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:

Related Animations

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.

We use exaggerated motion of a building (seismic station) to show how the ground moves during an earthquake, and why it is important to measure seismic waves using 3 components: vertical, N-S, and E-W. Before showing an actual distant earthquake, we break down the three axes of movement to clarify the 3 seismograms. 

A cow and a tree in this narrated cartoon for fun and to emphasize that seismic waves traveling away from an earthquake occur everywhere, not just at seismic stations A, B, C, and D. A person would feel a large earthquake only at station A near the epicenter. Stations B, C, D, and the cow are too far from the earthquake to feel the seismic waves though sensitive equipment records their arrival.

This companion to the animation "Four-Station Seismograph network"  shows the arrival of seismic waves through select wave paths through the Earth (P and S waves) and over the surface of the Earth. The movement at distant stations occurs at a microscopic scale. While that doesn't result in noticeable movements of the buildings, the arrivals are recorded on sensitive seismometers.

Seismograms of common events are compiled to show the different seismic signals recorded by ground-shaking events. Seismograms can record everything from nearby earthquakes to earthquakes on the other side of the world, plus anything that shakes the ground near the seismograph station like people walking, elk running, rocks falling and helicopters landing.

Related Software-Web-Apps

The IRIS Earthquake Browser (IEB) is an interactive map for exploring millions of seismic event epicenters (normally earthquakes) on a map of the world. Selections of up to 5000 events can also be viewed in 3D and freely rotated with the 3D Viewer companion tool.

Seismic Waves is a browser-based tool to visualize the propagation of seismic waves from historic earthquakes through Earth’s interior and around its surface. Easy-to-use controls speed-up, slow-down, or reverse the wave propagation. By carefully examining these seismic wave fronts and their propagation, the Seismic Waves tool illustrates how earthquakes can provide evidence that allows us to infer Earth’s interior structure.

jAmaSeis is a free, java-based program that allows users to obtain and display seismic data in real-time from either a local instrument or from remote stations.