What is shown on seismograms?

A seismogram is a record of the motion of the ground in the vicinity of the seismometer. For example, for a vertical-component seismogram, when the ground moves up, the line on the seismogram moves up; when the ground moves down, the line on the seismogram moves down. The motion during an earthquake is more complicated because the ground moves in three dimensions.

The animation below provides an example. The seismograms show the ground movements produced by the M 7.0 foreshock of the Great Tohoku earthquake of March 2011, recorded at Albuquerque, NM. Think of the red ball as the seismometer and watch how the motion of the ball changes as the red line sweeps across the seismograms shown below the ball. The three seismograms below are records of the up-down, north-south, and east-west motions of the ground near the seismometer. When the trace of the top seismogram is offset upwards, the ground moved up; when the trace of the middle seismogram is offset upwards, the ground moved to the north; when the trace of the bottom seismogram is offset upwards, the ground moved to the east.

The best way to interact with the animations is to pause them, and use the cursor keys to move through them at a variable pace.

Animation by Michael Cleveland, Penn State

What causes the vibrations? The ball bounces as seismic waves pass the location of the seismometer. The first seismic wave passes by at around 700 seconds - that's the start of the P waves; strong S waves arrive at about 1300 s; Love waves around 2000 s, and Rayleigh waves come in at about 2400 s. The shear-waves and surface waves are much larger for this earthquake, which is located at this distance (83 degrees, ~9,000 km, or ~5,700 miles).  None of these vibrations are large enough to be felt, but they are well above the normal level of Earth's background motions.

Another example is show below. These ground motions are from the 23 August, 2011 central Virginia earthquake. Mike Cleveland suggested adding the box to the animation to provide better depth perception. The observations are from central Pennsylvania (near Standing Stone Valley), a few hundred kilometers north of the earthquake.

The best way to interact with the animations is to pause them, and use the cursor keys to move through them at a variable pace.

 For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.