Swarm of quakes bedevils Hanford

by Eric Mortenson, The Oregonian
Friday March 20, 2009, 8:41 PM

It's been a jittery week at eastern Washington's Hanford Nuclear Reservation, where more than 100 small earthquakes have been detected in the past seven days.

The quakes are part of an earthquake "swarm" that has puzzled scientists since it began at the first of the year. As of Friday, monitors at Hanford had detected more than 700 earthquakes since Jan. 4, said Alan Rohay, senior scientist and seismologist with Pacific Northwest National Laboratory, which operates at Hanford.

The quakes haven't disturbed the extensive stores of radioactive waste at Hanford or interfered with cleanup operations there.

The plant processed plutonium for nuclear weapons during World War II and the Cold War.

Highly contaminated liquid material is stored in underground tanks that have a history of leaks, and critics are wary of leaks or spills that could migrate to the nearby Columbia River.

Some of the quakes jolted farms and vineyards across the Columbia River from Hanford, but most were not felt by people and only a half-dozen have approached magnitude 3, Rohay said. Still, a significant increase in quakes during the past week caused Rohay on Friday to deploy two additional seismic monitors in an attempt to better track the activity.

"It's of concern because sometimes in swarms the magnitude has gotten up to 4, or 4.5," Rohay said. "That's getting to the size that things could get knocked off shelves or electrical systems can chatter.

"It would not do damage to a building or a (storage) tank, but it could cause some electrical equipment to fail or chemicals on a lab bench might spill."

Storage tanks at Hanford were built in the 1950s, 1960s and 1970s, when earthquake standards were not as defined as they are today. Rohay said underground tanks such as those at Hanford generally aren't susceptible to earthquake damage, but a quake of magnitude 6 might cause material in the tanks to slosh and perhaps spill or leak.

Each increase in magnitude is a tenfold increase in power; the quakes registered at Hanford this year have been mild compared with a magnitude 6 event.

The earthquakes are centered near Wooded Island on the Columbia River, about eight miles north of Richland, Wash. Earthquake swarms were reported in the area in 1970, 1975 and 1988.
Scientists believe the quakes are occurring in layers of Columbia River basalt. The area is under steady geologic compression north and south, which causes brittle edges of the basalt layer to break. One researcher called it a "snap, crackle, pop" effect that results in a flurry of small earthquakes.

The seismic monitors Rohay deployed Friday may help pinpoint the depth at which the quakes are occurring. If the quakes are confined to a single layer of basalt, they are likely to remain small, he said.

"If they're within 100 meters of each other in depth, you might think they are in the same basalt flow, and you wouldn't expect them to get bigger," he said. If seven or eight layers are cracking, it could cause a magnitude 4 quake.

Swarms were first noticed in the area after nearby farms began using groundwater for irrigation.

Some researchers speculate the basalt layers may be sensitive to changes in the water table or water pressure.

Hanford shares monitoring equipment with the University of Washington's Pacific Northwest Seismic Network. Researchers at UW recorded 103 earthquakes in the past week; Rohay said his count shows more than 200, however, because Hanford's computers are set to recognize smaller events as earthquakes.
-- Eric Mortenson; ericmortenson@news.oregonian.com