The ground in the Puget Sound region didn’t just shake during the magnitude 6.8 Nisqually earthquake, it moved — literally.
In fact, measurements using global positioning system (GPS) data indicate that in most areas the ground shifted more during the Feb. 28 quake than it normally does in a year.
“Not only that, but it moved in completely the opposite direction of what we’ve observed from year to year,” said Anthony Qamar, a University of Washington research associate professor in Earth and space sciences and the state seismologist.
Qamar works on a project called PANGA, or Pacific Northwest Geodetic Array, that uses global positioning information to measure how much the ground in western Washington and Oregon move each year relative to a fixed point farther east. PANGA partners include the UW, Central Washington University, Rensselaer Polytechnic Institute in Troy, N.Y., Oregon State University, the U.S. Geological Survey, the U.S. Coast Guard and the Geological Survey of Canada.
PANGA’s measurements have shown that typically the central Puget Sound region moves to the east-northeast at about 3 to 5 millimeters per year. By contrast, at Neah Bay on the state’s northwest coast the movement is about 10 millimeters, or a half-inch, per year. That’s because the coast is much closer to the zone where the Juan de Fuca plate dives beneath the North American Plate, and the pressure moving the land surface is much greater than farther inland.
In the Nisqually earthquake, GPS sensors showed a Coast Guard station at Point Robinson on the east edge of Maury Island moved 8 millimeters to the south-southwest and the UW campus moved 5 millimeters — about two-tenths of an inch — south-southwest. The data showed that Satsop, which is about midway between the epicenter and the Washington coast, moved west about 6 millimeters and Pacific Beach, on the coast, moved northwest about 4 millimeters.
Though currently there are no measurements, Qamar also expects that data eventually will show that areas west of the earthquake’s focus deep beneath the Nisqually River delta north of Olympia rose as much as a half-inch in the quake. He expects that areas to the east will have dropped about one-third of an inch. (An earthquake’s epicenter is the area on the surface that lies directly above the hypocenter, or focus.)
The actual movement of the fault at the focus of the earthquake was probably about 1 meter, more than 3 feet, Qamar said. But the fact that the focus was some 34 miles deep in the Earth means the displacement at the surface is far less.
PANGA has about 20 permanent global positioning stations running in western Washington and Oregon. There also are 70 National Geodetic Survey sites permanently marked with metal plates that are in the process of being measured with portable GPS equipment to provide a more complete picture of what happened in the Nisqually quake. Those sites, a number of them lined up through the heart of the epicenter region, typically are measured every two years or so.
The purpose of PANGA is to allow scientists to see geographic positions changing over time. That happens as pressure is applied from the west by the interaction of the Juan de Fuca and North American plates, pushing this region east, and from the south by the movement of a large chunk of California against Oregon and Washington, pushing the region north. Eventually, those forces will counteract what happened in the Nisqually quake, Qamar said.
“I would expect that if we go back and measure the Satsop station in a year or two, we’ll see that it’s right back where it was before the earthquake,” he said.
For more information, contact Qamar at (206) 685-7563, (206) 543-7010 or http://www.geophys.washington.edu