Seismograms: Illustrated Guide to Reading a Seismogram (USGS)

Ever wondered how to read the data on a seismogram? 

Seismometers measure vibrations. More vibration… more wiggle. Some seismometers measure only up and down. Sometimes, they shake too much and data are off-scale. Some seismometers record in all three dimensions. Many webicorders display 15 minutes per line, alternating colors as time passes. Plots are automatically updated every few minutes.

Scientists use webicorders for a quick look at current activity, but webicorders are really just a blurry picture of the actual data To really understand what’s going on, they look at the real data, which is far more informative. Here is a small earthquake. It begins as a sharp crack as the rock breaks, and then quickly decays with time. In this plot, the data are clipped to allow viewing of other data. That can be very useful when there are lots of earthquakes, as in this seismic swarm, with multiple earthquakes of magnitude 2 to 3. 

Seismometers are sensitive enough to pick up vibrations from earthquakes around the world – if they’re big. This is what the giant 2011 Japan earthquake looked like on a seismometer in Wyoming, 5000 miles away. Compared to the P and S waves that travel through the earth, surface waves are slower and arrive later, but can continue to be recorded for hours as they propagate around the earth.

Seismometers record not only earthquakes but anything that can make the ground vibrate: Helicopters, Ocean waves, Traffic, Animals and Wind. Wind can be seen as noisy periods. Here, the wind picks up each day and drops off at night. Here, traffic noise shows up on a seismic station located a quarter mile from a road.

Seismometers are sensitive electronic instruments often deployed in harsh wilderness environment. They can and do break. Sometimes they just stop working, but sometimes they just become noisy and unreliable. Here’s a record from a three channel instrument, one of which is broken – guess which one? Nearby equipment and machinery can also create interference.

Worried about something on a webicorder? Then check readings from nearby instruments. Anything that shows up on only a single seismometer is either noise or earthquake activity too small to worry about. 

This animation was used with permission from the USGS, and can also be downloaded from their site: http://gallery.usgs.gov/videos/852#.VxkfnJMrLew

Total Time: 4min 26s
Level: Novice

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Seismic waves travel through the earth to a single seismic station. Scale and movement of the seismic station are greatly exaggerated to depict the relative motion recorded by the seismogram as P, S, and surface waves arrive.

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