Volcanoes and Harmonic Tremors
Laura Nielsen for Frontier Scientists –
Volcanoes are an awe-inspiring and hazardous part of our planet. And now, they're being made to sing. Scientists are turning seismic data into audio data in order to grasp just what's going on deep in the Earth's crust, and better forecast future explosive eruptions.
In Alaska, the March 2009 Redoubt eruption was preceded by a series of accelerating earthquakes that were picked up by seismic sensors placed on the volcano's flanks. The distinct, repetitive earthquakes increased in power and frequency. There was a short, calm pause. And then Redoubt erupted with magmatic explosions and emitted towering ash clouds. Twenty eruptions shook Redoubt's innards over the course of two weeks. And just before each eruption the earthquake tempo was intense: Alaska Volcano Observatory scientists could measure 30 new earthquakes starting every second. What caused this unusual eruptive behavior? Understanding more of the processes happening deep within an erupting volcano can improve models that predict the timing and characteristics of future volcanic activity.
That's important. Even volcanoes in remote and sparsely populated places such as Alaska's Aleutian Chain island volcanoes need to be monitored because they can eject large ash clouds into airspace, threatening aviation traffic in the North Pacific air routes connecting North America and Asia. Ash clouds are incredibly dangerous for airplane engines, and when the Alaska Volcano Observatory forecasts high-reaching volcanic ash the Federal Aviation Administration redirects flight paths.
Ash landing in cities poses a health risk to the population, and can cause millions of dollars in damages. The National Oceanic and Atmospheric Administration notes that after the 1992 eruption of Mount Spurr's Crater Peak, the Municipality of Anchorage reported nearly $2 million in damage. Volcanoes that erupt near population centers can release pyroclastic and lava flows, lahars (dangerous mudslides), ash, steam, and dangerous gasses.
Volcanic eruptions even have implications for climate, since tiny particles ejected high into the atmosphere can linger and reduce how much incoming solar radiation reaches the ground. Volcanic aerosols have the potential to affect temperatures and crop health across startlingly large regions of the globe.
The Alaska Volcano Service monitors northern volcanoes. The AVO is jointly operated by the U.S. Geological Society, the Alaska Division of Geological & Geophysical Surveys and the University of Alaska Fairbanks Geophysical Institute. It works in close conjunction with the National Oceanic and Atmospheric Administration's National Weather Service to provide early warning of volcanic activity.
Volcanoes are monitored and studied with a complex suite of tools. Seismic sensors on the ground feel for earthquakes, wile GPS systems note the swelling of mountain slopes that sometimes precede an eruption. Webcams point toward peaks to spot signs of fumes or ash being released. Information from meteorological weather offices can help track wind and ash directions. Satellites are used to notice hotspots or ash trails. Even word-of mouth information from plane pilots and fishermen help the AVO in their volcanic vigil. You can report eruption clouds or ash fall in Alaska online at https://www.avo.alaska.edu/ashfall/ashreport.php - "Is Ash Falling?"
Using sounds to understand magma
When volcanic eruptions emit noise from the conduits beneath volcanoes where magma travels, they generally emit sounds that are inaudible to the human ear - much too low pitched to hear. But Redoubt's series of earthquakes created sounds that ramped up high enough to (just barely) be heard by human beings. Redoubt is one of the many volcanoes located on Cook Inlet in Alaska It sits about 110 miles [177 km] from Anchorage. Sensors on its flanks recorded the activity. Volcanologists from the AVO and the University of Washington headed to Stanford University to study the recordings.
Pressure building in the ground under volcanoes can emit harmonic tremors: sustained vibrations that generally cause a very low tone in the range of 0.5 - 5 Hertz (Hz: a unit of frequency equal to one cycle per second useful for measuring sound). That's too low for humans to hear, but high-powered supercomputers can be used to convert seismic readings into audible sound, generally by speeding up the sound. It's called sonification. The human ear is a powerful tool, and it's easier to detect patterns by listening to a sound than it is to detect patterns by visually pouring over complex numerical data.
The sound helps scientists grasp what's going on deep underground. By listening to it, researchers hope to understand th processes that caused Redoubt's unusual sonic behavior. Eric Dunham, assistant professor of geophysics, explained to Stanford News: "The better we understand the plumbing system beneath the volcano and how magma is being transported, the more accurately we can develop models to predict the timing, duration and explosiveness of eruptions."
Reconstructing Redoubt's innards
Redoubt's grumbles during the March 2009 eruption have been described as popcorn pops, representing a stream of disparate eruptions, and as a silent scream, representing eruptions so close together that they merge into one roaring sound. When the seismic data is converted to sound and sped up many times to enter the human hearing range, you can literally hear the series of earthquakes that originated something over a mile [2 km] below the volcanic crater.
What caused these noises? Scientists speculate that some blockage in the behemoth's throat, Redoubt's narrow underground conduit that links deep-earth magma chambers with the surface, forced pressure to build up inside the volcanic peak. It might have been caused by rocks from a collapsed wall choking the passageway, or by a cap of cooling magma temporarily holding back the hotter magma below it. The popping noises likely represents explosive stages of movement when building pressure forced the blockage higher toward the surface in stages, scraping and squeezing it past narrow rock walls and causing shudders in the earth's crust due to the extreme pressure. Information on Redoubt's 'screams' and speculation concerning the geologic force involved were published July 14, 2013 in Nature Geosciences.
Our planet is a volatile place. Gaining a better understanding of the processes happening deep within volcanoes will give us better tools to predict the timing and magnitude of future volcanic activity, helping volcanologists protect lives and property across the globe.
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- 'Frictional-faulting model for harmonic tremor before Redoubt Volcano eruptions' Nature Geoscience, Ksenia Dmitrieva, Alicia J. Hotovec-Ellis, Stephanie Prejean, Eric M. Dunham (2013)
- 'New Tool for Reporting Alaska Volcanic Ash Fall Allows Residents to Assist Scientific Monitoring' U.S. Geological Society Newsroom (2013)
- 'Stanford scientists eavesdrop on erupting volcano's astonishing seismic sound' Stanford News (2013)
- 'Redoubt reported activity: March 2009' Alaska Volcano Observatory (2009)
- 'Volcanoes may reveal secrets through 'song'' New Scientist (2006)