A New Explanation for Volcanic Tremors
University of British Columbia geophysicists are offering a new explanation for seismic tremors accompanying
volcanic eruptions that could advance forecasting of explosive eruptions such as recent events at Mount Pinatubo
in the Philippines, Chaiten Volcano in Chile, and Mount St. Helens in Washington State.
All Explosive Eruptions Preceded by Tremors
All explosive volcanic eruptions are preceded and accompanied by tremors that last from hours to weeks, and a remarkably
consistent range of tremor frequencies has been observed by scientists before and during volcanic eruptions around the world.
What Causes the Tremors?
However, the underlying mechanism for these long-lived volcanic earthquakes has never been determined. Most proposed
explanations are dependent upon the shape of the volcanic conduit - the 'vent' or 'pipe' through which lava passes
through - or the gas content of the erupting magma, characteristics that vary greatly from volcano to volcano and are
impossible to determine during or after volcanic activity.
Published this week in the journal Nature, the new model developed by UBC researchers is based on physical properties
that most experts agree are common to all explosive volcanic systems, and applies to all shapes and sizes of volcanoes.
"All volcanoes feature a viscous column of dense magma surrounded by a compressible and permeable sheath of magma, composed
mostly of stretched gas bubbles," says lead author Mark Jellinek, an associate professor in the UBC Department of Earth and Ocean Sciences.
Tremors Caused by an Oscillating Magma Plug
"In our model, we show that as the center 'plug' of dense magma rises, it simply oscillates, or 'wags,' against the cushion of
gas bubbles, generating tremors at the observed frequencies."
"Forecasters have traditionally seen tremors as an important - if somewhat mysterious - part of a complicated cocktail
of observations indicative of an imminent explosive eruption," says Jellinek, an expert in Geological Fluid Mechanics. "Our
model shows that in systems that tend to erupt explosively, the emergence and evolution of the tremor signal before and
during an eruption is based on physics that are uniform from one volcano to another."
Understanding Problematic Tremor Origins
"The role of tremors in eruption forecasting has become tricky over the past decade, in part because understanding processes
underlying their origin and evolution prior to eruption has been increasingly problematic," says Jellinek. "Because our model
is so universal, it may have significant predictive power for the onset of eruptions that are dangerous to humans."
Research Leadership and Support
The research co-led by Prof. David Bercovici of Yale University and was supported by the Canadian Institute for Advanced Research,
the Natural Sciences and Engineering Research Council of Canada, and the U.S. National Science Foundation.
Update 2011-02-25 4:00pm PST....
Sensational Headlines and Unnecessary Panic
Prof. Mark Jellinek says sensational headlines such as "Mt. Baker overdue to erupt" has taken his research out of context, potentially
causing residents to panic over the "imminent" eruption of Mt. Baker.
"Mt. Baker is the youngest and second-most active volcano in the Cascade Volcanic Arc in Washington State. Based on its eruptive
history, it is indeed overdue for an eruption, all Cascade volcanoes are," says Jellinek.
"However, in geological terms, 'overdue' does not equate 'tomorrow,' or 'next month.' Indeed, it could be 500 to 1,000 years before
it's 'due' for an eruption."
"What's important for the public to note is that there are currently no precise ways of predicting such events, hence the
relevance of our research into the mechanics underlying the the eruptive behaviour of volcanoes," says Jellinek.
|The study found in one test that after the first large rainstorm in October, only 4 percent of the precipitation entering the soil ended up in the stream – 96 percent was taken up and held tightly by soil around plants to recharge soil moisture. A month later when soil moisture was fully recharged, 55 percent of precipitation went directly into streams. This is contrary to widely-held assumptions of how water behaves in soils. USGS image.
| A huge cloud of volcanic ash and gas rises above Mount Pinatubo, Philippines, on June 12, 1991. Three days later, the volcano exploded in the second-largest volcanic eruption on Earth in this century. Timely forecasts of this eruption by scientists from the Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey enabled people living near the volcano to evacuate to safer distances, saving at least 5,000 lives. . USGS image and caption.