A type of earthquake that rumbles deep below or near dormant volcanoes probably is not suggestive of potentially new eruptions, says a University of Oregon earth scientist.
In a study published May 15 in the journal Science, the UO’s Amanda Thomas and two U.S. Geological Survey colleagues argue that a transformation process of “second boiling” of magma may explain deep, long-period earthquakes. Such quakes, the team concludes, “may more commonly be indicative of stagnant, cooling magma.”
The findings emerged from the team’s identification and analysis of more than a million deep, long-period earthquakes, which ranged in magnitude from 1.2 to 1.5, over 19 years below Hawaii’s Mauna Kea volcano. The quakes occurred some 25 kilometers, about 15 miles, below sea level under the volcano, which last erupted about 4,500 years ago. They occurred on average every 10 minutes during the explored time period.
“The prevailing wisdom is that this enigmatic type of earthquake represents magma motion in the subsurface, and sometimes such events are used to forecast eruptions,” said Thomas, an assistant professor in the Department of Earth Sciences. “We found over a million of these earthquakes beneath a dormant volcano that will not, in all likelihood, erupt again.”
Instead of indicating moving magma, the activity is caused by exsolution, a process in which the repeated pressurization of volatiles separates cooling magma into new forms of crystalline minerals. The study is the first to link this process to deep, long-period earthquakes, which occur without the high-frequency energy of normal earthquakes, the researchers noted.
“It's unlikely that these quakes at Mauna Kea represent magma movement,” Thomas said. “Our interpretation is that they probably, here and elsewhere, represent cooling magma.”
That “everywhere,” she noted, includes the Cascade Range in Oregon and Washington. According to previously published research, 60 locations of deep, long-period earthquakes have been identified beneath six volcanoes in Oregon and Washington: Mount Baker, Glacier Peak, Mount Rainier, Three Sisters and Crater Lake. Mount Baker has recorded the most.
A group of these types of earthquake is also located under Oakridge, which, she said, she plans to eventually explore to see if the same process is occurring.
For Mauna Kea, the researchers wrote: “We propose that a continuous flux of volatiles exsolved from second boiling migrates upward through a matrix of fractures to repeatedly pressurize a complex reservoir system and generate (deep long-period earthquake) seismicity.”
To reach their conclusions, the researchers examined data on seismic waves using envelope cross-correlation, a data-mining technique that identifies more small earthquakes than is possible with algorithms used in traditional seismic monitoring, which often miss such low-energy events. They detected patterns of activity on time scales spanning minutes to years.
Aaron G. Wech of the USGS Alaska Volcano Observatory led the study. Weston A. Thelen of the USGS Cascades Volcano Observatory also was a co-author. The USGS Volcano Science Center and National Science Foundation (grant No. 1848302) supported the project.
—By Jim Barlow, University Communications