Swarm of small quakes occurring in Hanford area

HANFORD -- About 300 small earthquakes have been detected at southern Hanford since Jan. 4 and the intensity has increased over the past week, said Alan Rohay, a seismologist at Pacific Northwest National Laboratory.

The largest was a magnitude 2.9 quake measured Feb. 22, he said.

The swarm of quakes continued Wednesday, with the latest one at 2:19 p.m. a magnitude 0.4 earthquake.

There have been no reports of people feeling the earthquakes. But if someone had been within a mile of the fault during the quakes that approached magnitude 3.0, the person might have been able to feel them, Rohay said.

The quakes are occurring about eight miles north of Richland just west of the Columbia River about halfway between Hanford’s 300 Area and Energy Northwest. The closest landmark is Wooded Island in the Columbia River, which is just north and east of the fault.

It’s an area that has produced small quakes before, with similar swarms recorded in 1970, 1975 and 1988. In addition, smaller numbers of quakes -- 10 or 20 -- are recorded there every few years, Rohay said.

Monitoring for small quakes was not done for many years before 1970, so there’s no telling how long the swarms have been occurring.

When the swarms occur, they usually tend to build up gradually and then gradually die off. Consequently, there’s no telling if the earthquakes will continue to build or start to die off, Rohay said.

The quakes do not pose a risk to Hanford cleanup now, but if the national lab starts measuring quakes of a magnitude 4.0 -- which can cause moderate damage -- it will notify Hanford officials, he said. That’s not expected to happen.

The fault is likely about 300 by 300 feet and less than a mile deep, Rohay said. Because of the thickness of the basalt rock layer at that depth, there’s a limit to the magnitude of the quakes the fault produces, he said.

Bigger quakes would be more likely to result from faults that would be about six miles down where there is more crystalline rock such as granite.

The quakes near Wooded Island occur when basalt under stress reaches the breaking point and cracks. As one crack occurs, that shifts stress to adjoining rock that may also crack, creating a sort of chain reaction, Rohay said.

The Department of Energy’s Pacific Northwest National Laboratory operates a network of seismograph stations through Eastern Washington, mostly to provide information about the Hanford nuclear reservation.

Hanford produced plutonium for the nation’s nuclear weapons program and was left contaminated with radioactive and hazardous chemical waste.

“We monitor the pattern of small earthquakes and use it to estimate how frequently larger earthquakes may occur,” Rohay said.

The swarms this year near Wooded Island will add to the accumulated knowledge of the past 40 years, but it’s just one element of observing the pattern of earthquakes in the area, he said.

Instrumentation and analysis have advanced since the program’s early years, which will allow researchers to learn more about the swarms than they could about those recorded in the ‘70s.

Although Hanford has not been affected by the swarm of earthquakes, they have been noted at the Laser Interferometer Gravitational-Wave Observatory at Hanford. It uses highly sensitive equipment to detect movement caused by gravitational waves from space.