At any given time, somewhere in the world, 20 volcanoes are erupting. Of course, most do not pose the immediate threat to human life that, say, the Mount. St. Helens eruption did in 1980. That volcano killed 57 people, spawned mud flows that choked the Columbia River, leveled hundreds of square miles of forests and showered distant communities with volcanic ash. While most active volcanoes do not pose this degree of immediate danger, the Mount St. Helens eruption clearly illustrates the need to monitor and study each and every one of these giants, be they sleeping or restless.
A mountain of information.
In 1999, data collected from a Setra Systems pressure sensor gave seismologists new insights into the eruption of the Shishaldin volcano on Unimak Island, Alaska. Setra, based in Boxborough, MA, is a leading designer and manufacturer of pressure measurement devices. One of six volcanoes comprising this Unimak Island, Shishaldin is known to have erupted at least 29 times since 1755, making it one of the most active volcanoes in the state. Like many volcanoes in this remote region, Shishaldin poses little threat to human life or property. However, ash from potential eruptions pose a very real threat to aircraft as it sits beneath one of the heavily traveled north Pacific air routes. Encounters with volcanic ash by jet aircraft can cause serious damage, and even engine failure.
There are two populated regions near Shishaldin, the nearest being False Pass. Although just 32 km away, this town’s view of the volcano is blocked by another volcano called Roundtop. As a result, visual observations were extremely limited, available only from either Cold Bay, 90 km away, or from pilot reports. Poor weather also hindered eyewitness descriptions. As a result, capturing and understanding the eruptive behavior of Shishaldin volcano would rely heavily on the analysis of telemetered acoustic, seismic and satellite data.
Installed in the summer of 1997, the Setra Model 239 high accuracy pressure sensor was the source of much of the acoustic data detailing the eruption. Integrated into a network of short-period seismometers that were deployed between 5 and 19 km from Shishaldin’s primary vent, the Setra 239 was used to detect the infrasonic oscillation that was generated whenever the volcano erupted. Upon detection of the infrasonic oscillation by the sensor, it is automatically converted to a voltage, which is then transmitted by radio back to the Alaska Volcano Observatory.
Manufactured by Setra Systems, the Setra 239 is designed for simplicity and reliability, and features a stainless steel diaphragm and an insulated electrode, which forms a variable capacitor. The volcanic signals generate pressure fluctuations in the atmosphere which in turn cause the stainless steel diaphragm to vibrate. This change in the diaphragm results in a change in instrument capacitance. Because capacitance and voltage are directly related, the signal is thereby converted into a changing voltage. As pressure increases or decreases the capacitance changes. The Setra 239’s unique electronic circuit detects this change in capacitance and converts it to a linear D.C. high-level voltage output signal. It has a flat response for frequencies less than 100 Hz and sensitivity of 0.2 mV/Pa. The 239 is also known for its extreme robustness. The 239 not only survived the earth-shattering Shishaldin volcano eruption, it continued to work for a lengthy period of time following this seismic frenzy.
At Shishaldin, the Setra 239 was mounted on the eastern side of the instrument hut with a downward-pointing intake tube and a clear line of sight to the volcano’s summit.
Signaling three stages of activity.
Jackie Caplan-Auerbach, Ph.D is a seismologist with the Alaska Volcano Observatory, which is part of the United States Geological Survey (USGS). The observatory is also affiliated with the University of Alaska Fairbanks and the Alaska Division of Geological and Geophysical Surveys. In the most general terms, her job is to determine if, when, and in what way one of 44 Alaskan volcanoes is going to erupt. She and the other members of the seismology team check data on these volcanoes twice a day, seven days a week. If the volcanoes are in a state of unrest they are checked more frequently.
Caplan-Auerbach credits the Setra 239 sensor with being of great assistance in her team’s ability to both predict and monitor the behavior of the Shishaldin volcano. She said, “By sensing an atmospheric eruption, the 239 told us that a Strombolian eruption was on the way. We were able to monitor the volcano’s transitions from Strombolian (primarily lava) to Subplinian (plumes of ash) then back to Strombolian eruptive phases.”
According to Caplan-Auerbach, there were three different signals detected by the pressure sensor in 1999 that were associated with three stages of eruptive activity. These signals were: (1) a steady 2-3 Hz hum; (2) a diffuse broadband signal at the time of a Subplinian eruption and (3) discrete, repeating pulses lasting 1-3 seconds.
“We investigated each of these signals to determine their relationship to the volcano and what they indicate about the eruption,” Caplan-Auerbach said. “To verify the volcanic origin of the pressure changes, we attempted to draw correlations to signals found in the seismic record.”
The 2-3 Hz acoustic humming signal, which began on April 19 and most directly preceded the Subplinian eruption, was upon further analysis found to be composed of a series of explosion-like pulses, several seconds apart. The Shishaldin team believes that this signal may represent the release of a volume of gas from rising magma.
After more than 13 hours of continuous amplitude increase, the Setra 239 recorded a decline in the hum while picking up several low frequency (approximately 1-2 Hz) bursts. This time the signal coincided with an increase in seismic tremor amplitude and suggested a change in the physical process. Four minutes later the Setra 239 recorded a strong, 14-minute relatively broadband (1-10 Hz) background over which was superimposed a series of low-frequency pulses at 1-2 minute intervals. This pattern continued for 49 minutes, coinciding with the Shishaldin’s main ash-producing episode – the volcano’s Subplinian eruption phase.
Following this phase, the volcano returned to what appears to be a more common eruptive mechanism for Shishaldin – a Strombolian phase with massive gas bubble bursts. These explosions appeared as discrete, impulsive pulses in the Setra 239’s pressure record. Four episodes of this Strombolian activity continued April 19-20, one of which was not associated with any strong tremor and, thus, had not previously been recognized. An additional period of explosivity occurred on April 23, with explosions that were significantly larger and more frequent than similar events observed at other volcanoes.
According to Caplan-Auerbach, “Shishaldin exhibited an unusually wide range of eruptive behavior over a period of 4 days. Data collected by the pressure sensor provides us with new insights into this eruption, with its unique combination of lava flow and ash plume. Shishaldin represents the first time that a sensor was able to gather data about this particular pairing of volcanic activity.”
She concludes, “The Setra 239 was able to detect what was happening inside the volcano before the eruption occurred. It was able to pick up signals and record information that the seismometers could not. This pressure sensor did its job wonderfully. I also remain impressed that the Setra 239 actually survived the eruption and continued to function for a lengthy period of time following this period of immense volcano activity.”
