December 28, 2008
by Michael Milstein
The Oregonian
Every other day, on average, for the last two years, an earthquake has struck near the small town of Maupin, about 30 miles east of Mount Hood -- more than 360 in all.
The tremors are small -- fewer than 15 larger than magnitude 3 -- but persistent.
While scientists have yet to figure out just what's causing the rattles and shakes, they suspect some sort of changes in water levels about 10 miles below the surface. Oregon State University researchers presented their analysis at an annual meeting of the American Geophysical Union in San Francisco this month.
The continuing earthquake swarm stands out, because such concentrated tremors are unusual east of the Cascade Range, said Jochen Braunmiller, a research associate in OSU's College of Oceanic and Atmospheric Sciences, who led the studies.
"It just kind of keeps going," he said. "Overall, we know stress is being released so we think it will stop at some point. But we cannot say when that will happen or whether we have seen the largest one yet."
Scientists have noticed earthquake swarms in the area before -- once in 1976 and again in 1987, although geological instruments were more limited at the time and did not record as complete a picture as they have with the current swarm.
An earthquake swarm is generally a large number of tremors with no single event much larger than the rest. The Maupin swarm began in December 2006, and its largest quakes so far were a magnitude 3.9 tremor July 14 of this year and a 3.8 earthquake March 1, 2007. Two or three earthquakes have sometimes hit on the same day.
The earthquakes have been focused about 10 miles beneath the surface, which is relatively deep. They are located near the Deschutes River, amid thick layers of volcanic basalts that fill much of the Columbia Basin.
The tremors could be related to minor movement of molten rock deep underground, but Braunmiller said scientists think they're more likely related to changes in subterranean water levels. Water can reduce the resistance within rock, allowing it to shift.
He also suggested the seismic activity could be connected to something called the Eastern California Shear Zone, which runs parallel to the better-known San Andreas Fault. The San Andreas Fault is expanding to the north faster than the Shear Zone, which could create extra geological pressure northward into Oregon.
A similar earthquake swarm -- with some larger quakes -- is occurring around Reno, which lies along the Shear Zone. But those tremors are much shallower, hitting only two miles below the surface.
The best data on the Maupin swarm dates to 2007, when the National Science Foundation's Earthscope program deployed a temporary array of seismic monitors around the state. The program has its national office at OSU, but has since moved the instruments to other locations.
-- Michael Milstein; michaelmilstein@news.oregonian.com