DESCRIPTION:
Seismometers, Seismographs, and Seismograms, etc.

The mainstay of volcano monitoring is the continuous recording of seismic activity. Virtually all Hawaiian eruptions are preceded and accompanied by an increase in the number of shallow earthquakes. As magma moves into the reservoir during inflation, it must make room for itself by rupturing or crowding aside the solidified lava that surrounds the reservoir. Such underground ruptures produced seismic waves that travel through the volcano and are recorded by a network of seismometers placed on the volcano's surface. Ground motions sensed by the seismometer are converted into electronic signals, which are transmitted by radio and are recorded on seismographs located at the volcano observatory. The seismic data are analyzed to determine the time, location, depth, and magnitude of the earthquakes. Mapping the earthquake activity allows HVO scientists to track the subsurface movement of magma.

[Image,131K,GIF]
Seismogram from Garden Seismic Station, showing an increase in earthquakes during a dome-building eruption of Mount St. Helens.
-- USGS Photo by Lyn Topinka

[Graphic,20K,GIF]
Four major types of seismograms, or "seismic signatures," are recognized from seismometers in the vicinity of Mount St. Helens. Seismologists review and classify the seismic records daily.

Seismic signals from 5 seismometers on the flanks of Mount St. Helens, 1 in the crater, and 10 others within 40 kilometers of the volcano are radioed to the Geophysics Laboratory at the University of Washington. Signals from several of these stations are also radioed to and recorded at the Cascades Volcano Observatory. This detailed seismic network (Web note: publication date: 1984) enables seismologists to distinguish between different types of volcanic earthquakes and surface events. Although the seismic precursors to the May 18, 1980, eruption did not specify the time of its onset, seismologists have learned to recognize certain characteristic patterns of seismic activity that precede and accompany the subsequent eruptions. By plotting the cumulative seismic strain energy release of various types of seismic disturbances versus time, eruptions have been predicted from a few hours to several days in advance.

Earthquake data from the Mount St. Helens seismic network are stored on computer files at the University of Washington in Seattle. Seismologists review and classify the seismograms, or "seismic signatures," from several local stations each day; during periods of high earthquake activity, seismologists monitor the records 24 hours a day.

The following major types of seismograms have been recognized at Mount St. Helens: (1) deep earthquakes and those located away from the volcano, which produce high-frequency signatures and sharp arrivals similar to tectonic earthquakes, (2) shallow earthquakes, located under the dome at depths of less than 3 kilometers, which produce medium-to-low-frequency seismic arrivals, (3) surface events, such as gas and tephra events, rockfalls associated with dome growth, and snow and rock avalanches from the crater walls, which produce complicated signatures with no clear beginning or end, and (4) harmonic tremor, which is a long-lasting, very rhythmic signal whose origin is not well understood but which is often associated with active volcanoes.

The rate of activity of the various categories of seismic events is used to assist in predicting volcanic activity at Mount St. Helens. An increasing number of shallow volcanic earthquakes were observed several days to 2 weeks before each dome-building eruption from 1980 through 1982. As the number of earthquakes increase, total seismic energy release is calculated and plotted against time. The observation of a sudden upward turn in this smoothly accelerating curve a few hours before the eruption begins is the basis for relatively short-term predictions. Once the eruption is underway, shallow volcanic earthquakes cease, and surface events from rockfalls dominate the records.

Accelerograph:

• A seismograph whose output is proportional to ground acceleration (in comparison to the usual seismograph whose output is proportional to ground velocity). Accelerographs are typically used as instruments designed to record very strong ground motion useful in engineering design; seismographs commonly record off scale in these circumstances. Normally, strong motion instruments do not record unless triggered by strong ground motion. -- Excerpt from: Noson, Qamar, and Thorsen, 1988, Washington State Earthquake Hazards: Washington Division of Geology and Earth Resources Information Circular 85

Seismogram:

• A graph showing the motion of the ground versus time. -- Excerpt from: Noson, Qamar, and Thorsen, 1988, Washington State Earthquake Hazards: Washington Division of Geology and Earth Resources Information Circular 85

• Seismograms are the records (paper copy) produced by seismographs used to calculate the location and magnitude of an EQ. They show how the ground moves with the passage of time. On a seismogram, the HORIZONTAL axis = time (measured in seconds) and the VERTICAL axis= ground displacement (usually measured in millimeters). When there is NO EQ reading there is just a straight line except for small wiggles caused by local disturbance or "noise" and the time markers. -- Excerpt from: USGS Earthquake Hazards Program Website, 2002

Seismograph:

• A sensitive instrument that can detect, amplify, and record ground vibrations too small to be perceived by human beings. -- Excerpt from: Noson, Qamar, and Thorsen, 1988, Washington State Earthquake Hazards: Washington Division of Geology and Earth Resources Information Circular 85

• Seismographs are instruments used to record the motion of the ground during an EQ--installed in the ground throughout the world and operate as seismographic network. The first one was developed in 1890. The earliest "seismoscope" was invented by the Chinese philosopher Chang Heng in A.D. 132. This did not record earthquakes, however. It only indicated that there was one occurring. A seismograph is securely mounted onto the surface of the earth so that when the earth shakes, the entire unit shakes with it, EXCEPT for the mass on the spring which has inertia, and remains in the same place. As the seismograph shakes under (in the example below) the mass, the recording device on the mass records the realtive motion between itself and the rest of the instrument, thus recording the ground motion. In reality, these mechanisms are no longer manual, but instead work by measuring electronic changes produced by the motion of the ground with respect to the mass. -- Excerpt from: USGS Earthquake Hazards Program Website, 2002

Seismometer:

• A seismometer is the internal part of the seismograph, which may be a pendulum or a mass mounted on a spring; however, it is often used synonymously with "seismograph". -- Excerpt from: USGS Earthquake Hazards Program Website, 2002

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