What does a seismogram actually record?

A seismogram is a graphical chart that records ground motion over time as a function of three cartesian axes. Historically drawn on paper via rotating drums, these records are now mostly digital to simplify computer analysis. The ground movements they capture can stem from major earthquakes, explosions, or minor background disturbances like wind and traffic.

What we know

• A seismogram is a graph output by a seismograph that records ground motion at a measuring station as a function of time. [1, 2]
• Seismograms typically record motions across three cartesian axes, with the z-axis perpendicular to the Earth's surface and the x- and y-axes parallel to it. [2]
• Historically, seismograms were recorded on rotating paper drums using pens or light beams on photosensitive paper, but today they are practically all recorded digitally. [2]
• Earthquake magnitude scales determine an earthquake's overall size using measurements of its seismic waves taken from a seismogram. [4]
• Seismograms can record tiny background waves called microseisms caused by traffic, ocean waves hitting beaches, or wind. [2]

Where to go next

• how it workedHow do seismometers capture ground shaking?
• a case in pointHow are synthetic seismograms modeled?
• what followedHow do seismic magnitude scales measure earthquakes?
• the bigger pictureWhat is seismology?
• compared withHow do seismographs compare to seismic intensity scales?
• an unexpected connectionHow does ocean surf affect seismogram readings?Seismograms are sensitive enough to pick up tiny background microseisms generated by ordinary surface phenomena like ocean waves hitting a beach or nearby traffic.

Sources

• [1]seismogram (Wikidata Q510375) · CC0 1.0
• [2]Seismogram · CC BY-SA 4.0
• [3]Synthetic seismogram · CC BY-SA 4.0
• [4]Seismic magnitude scales · CC BY-SA 4.0
• [5]Seismometer · CC BY-SA 4.0