As magma moves beneath a volcano prior to an eruption, it often generates earthquakes, causes the surface of the volcano to swell, and causes the amount of gases emitted by the volcano to increase. By monitoring these changes, scientists are often able to anticipate eruptive activity and issue warnings of possible hazards. While not all of these changes are observed before every eruption, combining observations of each of these precursors often allows scientists to forecast eruptions.
Often the first indication of an impending eruption is an increase in earthquake activity. Generally, a network of six to eight seismometers are positioned around a volcano. Readings from each seismometer are continuously radioed to a central recording site where scientists determine the locations, sizes, numbers, and types of earthquakes. In the weeks or days prior to an eruption the number, size and type of earthquakes that occur beneath the volcano will often increase. In many cases, the earthquakes will move to progressively shallower depths beneath the volcanic vent. At some volcanoes, low-frequency earthquakes and a continuous seismic disturbance called tremor will occur shortly before an eruption.
As magma moves to shallower depths it can cause the surface of the volcano to swell. Scientists monitor the deformation of the ground surface using a variety of surveying techniques and instruments. Electronic distance meters (EDM) bounce infrared light off of targets on the volcano's surface to accurately determine the distance between the EDM and the target. Repeated measurements allow the displacement of the target to be measured. Scientists may also use the global positioning system (GPS, a network of precise navigational satellites) to monitor changes in the ground's surface. In some cases, instruments called tiltmeters are cemented to a volcano's sides. These instruments operate much like a carpenter's level and record the amount the volcano's surface bulges or tilts. Data from both tiltmeters and GPS receivers can be transmitted from the volcano to a central recording site for real-time analysis.
As magma nears the ground's surface it releases several types of gas. These include water (steam), carbon dioxide, and sulfur dioxide. Sulfur dioxide is the easiest of these to monitor. Measurements of sulfur dioxide concentrations are made using an instrument called a correlation spectrometer (COSPEC) which can measure the absorption of sunlight by sulfur dioxide. The COSPEC can be used from the ground or mounted in an airplane or helicopter that then circles the volcano.
AVO also observes volcanoes in Alaska and on the Kamchatka Pennisula in Russia using the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA-12 and NOAA-14 satellites. The images are received at a ground station at the Geophysical Institute, University of Alaska Fairbanks,and are analyzed daily to detect volcanic eruption clouds and thermal anomalies at volcanoes in the north Pacific region. This important tool allows AVO scientists to keep an eye on the many volcanoes not yet monitored by the current seismic network. The Okmok eruption of 1997 is a prime example of the importance of this rapidly expanding technology.
An important component in identifying hazards at a given volcano are geologic studies of the deposits from past eruptions. Frequently a given volcano will develop a characteristic eruptive style and size that will remain approximately the same for thousands of years. Identifying the types and sizes of past eruptions allows estimates of the areas and types of hazards that may be expected in the event of a future eruption at a given volcano.
Decades of monitoring many restless volcanoes around the world have shown that each volcano is different. Some reawakening volcanoes, such as Mount Spurr, show increased earthquake activity for many months before they erupt, while others, like Redoubt Volcano, may have less than a day of increased seismic activity. Some volcanoes will deform visibly, as at Mount St. Helens, and some will not. Other volcanoes, like Augustine, vent large quantities of gas before they erupt.
To successfully predict eruptions, scientists consider information from all the monitoring techniques. This information is generally analyzed on a high-speed system of networked computers. Once patterns emerge that suggest an eruption may be imminent, a warning will be issued. Advance warnings were made for a number of eruptive events during the 1989-90 eruptions of Redoubt Volcano, as well as two of the three eruptions of Mount Spurr in 1992.