Scientists Study Earthquake Swarm Near Maupin
(糖心视频Org.com) -- During the last two years more than 350 small earthquakes have been recorded just outside the small eastern Oregon town of Maupin and scientists are unsure what is triggering the activity.
The earthquakes began in December of 2006 and though only a dozen or so of the temblors exceeded magnitude 3.0, the largest of the events could be felt locally. Scientists from Oregon State University presented an analysis of this 鈥渆arthquake swarm鈥� at the annual meeting of the American Geophysical Union in San Francisco in December.
The earthquakes all originated within a very small area just a few miles southeast of Maupin and about 30 miles southeast of Mt. Hood, according to Jochen Braunmiller, a research associate in OSU鈥檚 College of Oceanic and Atmospheric Sciences.
鈥淪eismic activity in eastern Oregon is comparatively rare,鈥� Braunmiller said, 鈥渁nd when it does happen, it seems like it happens in these swarms. More common earthquake activity is to get a main shock and then a number of aftershocks. But these swarms lack a main event and instead are characterized by an ongoing series of small quakes.
鈥淭hese small earthquakes strike Maupin about every other day on average and one looks a lot like another,鈥� he added.
The largest events were a magnitude 3.9 earthquake on July 14 of this year and a 3.8 earthquake on March 1, 2007. Eleven other events had a magnitude of 3.0 or higher, according to data the scientists analyzed from the Pacific Northwest Seismic Network, which is based at the University of Washington and funded primarily by the U.S. Geological Survey; the Oregon Department of Geology and Mineral Industries is a contributing organization.
Occasionally, Braunmiller said, two or three quakes would occur on the same day, but never more than four. And rarely would more than a few days go by without an earthquake.
鈥淭hat鈥檚 rather unusual,鈥� he said.
A geologic map shows a small fault line near the region where most of the quakes have occurred, though it runs in a direction that doesn鈥檛 necessarily mirror the pattern of recorded earthquakes. However, Braunmiller pointed out, the exact origin of the earthquakes can be difficult to pinpoint.
Possible explanations for the quakes vary, scientists say. They could be associated with minor magma-related activity or triggered by fluid movement within the Earth鈥檚 crust. Braunmiller said it鈥檚 also possible that the seismic activity is an extension of disturbances along the Eastern California Shear Zone that runs roughly parallel to the San Andreas Fault, which is creeping northward at a faster rate than the shear zone, creating geologic pressure that may extend into Oregon.
A similar swarm of earthquakes, though some of them larger, has been taking place in Reno, which also is along the Eastern California Shear Zone. Those quakes are much shallower 鈥� about two miles below the surface compared to the 10 mile-deep Maupin events.
The largest instrumentally recorded earthquake in Maupin history was a 4.6 event in 1976 and the region also experienced a similar swarm of earthquakes in 1987. Knowledge about the area鈥檚 seismic activity before the first monitors were installed in the 1970s is limited.
The most comprehensive data on the Maupin swarm is from 2007, when the National Science Foundation鈥檚 Earthscope program, which has its national office at OSU, deployed a temporary array of seismic monitors throughout Oregon. Additional instrumentation helps scientists better hone in on the origin of individual earthquakes. That equipment since has been moved to other locations 鈥� part of a plan to gain better seismic understanding of the entire nation.
Braunmiller, who worked on the project with OSU colleagues Anne Trehu, John Nabelek and Mark Williams, said the seismic activity at Maupin is continuing.
鈥淵ou think at some point that the earthquakes are done,鈥� Braunmiller said, 鈥渂ut they keep cranking along. It isn鈥檛 likely that a major quake will follow, but you can never exclude that possibility. That uncertainty is why we need to do more research.鈥�
Provided by OSU College of Oceanic and Atmospheric Sciences
