What is Seismic Activity?
A vertical seismograph measures up-and-down motions.
CREDIT: R. Toro
Measuring earthquakes is a tricky business.
A primary instrument is a seismograph (or seismometer). But the problem with a single seismograph is that it is only able to track movement in one direction. Thus, three types of seismographs have to be used to get a complete picture.
One records the seismic waves running north-south; one tracks east-west movement; and the third measures the vertical, or up-and-down, movement. Each has a clock to record exactly when a movement is measured, as this enables important calculations (more about that later).
Horizontal seismographs record north-south and east-west movement. A recording drum spooled with paper sits parallel to the ground, connected to a unit with a base anchored to bedrock. A "mass" or weight hangs over the drum with a pen that extends farther down to make contact with the drum. The mass swings like a pendulum, mimicking the movement of the earth, and the pen records that as lines on the drum's paper.
Meanwhile, the vertical seismograph stands differently, with the recording drum perpendicular to the ground. Here, the mass hangs from a spring, allowing it to move up and down, reflecting the up and down movements of the earth. A pen hanging from the mass at a 90-degree angle draws up-and-down lines on the paper on the drum. [See also: Satellite 'Hears' Earthquake From Space]
Each seismograph records two types of waves. The tight zigzag lines at the beginning of a seismograph reading represent the primary, or P, waves, which squeeze and stretch rock as they pass through the earth. The wider zigzags are the secondary, or S, waves. They ripple at right angles to the direction of the P waves and represent the back-and-forth shaking of a quake.
Remember those clocks we mentioned? Seismologists use them to calculate the lag time between the P and S waves, which allows them to determine where the earthquake originated.