Interpreting Waveforms

"Webicorder" images digitally simulate seismograms recorded at the Earthquake Studies Office. Seismograms are records of ground motion at seismograph stations in the Montana Seismograph Network made each 24-hour period. Seismograms are "read" like a book, from left to right, and then from top to bottom (these are the directions on the images that time increases). As with a book, the right end of any horizontal line "connects" with the left end of the line below it. The color of each horizontal line has no significance, but makes the "webicorder" image (seismogram) easier to read.

The vertical lines are spaced at one-minute intervals. The time of day (Hours:Minutes) is indicated in the left margin in Mountain Daylight Time (On the hours) and at the right margin in Universal Coordinated Time (Greenwich Time) (On the quarter-hours).

When a seismic event occurs, seismograms show ground motion at the location of the recording instrument that typically lasts from several tens of seconds to many minutes, depending on the size of the event and its distance from the recorder. The height of the waves on the seismogram (wave amplitude) is a greatly magnified representation of the actual ground motion. The magnification of some stations may be greater than 100,000 times. An earthquake recorded on a seismogram has recognizable characteristics: Typically, you can see the arrival the Compressional Wave, P, (travels fastest through the earth), and S, Shear Wave (somewhat slower).

On our seismograms you may see local earthquakes in Western Montana, regional earthquakes, from surrounding states and the Pacific Northwest, and earthquakes elsewhere in the world having a magnitude greater than 5.5 on the Richter scale. For distant earthquakes be sure to look at the "broad band" seismogram (BOZ). Broad band seismographs are equally sensitive to high-frequency shaking from local earthquakes and to low-frequency ground motions generated by distant earthquakes. The lower frequency surface waves generated by a distant earthquake will only be visible on broad band seismograms.

Not all the waves on seismograms are caused by earthquakes. Anything that produces ground vibrations is recorded. For example: vehicles on nearby roads (why we try to locate our seismometers well away from roads), wind noise and thunder, or animals passing nearby (a herd of elk for example). Man-made mine blasts are also recorded, such as the ones from: Montana Tunnels, Golden Sunlight, the Continental Pit, and Yellowstone Talc. The uniform amplitude ground vibration at certain stations, (BZMT, STMT) is caused by nearby equipment.

Some other non ground motion signals are common: pulses at regular intervals during daylight hours on some stations, (for example SLMT, BGMT) are caused by the solar charging system voltage regulator. There is also radio noise. Since the seismic signals from the seismometers are typically transmitted to the Earthquake Studies Office using low-power FM radio newslinks, other radios with similar frequencies may interfere with our signal. Interference is usually easy to distinguish from earthquakes because the the interference is often "spiky" or "one-sided", and begins and ends abruptly.