling and the shearing behavior of the S wave perpendicular to the direction of wave travel. Although the arrow shows gross movement, remember that this happens on a microscopic scale in the Earth.
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Horizontal Seismograph
Animation of the principles of a drum-style horizontal seismograph station that records back- and-forth (N–S, E–W) movement. The suspended mass remains at rest as the spring and pivoting knife absorb most of the motion during an earthquake. The instrument shifts back and forth due to the oblique travel direction of the P wave and the shearing movement of the S wave. The stylus (pen) affixed to the mass, records the relative motion between itself and the rest of the instrument, thus recording the ground motion.
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Drum Seismograph
Movie clip of an actual seismograph drum at the Hawaiian Volcano Observatory recording a small local earthquake. Although most Observatory seismometers are now computerized, the rotating drums are still used in displays for visitors. (Used with permission from Volcano Video Productions.)
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More About Seismographs
Seismographs operate on the principle of inertia. Stationary objects, such as the mass in the above animation, remain stationary unless a force is applied to them. A seismometer senses ground vibrations too small to be perceived by human beings. This helps us to detect the half million detectable earthquakes that occur every year. To learn more about how world-wide earthquakes are monitored, go to IRIS' Stations & Instrumentation.
Seismograph Principle
Seismographs operate on the principle of inertia of stationary objects, such as the inertial mass in the above animation, remain stationary unless a force is applied to them. The weight thus tends to remain stationary following sudden movement, while the frame and drum are moving. Seismometers used in earthquake studies are designed to be highly sensitive to ground movements, so that movements as small as 1/10,000,000 centimeters (distances almost as small as atomic spacing) can be detected at very quiet sites. The largest earthquakes, such as the magnitude 9.1 Sumatra-Andaman Islands earthquake in 2004, create ground motions over the entire Earth that can be several centimeters high. [From IRIS Onepager # 7, "How Does a Seismometer Work?"]
Seismograms are the graphs of the motion of the ground versus time (figure 1). They are the squiggles left by the pen or produced by digital computer records. Seismograms are used to calculate the location and magnitude of an earthquake. - (Excerpt from: USGS Earthquake Hazards Program Website)
FIGURE 1 - Seismogram
The horizontal axis = time (measured in seconds)
and the vertical axis = ground displacement usually
measured in fractions of millimeters (nanometers).
When there is NO earthquake reading there is just a
straight line except for small wiggles caused by local
disturbance or “noise.” Also, old-style rotating drums
make a small marker jump every minute.
Three-component Seismometers
Following an earthquake, the ground responds to P, S, and surface waves by moving in all directions, not just up and down (Figure 2). Most modern seismometers include three separate elements (Figure 3) that record movement in three different directions: up-down, north-south, and east–west. Those seismometers record three seismograms simultaneously, such as the seismogram in figure 2.
FIGURE 2 - This three-component was recorded
from an earthquake about 10,000 km (6,000 miles)
from the seismometer. The initial wave (P wave)
arrives at about 6 minutes from the start of the
plot. The signal is largest on the vertical component
showing that the movement is largely up-down. The
subsequent S wave, at about 15 minutes, is largest
on the two horizontal components (north-south and
east-west), which shows that this shear wave has
more sideways movement than vertical movement.
The surface waves are much slower and arrive
about 15 minutes later. The first surface wave, at
about 30 minutes, includes primarily horizontal
motion (Love wave) while the second surface wave
at 35 minutes includes both horizontal and vertical
motion (Rayleigh wave).
Larger Image
FIGURE 3 (left)
Photograph of a modern three-component
borehole seismometer. Three seismometers are stacked
inside a tube with electronic circuitry. The instrument is
encased in a vertical drill hole. Larger Image
FIGURE 4 (right)
Photograph of a
simple AS-1 vertical
seismometer used in
IRIS’ Seismographs
in Schools program.
Learn more.
How does a seismic Station work?
Previous Animations of the Month
Previous Animations of the Month can be found on the archive page.
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Animations By Jenda Johnson
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