by Mary Caperton Morton Tuesday, July 16, 2013
In March 2009, Alaska’s Mount Redoubt awoke from two decades of silence with something to say: A series of small earthquakes leading up to the eruption produced a seismic sound that staff at the U.S. Geological Survey's Alaska Volcano Observatory nicknamed “the screams.” Now, two new studies are eavesdropping on Redoubt’s inner workings and quantifying the forces needed to produce the unusual harmonic tremors.
The most recent eruption of Redoubt wasn’t just one and done. The volcano, located 180 kilometers southwest of Anchorage, exploded 20 times over a two-week period from late March into April 2009, spewing ash and gas 10 kilometers into the atmosphere.
Before six of the explosions — including five successive events — the seismic network on the volcano’s slopes detected a swarm of magnitude-0.5 to magnitude-1.5 earthquakes originating two kilometers below the crater, approximately where the magma chamber connects to the conduit in the neck of the volcano.
In the final minute before the explosions, the seismic noise reached a crescendo with as many as 30 tiny earthquakes initiating every second. Then, 30 seconds of foreboding seismic silence, just before Redoubt blew its top.
This unusual seismic pattern, classified as a harmonic tremor, was reported in the Journal of Volcanology and Geothermal Research by a team led by Alicia Hotovec-Ellis, a doctoral student in volcano seismology at the University of Washington. “The frequency of this tremor is unusually high for a volcano, and it’s not easily explained by many of the accepted theories,” she says.
Harmonic tremors are fairly common in volcanic eruptions, says Eric Dunham, a geophysicist at Stanford University in Palo Alto and an author of the other new study published in Nature Geoscience, on which Hotovec-Ellis is also a co-author.
“When Redoubt’s seismic recording is sped up to frequencies that the human ear can hear, the sound produced by these tiny earthquakes is very harmonic, indicating events are becoming more closely spaced,” Dunham says.
Lower-frequency harmonic tremors preceding volcanic explosions have been recorded at Arenal Volcano in Costa Rica and at the Soufrière Hills Volcano on Montserrat in the Caribbean. “But Redoubt is pretty unique in having an extremely high-frequency tremor, as high as 20 to 30 Hertz,” Dunham says.
Lower-frequency harmonic tremors are thought to be produced by the discharge of highly pressurized gases through small cracks, Dunham says. The link between repeating earthquakes and high-frequency harmonic tremor has been proposed before, but has never been well documented, he says.
To determine what might be producing the high-frequency earthquakes, Dunham and colleagues created a mathematical model of the forces involved in producing such seismic noise and found that the stress and deformation rates would have to be 10 orders of magnitude higher than those driving the San Andreas Fault.
“If you were to start at the surface of the ocean and dive down to one kilometer in one second, that’s the rate at which stresses are increasing on these faults,” Dunham says. “These forces are incredibly strong.”
The next step will be to determine the underlying mechanisms that produced the earthquake swarms, Dunham says. “It was beyond the scope of the paper to evaluate what might be causing those forces, but it is fun to speculate,” he says.
Dunham says a crystalline plug may have been stuck in the volcano’s conduit, “plugging the volcano like a cork in a champagne bottle.” As pressure built up from below, the obstruction would have been forced upward, grinding along the walls of the conduit, producing the series of earthquakes. As the magma pushed harder on the plug, the quakes would become more frequent. Once the obstruction was clear, the seismic noise would have stopped, producing the 30 seconds of silence just before the blast.
“This particular type of tremor has rarely been reported at other volcanoes,” Dunham says, “but I wouldn’t be surprised to see it in other eruptions.”
The work could provide some valuable insights into future eruptive patterns at Redoubt, which is one of Alaska’s more dangerous volcanoes, says John Power, a geophysicist at the Alaska Volcano Observatory in Anchorage who was not involved with the new studies. “There have been other examples of volcanic tremor but this study takes the modeling to another level,” he says.
The screams aren’t likely to be useful in predicting eruptions: By the time a volcano is exhibiting high-frequency tremors, more than enough warning signs have raised flags that an eruption is likely imminent, Power says. “But these kinds of observations and discoveries have the potential to teach us more about what drives the volcano’s eruptive cycle, which may indirectly help provide a more extended forecast.”
Since 1900, Mount Redoubt has erupted five times: in 1902, 1922, 1966, 1989 and 2009. It currently shows no signs of an impending eruption. “Redoubt has the potential to affect a lot of people and a lot of industries so it’s very well instrumented,” Power says. “Still, we’re always upping our game for the next one.”
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